(From Pilbeam SP: Ventilator graphics. In: Pilbeam SP, Cairo JM (editors): Mechanical Ventilation: Physiologic and clinical applications, ed 4, St Louis, 2006, Mosby.)

Figure 15-2 Analysis of key points in a flow/time graph. Shown is the inspiratory rectangular waveform during volume ventilation and exhalation of the tidal volume. A, Start of inspiratory flow. B, Peak inspiratory flow of 60 L/min (1 L/sec). C, End of inspiratory flow. Total inspiratory flow time is found between points B and C (0.5 sec) when the tidal volume is delivered. The tidal volume can be calculated at 500 mL. (1 L/sec of flow × 0.5 sec of inspiratory time = .5 L [500 mL] tidal volume.) D and E, Inspiratory hold (inflation hold) of 0.25 second after the end of inspiration. Without inspiratory hold, the patient would begin exhalation at point D. Total inspiratory time is 0.75 second (0.5 sec inspiratory flow time + 0.25 inspiratory hold time = 0.75 second). F, Peak expiratory flow of −80 L/min (−1.33 L/sec). G, End of expiratory flow. Expiratory flow time is 0.75 second. H, Start of the next inspiration. G and H, 0.75 second of zero flow time. Total expiratory time is 1.5 second (0.75 sec expiratory flow time + 0.75 sec zero flow time). Inspiratory/expiratory (I : E) ratio is 1 : 2 (0.75 sec of total inspiratory time/1.5 sec of total expiratory time).

(Modified from Pilbeam SP: Ventilator graphics. In: Pilbeam SP, Cairo JM (editors): Mechanical ventilation: physiologic and clinical applications, ed 4, St Louis, 2006, Mosby.)

b. Perform the procedure to measure ventilator pressure/volume and flow/volume loops (WRE Code: IB9o) [Difficulty: WRE: R, Ap, An]

Follow the ventilator manufacturer’s steps to direct the unit to create flow, volume, and pressure waveforms. The patient can be instructed, based on the breathing test, to perform a breathing maneuver actively or to lie passively as the ventilator delivers a breath.

c. Select ventilator graphics (Code: IIID3) [Difficulty: ELE: R, Ap; WRE: An]

The operator typically can select any two of the following for display on the monitor: time, flow, pressure, and/or volume. Certain combinations are selected to best present the needed information. For example, air trapping is best shown by comparing flow vs. time (Figure 15-3) peak and plateau pressures are best shown by comparing pressure vs. time (Figure 15-4), and lung inflection points are best shown by comparing volume vs. pressure (discussed later). Note examples of ventilator graphics throughout this chapter.

Figure 15-3 Two graphic tracings of expiratory air trapping. Top, Volume vs. time. A shows that the exhaled tidal volume tracing does not reach the baseline. Inspiratory tidal volume is greater than expiratory tidal volume, indicating air trapping. Bottom, Flow vs. time. Inspiratory flow is the tracing above the horizontal baseline of zero flow. Expiratory flow is the tracing below the horizontal baseline. The patient’s expiratory flow does not return to zero; this indicates air trapping. The higher the flow rate, the more air trapping is present. Air trapping leads to auto–positive end-expiratory pressure. It can be minimized by increasing expiratory time, giving an aerosolized bronchodilator to treat bronchospasm, or suctioning out any secretions.

(Reprinted by permission of Nellcor Puritan Bennett LLC, Boulder, CO, part of Covidien.)

Figure 15-4 Decreased dynamic compliance (Cdyn) with a stable static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

d. Interpret ventilator pressure/volume and flow/volume loops (WRE code: IB10o) [Difficulty: WRE: R, Ap, An]

A number of flow, volume, and pressure waveforms have been included in this chapter for practice. In addition, review Figures 4-1 and 4-10 for examples of pulmonary function test waveform tracings. The examples in this chapter include common clinical situations and are accompanied explanations to help with interpretation.

Exam Hint 15-1 (WRE)

Expect to see at least one examination question that describes a waveform or shows a waveform that must be analyzed.

2. Review the patient’s chart for information on the work of breathing (Code: IA7d) [Difficulty: ELE: R; WRE: Ap]

Work of breathing (WOB) normally refers to how much energy the patient has to expend to inhale. Patients with stiff lungs or high airway resistance or both have an increased WOB. Exhalation normally is passive and requires no work. However, some patients with high airway resistance have to work to exhale. Look in the patient’s chart for information on patient complaints of shortness of breath and easy tiring as signs of increased WOB.

A patient who has been intubated and placed on a modern mechanical ventilator with a microprocessor and graphics software can have WOB measured. See Figure 15-5 for a pressure/volume loop tracing that shows a patient’s WOB. WOB is minimized when the ventilator is set to minimize the negative pressure and inspiratory flow the patient has to generate.

Figure 15-5 Two graphic tracings of pressure support ventilation. Top graphic shows pressure/volume loop of a patient triggering a breath. A, Amount of work patient has to provide. Shaded area on right side indicates how much work was provided by ventilator. Bottom graphic shows pressure (Paw) vs. time and flow (V.) vs. time tracings. A, Where patient initiated a breath. B, Where ventilator cycled off when pressure support level of 15 cm water was reached. C, High peak flow at start of the breath. D, How flow rate decreases as pressure support level is reached.

(Reprinted by permission of Nellcor Puritan Bennett Inc LLC, Boulder CO, part of Covidien.)

If the breathing of an intubated and ventilated patient appears to be unsynchronized with the ventilator, his or her WOB should be evaluated. Ask the conscious patient simple questions to try to determine what the problem is. Observe the patient for use of accessory muscles as a sign of increased WOB. If the ventilator is capable, program it to perform a pressure/volume loop of the patient’s WOB. Be prepared to adjust parameters such as the machine’s sensitivity and inspiratory flow to minimize the patient’s workload.

3. Airway resistance

a. Review the patient’s chart for information on airway resistance (Code: IA7d) [Difficulty: ELE: R; WRE: Ap]

Airway resistance (Raw) may have been measured earlier in the pulmonary function laboratory or on the ventilator. Compare any earlier values with new measurements. This is important for understanding the patient’s trends toward an improving or worsening pulmonary condition.

Airway resistance is measured in units of centimeters of water per liter per second at a standard flow rate of 0.5 L/sec (30 L/min). The normal spontaneously breathing adult’s Raw is 0.6 to 2.4 cm water/L/sec; the normal 3-kg infant’s Raw is 30 cm water/L/sec. Do not forget that this procedure is being performed on a patient with an intubated airway on a ventilator. The endotracheal tube adds to the patient’s total Raw. The smaller the tube, the greater is the resistance that it offers to gas flowing through it. Altering inspiratory flow has an influence on the peak pressure measured for the calculation. As flow is reduced, gas turbulence is reduced and peak pressure is reduced. Conversely, a higher flow creates more turbulence and a higher peak pressure is seen.

b. Recommend measurement of the patient’s airway resistance (Code: IC7) [Difficulty: ELE: R, Ap; WRE: An]

This calculation is important because it provides valuable information on the patient’s pulmonary condition. The Raw value indicates how difficult it is to move the tidal volume through the patient’s airways and whether aerosolized bronchodilating medications are effective.

It is reasonable to recommend an airway resistance measurement on any patient who has an increased airway resistance problem such as chronic obstructive pulmonary disease (COPD), asthma, bronchospasm, or wheezing breath sounds. It also is reasonable to measure airway resistance before and after a bronchodilator medication is given to determine whether it had any benefit.

c. Determine the patient’s airway resistance (Code: IB9n) [Difficulty: ELE: R, Ap; WRE: An]

Most of the microprocessor ventilators offer software for calculating all of these values. However, in other situations, they must be calculated manually.

1. Procedure for calculating airway resistance

1. Cycle a tidal volume. The patient should be breathing passively; fighting the breath will result in an erroneously high peak pressure, and assisting with the breath will result in an erroneously low peak pressure.

2. Note the peak airway pressure on the manometer.

3. Briefly prevent the tidal volume from being exhaled. No air should be moving. Note that the pressure manometer shows a peak pressure and then a static or plateau pressure that is stable as long as the tidal volume is held in the lungs. Record the plateau pressure.

4. Calculate the flow in liters per second by taking the flow in liters per minute and dividing it by 60 seconds.

5. Place the peak airway pressure, plateau pressure, and flow into the following formula and solve for airway resistance:

Exam Hint 15-2 (ELE)

The airway resistance calculation has been tested on previous examinations, so review the math.

EXAMPLE

A mechanically ventilated patient has a peak airway pressure of 30 cm water and a plateau pressure of 20 cm water. The peak flow is set at 60 L/min.

Calculate peak flow in liters per second:

Calculate airway resistance:

This value is greater than normal for a patient who is breathing spontaneously. However, remember that the patient’s airway is intubated. This results in a smaller airway diameter and greater resistance. Some practitioners use the calculated airway resistance as the basis for setting the pressure support ventilation (PSV) level (discussed later). Increased airway resistance may indicate bronchospasm or secretions in the airways. Delivering an aerosolized bronchodilator or suctioning should result in the return of resistance to the original level.

4. Auto-PEEP detection

a. Perform the procedure to detect auto-PEEP (WRE code: IB9w) [Difficulty: WRE: R, Ap, An]

Auto-PEEP is positive end-expiratory pressure in the lungs that cannot be seen on the ventilator’s pressure manometer. (The terms inadvertent PEEP and intrinsic PEEP also are used.) Auto-PEEP is caused by air trapping resulting from an inadequate expiratory time. It becomes more likely when the inspiratory time is increased or the expiratory time is decreased, or in patients with long time constants of ventilation. Simply put, the next breath is delivered before the patient has exhaled completely (see Figure 15-3). This problem is seen frequently in patients with status asthmaticus or COPD because of early small airway closure. In patients with acute respiratory distress syndrome (ARDS) who are receiving pressure-controlled inverse ratio ventilation (PCIRV), the long inspiratory times used increase the risk of expiratory air trapping. Auto-PEEP is more likely to be found when the inspiratory/expiratory (I : E) ratio becomes 2 : 1 or greater.

The level of auto-PEEP can be determined in different ways, depending on the type of ventilator that is being used. It can be measured on the pressure manometer of most ventilators. The trapped expiratory gas also can be seen on the graphic display of all current generation microprocessor-type ventilators. The following procedure can be followed for determining the presence or level of auto-PEEP:

1. Note the delivery of a tidal volume.

2. Watch the pressure gauge as it decreases to zero (or the level of therapeutic PEEP) at the end of exhalation. Make sure that the patient is exhaling passively to obtain an accurate reading.

3. Reduce the rate control to delay the next breath.

4. Occlude the expiratory tubing (or push the expiratory hold button or add inflation hold on other ventilators) to prevent any further exhalation for about 3 to 5 seconds. See Figure 15-6.

5. If the pressure gauge is being monitored, note any pressure increase above the baseline pressure, or, if the ventilator has a graphics monitor, note the failure of the exhaled tidal volume or expiratory flow to return to baseline to confirm the presence and amount of auto-PEEP (see Figure 15-4).

6. Listen to the patient’s breath sounds during a standard breath and during the prolonged exhalation. It is likely that during the standard breath, expiratory sounds will be heard until inspiratory sounds begin. During prolonged exhalation, the expiratory sounds continue for a longer time and then end with silence. This silent pause time indicates that no more expiratory airflow is occurring.

Figure 15-6 Identification of auto–positive end-expiratory pressure (PEEP) from air trapping. A, A normal patient will completely exhale before the next tidal volume breath is delivered. The manometer shows a pressure of zero (ambient, atmospheric pressure), which matches the patient’s airway and lung pressures. B, A patient with increased airway resistance (asthma or chronic obstructive pulmonary disease [COPD]) who is not able to exhale completely before the next breath is started. Even though the manometer shows zero pressure, pressure from the trapped gas is increased within the airways and lungs. C, Auto-PEEP can be measured by closing the exhalation valve at the end of inspiration and stopping the next tidal volume breath. During this prolonged expiratory time, flow and pressure equilibrate. The manometer pressure now shows the actual increased pressure with the patient’s airways and lungs. Reduce auto-PEEP by reducing airway resistance and increasing the expiratory time.

(Redrawn from PePe PE, Marini JJ: Am Rev Respir Dis 126:166, 1982, in Pilbeam S, Cairo J [editors]: Mechanical ventilation, ed 4, St Louis, 2009, Mosby.)

It is important to add any auto-PEEP to the amount of therapeutic PEEP the patient has. This should be recorded as the total PEEP. For example, the patient has 5 cm of therapeutic PEEP and 2 cm of auto-PEEP for 7 cm of total PEEP. It may be thought that the total PEEP level places the patient at risk for volutrauma or decreased venous return and lowered cardiac output. The amount of auto-PEEP can be reduced by decreasing the inspiratory time, increasing the expiratory time, or decreasing the tidal volume. Lack of auto-PEEP can be confirmed by this procedure. If the auto-PEEP cannot be eliminated, therapeutic PEEP can be added to match it. By increasing the baseline pressure, the patient can more easily trigger an assisted or synchronous intermittent mandatory ventilation (SIMV) breath. It is especially important to decrease auto-PEEP and therapeutic PEEP levels as the patient’s lung compliance improves and airway resistance returns to normal.

b. Interpret ventilator graphics to detect auto-PEEP (WRE code: IB10w) [Difficulty: WRE: R, Ap, An]

Figure 15-3 demonstrates two ways that air trapping on exhalation can be identified as auto-PEEP (unintended positive end-expiratory pressure). Note in Figure 15-3 (bottom) that the patient’s expiratory flow does not reach baseline pressure before another breath is delivered. This proves that air trapping has occurred. The larger the gap between the pressure at the end of expiration and at baseline, the greater is the air trapping.

5. Determine the patient’s plateau pressure (Code: IB9n) [Difficulty: ELE: R, Ap; WRE: An]

The plateau pressure on the ventilator is determined for evaluation of the patient’s lung compliance (C_LT). A change in the patient’s lung compliance will correlate with changes in the patient’s lung condition. The following two areas of discussion cover lung compliance, performing the lung compliance procedure, and interpreting the results.

There are two ways to determine the plateau pressure. First, have the patient lie passively and set the ventilator to prevent a delivered tidal volume from being exhaled (inflation hold). Second, have the patient lie passively, and manually cover the end of the expiratory tubing to prevent a delivered tidal volume from being exhaled.

The hold should be held for about 2 seconds so that the pressure manometer value is stable. Note the pressure as the plateau pressure (see Figure 15-4, A). After the plateau pressure has been determined, the patient must exhale completely. It may be necessary to delay the next timed ventilator tidal volume breath to avoid “stacking” a new breath when the first has not yet been exhaled. It is recommended that the plateau pressure procedure be repeated to ensure that the measured pressure is accurate.

6. Lung compliance

a. Review the patient’s chart for information on lung compliance (Code: IA7d) [Difficulty: ELE: R; WRE: Ap]

This value may have been measured earlier in the pulmonary function laboratory or on the ventilator. Compare any earlier values with new values. This is important for determining the patient’s trends toward an improving or worsening pulmonary condition.

The compliance values indicate how easily the tidal volume can be delivered into the lungs. Static compliance (Cst) is the measurement of work required to overcome the elastic resistance to ventilation. It is a measurement of the compliance of the lungs and thorax (C_LT). Static compliance is measured in units of milliliters per centimeters of water pressure. The normal adult’s static compliance is 100 mL/cm water; the normal 3-kg infant’s static compliance is 5 mL/cm water.

Dynamic compliance (Cdyn), sometimes called dynamic characteristic, is the measurement of the combination of the patient’s static compliance and airway resistance. As was discussed previously, airway resistance is the pressure required to move a tidal volume through the airways. It also is known as nonelastic resistance to ventilation.

b. Recommend measurement of the patient’s lung compliance (Code: IC7) [Difficulty: ELE: R, Ap; WRE: An]

It is reasonable to recommend that lung compliance should be measured on any patient who has clinical evidence of a significant change in lung compliance. This may occur in conditions such as ARDS, pneumonia, pulmonary edema, or pulmonary fibrosis. Compliance can be measured to document worsening of the patient’s condition or to determine whether compliance is improving with treatment.

c. Determine the patient’s plateau pressure and lung compliance (Code: IB9n) [Difficulty: ELE: R, Ap; WRE: An]

Most microprocessor ventilators offer software that can be used for calculating all of these values. However, in other situations, they must be calculated manually, as follows.

1. Procedure for calculating the tubing compliance factor

Before the actual calculation of the patient’s static and dynamic compliance can be performed, the compliance of the breathing circuit should be determined. During positive-pressure ventilation, some of the set tidal volume never reaches the patient because it is “lost” in the circuit. Remember that when a positive-pressure breath is delivered, the ventilator circuit will be expanded and the gas within the tubing will be compressed. The term compressed volume is commonly used to describe this lost volume. Subtract the compressed volume from the exhaled volume coming from the ventilator to determine the patient’s actual tidal volume. (Some of the current generation of microprocessor ventilators will calculate the compressed volume. The therapist can have the ventilator compensate for the lost volume and deliver a tidal volume that meets the set volume on the ventilator.)

For greatest accuracy in the calculation of static and dynamic compliance and the calculation of actual tidal and sigh volumes, any lost volume must be subtracted from the exhaled tidal volume to find the actual tidal volume. The tubing compliance factor is used in the calculation to determine the compressed volume, through the following procedure:

1. Remove the patient from the ventilator and manually ventilate him or her during this procedure.

2. Set the pressure limit as high as possible.

3. Block the breathing circuit at the patient connector. Make sure there are no leaks.

4. Cycle a tidal volume. If possible, set inflation hold for 2 seconds to see a stable pressure.

5. Perform either of the following: (a) note the pressure developed in the circuit as the tidal volume stretches out the circuit; or (b) if the ventilator’s peak pressure hits the pressure limit, note the delivered tidal volume and the pressure limit. In both cases, measure the exhaled tidal volume.

Note: If the patient is receiving PEEP therapy, subtract the PEEP level from the measured pressure to find the true pressure.

6. The compliance factor is found by dividing the exhaled tidal volume by the pressure.

7. Return the patient to the ventilator.

EXAMPLE

The patient has a set tidal volume of 600 mL. By using step 5b, the pressure is found to be 80 cm water, and the measured tidal volume is found to be 320 mL.

The compressed volume is found by multiplying the compliance factor by either the peak or the plateau pressure when a tidal volume is delivered. The compressed volume is then subtracted from the exhaled tidal volume to determine the patient’s actual tidal volume.

Exam Hint 15-3 (ELE, WRE)

Commonly, examinations have had a question requiring the calculation of static compliance or dynamic compliance or both. Be able to perform the following calculations.

2. Procedure for calculating static compliance

1. Determine the compliance factor of the breathing circuit (as described previously).

2. Reattach the patient to the ventilator. Reset all controls to their ordered or preset positions.

3. Cycle a tidal volume. The patient should be breathing passively; fighting the breath will result in an erroneously high peak pressure, and assisting with the breath will result in an erroneously low peak pressure.

4. Briefly prevent the tidal volume from being exhaled. No air should be moving. Note that the pressure manometer shows a peak pressure and then a static or plateau pressure that is stable as long as the tidal volume is held in the lungs. Note the plateau pressure.

5. Calculate the Cst by using this formula:

in which compressed volume is Compliance factor × Plateau pressure.

3. Procedure for calculating dynamic compliance

1. Determine the compliance factor of the breathing circuit (as described previously).

2. Reattach the patient to the ventilator. Reset all controls to their ordered or preset positions.

4. Note the peak pressure on the manometer. If the pressure at the end of inspiration is less than the peak pressure, the pressure at the end of inspiration should be used in the calculation.

5. Calculate the dynamic compliance (Cdyn) by using this formula:

in which compressed volume is Compliance factor × Peak pressure.

EXAMPLE

Calculate the static and dynamic compliance on a ventilated patient without PEEP therapy.

The patient has an exhaled tidal volume of 600 mL. The peak pressure is 30 cm water, and the static or plateau pressure is 20 cm water. The compliance factor has been determined to be 4 mL/cm water. The compressed volume at the plateau pressure is determined to be 80 mL (4 mL/cm compliance factor × 20 cm). The compressed volume at the peak pressure is determined to be 120 mL (4 mL/cm compliance factor × 30 cm).

EXAMPLE

Calculate the static and dynamic compliance on a ventilated patient with PEEP therapy.

The same patient has an exhaled tidal volume of 600 mL. Because of refractory hypoxemia, 10 cm of PEEP therapy is started. The peak pressure is now 36 cm water, and the static or plateau pressure is now 25 cm water. The compliance factor has been determined to be 4 mL/cm water. The compressed volume at the plateau pressure is determined to be 60 mL [4 mL/cm compliance factor × 15 cm (25 cm − 10 cm PEEP)]. The compressed volume at the peak pressure is determined to be 104 mL [4 mL/cm compliance factor × 26 cm (36 cm − 10 cm PEEP)].

7. Interpret the patient’s lung mechanics results: plateau pressure, airway resistance, dynamic lung compliance, and static lung compliance values on the ventilator (Code: IB10n) [Difficulty: ELE: R, Ap; WRE: An]

Exam Hint 15-4 (ELE, WRE)

Every examination has had questions that deal with the interpretation of increasing or decreasing peak pressure (changing airway resistance) or increasing or decreasing plateau pressure (changing lung compliance). Be able to identify a clinical change, possible causes of the change, and corrective actions.

Any increase in airway resistance or decrease in lung compliance or both creates an increase in the patient’s WOB. Examples of conditions or situations in which an increased airway resistance is found include bronchospasm, secretions, mucosal edema, airway tumor, placement of a small endotracheal tube, and biting or kinking of the endotracheal tube. Lung compliance is decreased by pneumonia, pulmonary edema, ARDS, pulmonary fibrosis, atelectasis, consolidation, hemothorax, pleural effusion, air trapping, pneumomediastinum, and pneumothorax. Examples of chest wall and abdominal conditions that reduce compliance include various chest wall deformities, circumferential chest or abdominal burns, enlarged liver, pneumoperitoneum, peritonitis, abdominal bleeding, herniation, and advanced pregnancy. Correction of the problem should return the patient’s ventilator pressure(s) to baseline and normalize the patient’s WOB.

Six possible combinations of increasing or decreasing static and dynamic lung compliance exist. Each has its own possible causes and is discussed in turn. The patient must be passive on the ventilator for the measured values to be accurate. Check two or three breaths for increased accuracy. Let the patient have a normal breath or two between each of the peak and plateau pressure measurement breaths.

a. Decreased dynamic compliance with stable static compliance

Decreased dynamic compliance with stable static compliance is noticed as an increase in the peak pressure with an unchanged plateau pressure (see Figure 15-4). It is caused by increased airway resistance (bronchospasm, secretions, water in the circuit, kinked circuit, or endotracheal tube). Correction of the underlying problem results in return of the peak pressure to the original level.

Note that the inspiratory resistance has doubled from the original 10 to 20 cm, while the plateau pressure has not changed. This confirms that the problem originates in the airway or breathing circuit. The patient’s lung compliance has not changed.

b. Increased dynamic compliance with stable static compliance

Increased dynamic compliance with stable static compliance is noticed as a decrease in peak pressure with an unchanged plateau pressure (Figure 15-7). This represents an improvement in the patient’s airway resistance from the original condition. Secretions can be diminished, mucus plugs cleared, bronchospasm corrected, and so forth.

Figure 15-7 Increased dynamic compliance (Cdyn) with a stable static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

Note that the inspiratory resistance has decreased from the original level of 20 cm to just 5 cm. This confirms that the patient’s airway resistance has decreased. The patient’s lung compliance has not changed.

c. False decreased dynamic compliance with true decreased static compliance

False decreased dynamic compliance with true decreased static compliance is noticed as an increase in both peak and plateau pressures (Figure 15-8). This is seen when the patient’s lung/thoracic compliance worsens. The plateau pressure is elevated, and the static compliance is decreased.

Figure 15-8 False decreased dynamic compliance (Cdyn) with true decreased static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

As an artifact of the stiffer lungs, the peak pressure also is elevated and the dynamic compliance is decreased. However, the difference between peak and plateau pressures remains the same. This demonstrates that no real increase in the patient’s airway resistance has occurred.

d. True decreased dynamic compliance with true decreased static compliance

True decreased dynamic compliance with true decreased static compliance also is noticed as an increase in both peak and plateau pressures (Figure 15-9). This is seen with the combination of decreased lung compliance and increased airway resistance. Causes of both of these problems were discussed earlier.

Figure 15-9 True decreased dynamic compliance (Cdyn) with true decreased static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

e. False increased dynamic compliance with true increased static compliance

False increased dynamic compliance with true increased static compliance is noticed as a decrease in both peak and plateau pressures (Figure 15-10). This is seen when the patient’s lung/thoracic compliance improves. The plateau pressure decreases, and, as an artifact, the peak pressure also decreases. Notice that the difference between peak and plateau pressures remains the same. This indicates that the patient’s airway resistance is unchanged.

Figure 15-10 False increased dynamic compliance (Cdyn) with true increased static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

f. True increased dynamic compliance with true increased static compliance

True increased dynamic compliance with true increased static compliance also is noticed as a decrease in both peak and plateau pressures (Figure 15-11). This is seen when the patient’s airway resistance and his or her lung/thoracic compliance improve. Notice that the plateau pressure has decreased, thus indicating more compliant lungs. Also notice that the difference between peak and plateau pressures has decreased. This demonstrates that the airway resistance also has decreased.

Figure 15-11 True increased dynamic compliance (Cdyn) with true increased static compliance (Cst). A, Original pressure manometer reading and pressure/volume curve. B, Altered pressure manometer reading and pressure/volume curve.

All six examples of increasing or decreasing static or dynamic lung compliance or both make use of a single tidal volume that is analyzed for peak and plateau pressures. Some practitioners advocate using several different tidal volumes (e.g., 8, 10, and 12 mL/kg of ideal body weight) when measuring dynamic and static pressures. The measured values are plotted on a graph to find the patient’s optimal tidal volume that results in the highest static compliance value. Figure 15-12 shows a series of these graphs. The curves for diseased lungs and airways are quite different from those of a normal person or a patient with a pulmonary embolism. Because of this, a pulmonary embolism should be considered if the patient’s condition deteriorates rapidly and no change in dynamic or static compliance values is observed.

Figure 15-12 Pressure/volume curves for normal airways and lungs, pulmonary embolism (no change in airway resistance or lung compliance), increased airway resistance, and decreased lung compliance.

(From Bone RC: Respir Care 28[5]:597, 1983.)

MODULE B

Provide conventional mechanical ventilation to adequately oxygenate and ventilate the patient.

Conventional mechanical ventilation is defined here as the use of a single ventilator that can provide the customary modes and options needed by the large majority of patients. This ventilatory support is provided through an endotracheal tube or a tracheostomy tube. Several physiologic criteria have been compiled to help the clinician determine when a patient is in respiratory or ventilatory failure (Box 15-1) and needs ventilatory support. Remember that the patient may not fail each and every criterion; however, the patient often will fail one or more criteria in each category.

BOX 15-1 Indications for Ventilatory Support

VENTILATION

Apnea

PaCO₂ ≥55 torr in a patient who is not ordinarily hypercapneic

Dead space: tidal volume (V_D/V_T ratio) of >0.55–0.6 (55%-60%)

OXYGENATION

Pao₂ <80 torr on 50% oxygen or more

P(A-a)O₂ <300-350 torr on 100% oxygen

Intrapulmonary shunt >5%-20%

PULMONARY MECHANICS

Spontaneous tidal volume 3-4 mL/lb or 7-9 mL/kg of ideal body weight

Vital capacity <10-15 mL/kg

Maximum inspiratory pressure (MIP) <-20 to −25 cm water pressure

Forced expiratory volume in 1 second (FEV₁) <10 mL/kg

Respiratory rate <12 breaths/min or >35 breaths/min in an adult

Rapid, shallow breathing index (breaths/minute divided by tidal volume in liters) >105

MISCELLANEOUS

Unconscious patient

Unstable and unacceptable vital signs

Unstable cardiac rhythm caused by hypoxemia and/or acidosis

Worsening cardiopulmonary or other major organ system

1. Perform the following procedures to make sure that the patient is adequately oxygenated

a. Minimize hypoxemia by positioning the patient properly (WRE code: IIID7) [Difficulty: Ap, An]

Most patients who receive mechanical ventilation will be in the supine position. However, if a patient is short of breath when lying supine, he/she should be repositioned to sit more upright in a Fowler’s or semi-Fowler’s position. This seems to work best in patients with bilateral pulmonary problems such as congestive heart failure or pneumonia. If the patient cannot sit up and the lung problem is one-sided, roll the patient so that the more functional lung is down. The good lung should be positioned up in the following exceptions:

• Undrained pulmonary abscess that should not be drained into the good lung

• Neonatal congenital diaphragmatic hernia in which the good lung should not be compressed by the bowel in the chest cavity

• Pulmonary interstitial emphysema in which, by lying on the bad lung, the air leak and the functional residual capacity can be reduced.

In either case, always ask the patient whether the new position helps to make breathing easier. If not, reposition the patient until breathing is more comfortable with less shortness of breath. In general, Trendelenburg is not well tolerated. A patient with a closed head injury and brain edema should not be put into the Trendelenburg position. Recent research has shown that some patients with ARDS will be better oxygenated when placed in the prone position.

b. Administer oxygen, as needed, to prevent hypoxemia (ELE code: IIID6) [Difficulty: ELE: R, Ap, An]

Oxygen administration and adjustment were discussed in Chapter 6. In brief, the goal of oxygen administration is to keep the Pao₂ level of most patients between 60 and 90 torr and the Spo₂ level greater than 90%. Exceptions are the patient with COPD who is breathing on hypoxic drive and the patient who is in a cardiac arrest situation. The following formula can be used to help guide the use of supplemental oxygen in most stable patients:

EXAMPLE

Your patient has a Pao₂ level of 55 mm Hg on 30% oxygen. The clinical goal is a Pao₂ level of 90 mm Hg. What oxygen percentage should the patient have?

Those patients who have refractory hypoxemia (e.g., ARDS) will not respond with a normal increase in Pao₂ level as the oxygen percentage is increased. In the short term, use whatever oxygen percentage is needed to achieve the clinical goal. The risk of oxygen toxicity increases when the Fio₂ is greater than 0.5 for periods of longer than 48 hours. Always recheck the patient’s arterial oxygen level after a change in the Fio₂ has been made.

Either of the following formulas can be used in the special situation of determining the flows of air and oxygen into a “bleed-in” type of intermittent mandatory ventilation (IMV) or continuous positive airway pressure (CPAP) system to obtain an ordered Fio₂. Either version can be used for determining gas flows, total flow, and oxygen/air ratio through an air entrainment (Venturi) mask.

The first formula follows:

The second formula follows:

in which F₁ is the flow of first gas (oxygen), C₁ is the concentration of oxygen in the first gas (1.0 for pure oxygen), F₂ is the flow of second gas (air), C₂ is the concentration of oxygen in the second gas (0.21 for air), F_T is the total flow of both gases, and C_T is the concentration of oxygen in the mix of both gases. Use algebraic manipulation to solve for the unknown.

EXAMPLE

Determine the oxygen percentage through a bleed-in system that has an oxygen flow of 10 L/min and an airflow of 15 L/min. Determine the total flow through the system. Determine the ratio of oxygen to air.

c. Adequately oxygenate the patient to prevent accidental hypoxemia before and after suctioning, changing the ventilator circuit, or performing other procedures in which the patient is disconnected from the ventilator (ELE code: IIID9) [Difficulty: ELE: R, Ap, An]

Ensuring adequate oxygenation during suctioning is discussed in Chapter 13. In brief, remember to give the adult patient 100% oxygen for at least 30 seconds before suctioning. Perform the task as quickly and safely as possible to minimize time off the ventilator. Leave the patient on 100% oxygen for at least 1 minute after the procedure is completed, or until he or she returns to a stable condition as before the procedure. Children younger than 6 months can have the F_Io₂ increased by 10% for the procedure.

In an adult, it is acceptable to increase the inspired oxygen up to 100% before and after a procedure that requires disconnection from the ventilator. The goal is to prevent hypoxemia. The patient should be ventilated manually with a resuscitation bag if indicated. Always remember to return the patient to the original oxygen percentage when clinically indicated.

2. Initiate and adjust continuous mechanical ventilation settings (Code: IIID2b) [Difficulty: ELE: R, Ap; WRE: An]

a. Sensitivity

Sensitivity is the term used to describe the amount of work or effort the patient must perform to trigger the ventilator to deliver a tidal volume breath. Many ventilators require the patient to generate a negative pressure to trigger the unit. The term pressure triggering is used often to describe this type of sensitivity. With these machines, the sensitivity usually is set at about −1 to −2 cm water pressure.

Several of the newer ventilators also have an option in which the patient generates an inspiratory flow to trigger a ventilator tidal volume breath. The term flow-triggering often is used to describe this sensitivity option on the ventilator. With a ventilator using flow triggering sensitivity, the patient initiates a tidal volume when his or her inspiratory flow is about 2 L/min (33 mL/sec) less than the set baseline flow through the circuit. Very weak patients find that it requires less effort with flow sensitivity than with pressure sensitivity to initiate a ventilator breath.

b. Flow

Flow is adjusted to set the inspiratory time and the I : E ratio. In addition, flow is set to meet the patient’s needs. Inspiratory flow should be great enough to minimize the WOB. Increase flow if the patient has signs of greater demand, such as using accessory muscles of inspiration or lack of synchrony with the ventilator, or if the pressure manometer deflects greatly below the baseline pressure or shows a low initial increase in inspiratory pressure.

In addition, most current generation ventilators offer more than one inspiratory flow pattern (see Figure 15-1). The sine wave is most physiologically like a normal, spontaneous inspiration. The other waveforms can be compared with the sine wave to determine which one best meets the patient’s needs. Ideally, the best flow pattern is one in which the patient’s peak and mean airway pressures are lowest, exhalation is complete, breath sounds are improved bilaterally, heart rate and blood pressure are stable, and the patient feels most comfortable.

4. Synchronous intermittent mandatory ventilation

5. Pressure support ventilation

Pressure support ventilation (PS or PSV) is similar to intermittent positive-pressure ventilation (IPPV) in that when the patient initiates a ventilator breath, a preset pressure is delivered to the airway. The patient has the flexibility to determine the respiratory rate. The physician orders a PSV level for one of two reasons, depending on the clinical goal. First, enough PS is ordered to overcome the patient’s calculated airway resistance. This usually is done when the patient has an increased work of breathing from a smaller than ideal endotrachal tube. Second, PS is ordered to deliver a targeted tidal volume. In most cases, the tidal volume will be stable if the patient passively takes the PSV breath, or it can be larger if the patient interacts actively with the pressure that is delivered (Figures 15-5 and 15-13, E show the pressure/time curve).

Because PS utilizes a set delivered pressure, the patient’s tidal volume can vary with changing lung compliance and/or airway resistance. To bring stability to the tidal volume under changing patient conditions, several ventilator manufacturers have developed automatic compensation systems. They are designed to provide additional volume if the patient’s lung compliance and/or airway resistance should worsen. Conversely, they will reduce automatically the set pressure when the patient is easier to ventilate, so that too large a tidal volume will not be delivered. All manufacturers have their own name for this compensation system and have developed different ways to accomplish the goal of a stable tidal volume. Examples include pressure augmentation on the Bear 1000, volume-assured pressure support on the Bird 8400, dual-control pressure ventilation on the Puritan Bennett 840, volume support on the Servo-i, and AutoFlow on the Dräger Evita 4. It is beyond the scope of this text to cover each of these. However, the practitioner is encouraged to understand their essential features and to know that they are added to the pressure support mode to ensure a stable, ordered tidal volume.

6. Pressure control ventilation

Pressure control ventilation (PC or PCV) involves the delivery of tidal volume breaths that are pressure limited and time cycled. A set ventilator rate can be set and the patient can trigger additional breaths. Because the pressure is limited, tidal volumes may vary. This must be monitored closely in patients with frequently changing lung compliance and airway resistance. As the inspiratory time is increased, it can become longer than the expiratory time. This results in pressure control inverse ratio ventilation (PCIRV). Figure 15-14 shows the volume, flow, and pressure tracings.

Figure 15-14 Two sets of graphic tracings of pressure control ventilation. A, Two tracings show volume (V_T) vs. time and flow (V.) vs. time. A shows tidal volume; B shows inspiratory flow. Note how tidal volume increases as inspiratory time is increased. However, as is shown in the lower right tracing, as flow decreases to zero, no more volume is delivered. B, Two tracings show pressure vs. time and flow vs. time. A shows the pressure control level being reached with an inspiratory plateau or “square wave” appearance. B shows declining inspiratory flow. C shows final flow when inspiration is time cycled off and exhalation begins. This graphic can be used to help adjust a longer inspiratory time. If pressure control inverse ratio ventilation (PCIRV) were being optimally adjusted, the inspiration time could be increased until inspiratory flow reached zero (indicated by D).

(Image used by permission from Nellcore Puritan Bennett, LLC, Boulder CO, part of Covidien.)

As was discussed earlier with the PS mode, the patient’s delivered tidal volume can vary with the PC mode when lung compliance and/or airway resistance is changing. Review the previous discussion on PS for the automatic compensation systems that manufacturers have added to their ventilators. Use of these ensures that the delivered tidal volume is stable in the PC mode despite changing patient conditions.

7. Airway pressure release ventilation

Several ventilator manufacturers offer the airway pressure release ventilation (APRV) mode (as well as other modes). These ventilators are microprocessor controlled and include a monitor for patient data and graphics. The APRV mode has been used with success in patients with ARDS who have not responded well to constant volume ventilation. APRV can be described simply as a mode in which the patient can breathe spontaneously at two different levels of CPAP. A difference from conventional CPAP is that the two levels are held for set periods. The ventilator options for this mode are simple. The practitioner sets the low pressure (P_low), the high pressure (P_high), and the times that the patient will be at those pressure levels. Low pressure sometimes is referred to as CPAP and high pressure as release pressure. The timing changes from low pressure to high pressure and back to low pressure effectively deliver a tidal volume. (See Figure 15-15 for a pressure/time tracing.)

Figure 15-15 Airway pressure release ventilation (APRV) involves spontaneous breathing under one or two elevated baseline pressures. In the top graph, the patient can breathe during ambient pressure and during continuous positive airway pressure (CPAP). In the bottom graph, the patient breathes during a lower CPAP level and a higher CPAP level. In both situations, the patient breathes spontaneously at a high pressure level for a prolonged, adjustable length of time. Then, for a short, adjustable time, the higher pressure drops to the lower pressure level. This allows the patient to exhale a larger than usual volume. The higher pressure then is restored. This causes a large tidal volume to be inhaled.

(From Dupuis Y: Ventilators: theory and clinical applications, ed 2, St Louis, 1992, Mosby.)

When APRV is compared with other modes of ventilation, several similarities are evident between it and PCIRV with PEEP and bilevel ventilation. All have an elevated baseline pressure that allows the patient to breathe spontaneously. All have variable inspiratory times for the higher pressure level. None delivers a set tidal volume to the patient. The only real difference seems to be that the patient can breathe spontaneously at the higher pressure level only with APRV. If the patient does not make any respiratory efforts, APRV functions like PCV or bilevel ventilation.

A set of blood gases, vital signs, pulmonary artery catheter values, and so forth should be obtained on the current constant volume ventilator settings as a baseline before APRV is started. The following suggestions for initiating APRV are similar to those listed earlier for starting PCIRV:

1. Set the high pressure (release pressure) at the patient’s optimal PEEP/CPAP level. Often this is in a range between the patient’s static lung compliance pressure and peak pressure (Figure 15-16). The clinical goal is to maximize safely the patient’s functional residual capacity (FRC).

2. Set the low pressure (CPAP) at the level of PEEP that was used on the constant volume ventilator. This is done to maintain the patient’s FRC. See point A of Figure 15-16.

3. Set the inspired oxygen in the same way as before. Some may prefer to set it at 100%, as with PCIRV, until blood gas results show that it can be lowered.

4. Set the timing of the high pressure and low pressure values so that more time is spent at the high pressure than at the low pressure. Often the low pressure time is limited to 1 to 1.5 seconds. Keep the same respiratory rate as before. Even if the patient is apneic, as the ventilator switches from high pressure to low pressure, the patient exhales a tidal volume to blow off carbon dioxide.

Figure 15-16 Pressure/volume loop showing lower and upper inflection points in a patient with acute respiratory distress syndrome (ARDS). A, Lower inflection point during inspiration that indicates the opening pressure of alveoli (15 cm water). This is the minimum positive end-expiratory pressure level needed to maintain a patient’s functional residual capacity. B, Upper inflection point during inspiration that indicates that the lungs are being overstretched (40 cm water). This should be the upper limit of peak airway pressure to avoid overdistention and volutrauma. Flattening of the pressure/volume loop to the right of point B shows that little volume is delivered despite increased pressure. C, Upper inflection point of expiration (20 cm water). As the airway pressure drops below this point, alveolar collapse can occur.

Monitor the patient’s ventilator delivered and spontaneous tidal volumes and rates. Check vital signs. Get an arterial blood gas (ABG) sample in about 15 minutes.

If the patient’s initial blood gas results on APRV show hypoxemia, the following options are available: (1) increase the inspired oxygen percentage if it is not already at 100%, (2) increase the low pressure level, (3) increase thehigh pressure level, or (4) increase the time the patient is kept at the high pressure level. Reducing any or all of these options will decrease the patient’s Pao₂ if it is too high.

If the blood gas results show hypoventilation, the following options are available: (1) increase the high pressure level, or (2) decrease the time at the high pressure level to increase the respiratory rate. Do the opposite to increase the Paco₂ if the patient is being hyperventilated.

As with the previous ventilator modalities, the patient should be monitored closely and should have an ABG drawn for evaluation of every change.

Exam Hint 15-6 (ELE, WRE)

The NBRC examinations have used a variety of terms and phrases to describe modes or modifications of modes of ventilation, including the following:

3. Begin and modify combinations of ventilatory techniques to oxygenate the patient adequately: synchronous intermittent mandatory ventilation, pressure support ventilation, pressure control ventilation, and positive end-expiratory pressure (Code: IIID2b) [Difficulty: ELE: R, Ap; WRE: An]

The current generation of mechanical ventilators offers the physician and the practitioner a number of options for how best to tailor ventilatory support to meet the patient’s needs. The following should be considered when one is deciding what modes to use and combine.

a. Increased work of breathing

Patients who show increased WOB may have a very high airway resistance, as in status asthmaticus, or may have a very low lung/thoracic compliance, as in ARDS. Some practitioners believe that the C or A/C mode, when applied properly to a sedated patient, is best for these problems because the patient’s breathing efforts are almost eliminated. Other practitioners believe that IMV and SIMV are physiologically superior modes of ventilation. More recently, PSV has been shown to be beneficial to patients with increased efforts at breathing from the high airway resistance caused by a small-diameter endotracheal tube.

b. Hypercapnia

A patient may have hypercapnia (a high carbon dioxide level) because of sedation from a morphine or heroin overdose or may have COPD with worsening of the chronic hypercapnia. In either case, the patient becomes progressively more hypoxemic (unless given supplemental oxygen) as the carbon dioxide level increases. Control or A/C modes are best for setting a minimum minute volume to determine the maximum carbon dioxide level. As the patient recovers, SIMV or PSV allows the gradual reduction of ventilatory support. See Box 15-2 for indications of SIMV tolerance.

BOX 15-2 Indications of Intermittent Mandatory Ventilation (IMV)/Synchronous Intermittent Mandatory Ventilation (SIMV) Tolerance

INDICATIONS THAT IMV/SIMV IS BEING WELL TOLERATED

Stable spontaneous respiratory rate

Stable heart rate

Stable spontaneous tidal volume

Stable vital capacity, minimal inspiratory pressure (MIP), and/or forced expiratory volume in 1 second (FEV₁)

No use or stable use of accessory muscles of ventilation

Patient indicates that he or she is comfortable

Stable blood gases

INDICATIONS THAT IMV/SIMV IS NOT BEING WELL TOLERATED

Increased spontaneous respiratory rate

Tachycardia or dysrhythmias such as premature ventricular contractions

Decrease in spontaneous tidal volume

Decrease in vital capacity, MIP, and/or FEV₁

Beginning or increased use of accessory muscles of ventilation

Patient complaints of dyspnea

Deterioration of blood gases as seen by a falling Pao₂ or Spo₂ and a rapidly falling or rising PaCO₂

c. Hypoxemia

If hypoxemia is secondary to a decreased FRC, as in ARDS or atelectasis, the treatment of choice for hypoxemia is CPAP on a free-standing system or PEEP on a conventional volume-cycled ventilator. If the problem results from an increased intrapulmonary shunt, the patient may need PEEP or CPAP, as well as up to 100% oxygen. See Box 15-3 for patient monitoring during PEEP and CPAP. PCIRV and high-frequency ventilation (HFV) have been used with success in hypoxemic patients with a pulmonary air leak for whom conventional volume ventilation has failed.

BOX 15-3 Patient Monitoring During Therapy

Often, when a patient has more than one problem, more than one solution will be needed. The following are combinations of modes from which to choose.

1. Mandatory minute ventilation

Mandatory minute ventilation (MMV) is a relatively new variation on the SIMV mode. It has been used as a weaning mode that limits the increase in carbon dioxide if the patient should tire. With MMV, the patient is assured of a preset minute volume regardless of his or her spontaneous breathing. It has been proposed as an effective way to ventilate and wean patients who can breathe spontaneously but who have an unreliable respiratory drive and unstable tidal volume. Examples include patients who have received narcotic, sedative, anesthetic, or neuromuscular blocking medications. Patient conditions for which MMV is indicated include encephalopathy and cerebral disorders such as stroke. In addition, MMV may be used during the recovery period of a neuromuscular disease. Ventilators that include the MMV mode all are controlled by a microprocessor that monitors the ventilator’s and the patient’s tidal volume and rate. The following guidelines have been recommended for the initiation of MMV:

1. Set the ventilator tidal volume according to established guidelines (10 to 15 mL/kg of ideal body weight).

2. Spontaneous breaths may be taken through a demand valve or may be pressure supported.

3. Determine the minimum minute volume according to the patient’s preexisting condition and the clinical goals:

The patient who has been on SIMV should have the MMV set at 90% of the SIMV-delivered minute volume. For example, the patient has an SIMV rate of 5 breaths/min and a tidal volume of 800 mL. Therefore the ventilator is delivering 4 L of minute volume (5 × 800 mL), and the MMV would be set at 3600 mL (90% of 4 L).

The patient who has been on A/C should have the mandatory minute volume set at 80% of the A/C-delivered minute volume. For example, the patient has an A/C rate of 10 breaths/min and a tidal volume of 1000 mL. Therefore the ventilator is delivering 10 L of minute volume (10 × 1000 mL), and the MMV would be set at 8 L (80% of 10 L).

Ideally MMV establishes a minimum safe volume of ventilation. If the patient inhales less than this volume, the ventilator delivers as many breaths as necessary at the preestablished tidal volume to make up the difference. Be aware that a patient who is breathing rapidly with a small tidal volume may move enough gas to exceed the minimum minute volume. Because of this risk, it is important to set a low tidal volume alarm or a high respiratory rate alarm or both to give warning. Do not let the programming of an MMV create a false sense of security with these patients.

2. Pressure control/pressure control inverse ratio ventilation, synchronous intermittent mandatory ventilation, and positive end-expiratory pressure

PCV or, if necessary, PCIRV has been used with success in patients with low compliance (ARDS) or a pulmonary air leak. When the peak pressure is limited, less air seems to leak out and the tissues are more likely to heal. Therapeutic PEEP is applied to increase the patient’s FRC to correct hypoxemia. The SIMV feature is added to let the patient breathe spontaneously if desired and to stay more synchronized with the ventilator. With lung healing, the PEEP level is decreased and the inspiratory time is shortened. SIMV with a constant tidal volume may be used during the weaning phase. Figure 15-17 shows pressure and flow tracings during PCV.

Figure 15-17 Modification of combinations of intermittent mandatory ventilation (IMV)/synchronous intermittent mandatory ventilation (SIMV), pressure support ventilation (PSV), and positive end-expiratory pressure (PEEP). A, IMV/SIMV with PSV and PEEP. B, IMV/SIMV with PEEP. C, IMV/SIMV with PSV. D, PSV with PEEP. See text for descriptions of various combinations and their clinical applications.

3. Synchronous intermittent mandatory ventilation with pressure support ventilation and positive endexpiratory pressure

SIMV is used to give the patient a controlled number of deep tidal volume breaths. The patient can breathe as often as desired between the mandatory breaths. The patient’s total minute volume can be determined by adding the combination of SIMV and pressure-supported breaths. A maximum acceptable Paco₂ can be established with the proper combination of SIMV breaths and pressure support (PS) level. A PS level of more than 10 cm water may be needed. In addition, the PS ensures that the airway resistance of the endotracheal tube is overcome. (See the airway resistance calculation earlier in the chapter.) PEEP therapy is applied to the level necessary to obtain a clinically safe Pao₂ at the lowest possible Fio₂. Figure 15-17, A shows the pressure/time curve.

Patients who have both a ventilation problem and an oxygenation problem benefit from these modes of ventilation. They have the desire to breathe on their own but a very limited ability to do so. All three modes can be adjusted independently for more or less support, as indicated by the patient’s clinical condition and blood gas results.

4. Synchronous intermittent mandatory ventilation with positive endexpiratory pressure

IMV/SIMV and PEEP therapy are applied as indicated. Pressure support is not needed if the patient is strong enough to overcome the airway resistance of the endotracheal tube and breathe with a clinically acceptable tidal volume. Figure 15-17, B shows the pressure/time curve.

5. Synchronous intermittent mandatory ventilation with pressure support ventilation

SIMV and PS levels are increased or decreased on the basis of the factors discussed earlier. This patient has the ability to provide some, but not all, of his or her own ventilation. The Pao₂ is clinically acceptable at an oxygen percentage probably no higher than 40%.

A fairly common clinical situation is seen in which the recovering patient does well on a gradually decreasing number of SIMV breaths until he or she can go no lower. The barrier seems to be the airway resistance of the endotracheal tube. (See the airway resistance calculation earlier in this chapter.) The addition of some PS overcomes that resistance so that the SIMV level can be reduced further. When the SIMV frequency is down to four or less, the patient is providing almost all of his or her minute volume. The greatest barrier to breathing is likely to be the resistance of the endotracheal tube. The decision then can be made to extubate the patient. Figure 15-17, C shows the pressure/time curve.

6. Pressure support ventilation with positive end-expiratory pressure

PSV and PEEP are applied as discussed earlier. This patient has the drive to breathe on his or her own; however, he or she has some limitation in the ability to overcome the resistance of the endotracheal tube or to generate a consistently large enough tidal volume. (See the airway resistance calculation earlier in the chapter.) In addition, the patient has a significant oxygenation problem and needs some PEEP therapy. With recovery, both PS and PEEP can be reduced. They may be reduced individually or simultaneously as the patient’s strength and/or oxygenation improves. Figure 15-17, D shows the pressure/time curve.

Exam Hint 15-7 (ELE, WRE)

Usually several questions require the therapist to evaluate a patient and make a change in the mode of ventilation. Select “Assist/control” if the patient needs full support. Select “SIMV” if the patient needs to be partially supported. Frequently, one of the questions deals with identifying the need to increase the pressure support (PS) level to overcome the resistance of the endotracheal tube.

4. Choose and adjust the tidal volume for mechanical ventilation

A spontaneously breathing person exhales a large enough tidal volume (at the necessary respiratory rate) to remove carbon dioxide as fast as it is produced by his/her metabolism. This results in a normal carbon dioxide level and acid-base balance. Obviously, a person’s tidal volume and respiratory rate will vary considerably with activity level. A 70-kg (154-lb) adult with normal lungs and metabolism needs a tidal volume of about 7 mL/kg (3 mL/lb) of ideal body weight to remove carbon dioxide adequately. So, this person would have a spontaneous tidal volume of about 500 mL (70 kg × 7 mL/kg). The Radford nomogram can be used for predicting normal spontaneous tidal volumes and rates on the basis of body weight (Figure 15-18). It can be used to help establish an initial target for the ventilator tidal volume for many patients with normal lungs. However, because compressed volume within the ventilator circuit causes some of the set tidal volume to not reach the patient’s lungs, a larger set tidal volume usually is needed. (Review “Procedure for calculating the tubing compliance factor” earlier in the chapter.) In addition, patients with lung problems will have different tidal volume requirements. Following are the current tidal volume recommendations based on the patient’s pulmonary condition:

• Normal lungs: Set ventilator tidal volume of 10 to 12 mL/kg of ideal body weight (IBW).

Figure 15-18 Radford nomogram. To use the nomogram, place the left edge of a ruler on the patient’s estimated body weight and move the right edge to the patient’s breathing frequency. The predicted basal tidal volume now can be found. For example, a 150-pound male who is breathing 12 times per minute will have a predicted tidal volume of 500 cc (mL).

(From Radford EP Jr: Ventilation standards for use in artificial respiration. J Appl Physiol 7[4]:451, 1955.)

This might be a patient with a normal cardiopulmonary system who is receiving mechanical ventilation because of a neurologic problem. For example, the previously mentioned 70-kg (154-lb) adult with normal lungs needs a set mechanical ventilator tidal volume in the following range:

• Restrictive lung disease: Set ventilator tidal volume of 4 to 8 mL/kg of ideal body weight (IBW).

This might be a patient with ARDS who is at risk for pulmonary barotrauma/volutrauma. For example, the previously mentioned 70-kg (154-lb) adult with stiff lungs needs a set mechanical ventilator tidal volume in the following range:

• Obstructive lung disease: Set ventilator tidal volume of 8 to 10 mL/kg of ideal body weight (IBW).

This might be a patient with COPD or status asthmaticus who is at risk for air trapping on exhalation. For example, the previously mentioned 70-kg (154-lb) adult with overstretched lungs needs a set mechanical ventilator tidal volume in the following range:

Remember that the set tidal volume and measured exhaled tidal volume are greater than the patient’s actual tidal volume that reaches the lungs. This is because the compressed volume is lost in the circuit and does not go into the patient. So, in all of the above examples, the set tidal volumes are greater than the patient’s actual tidal volume.

It is common practice to get a set of ABG values after the patient is stable on the ventilator. The tidal volume can be adjusted within the range depending on whether the patient’s arterial CO₂ pressure (Paco₂) value is too high or too low for the therapeutic goal. The most direct way to change alveolar ventilation is to modify the delivered tidal volume. If everything else remains the same, a larger tidal volume results in a lower Paco₂ value. Conversely, although everything else remains the same, a smaller tidal volume results in a higher Paco₂ value.

The following formula can be used to help predict what tidal volume produces a desired Paco₂ value:

in which

• V_T = current tidal volume

• Vd_anat = anatomic dead space (This is calculated at 1 mL/lb or 2.2 mL/kg of ideal body weight.)

• Vd_mech = added mechanical dead space

• f = respiratory (ventilator) rate

• Paco₂ = actual patient Paco₂ value

• V_T′ = desired tidal volume

• Paco₂′ = desired patient Paco₂ value

Note that other, simpler formulas are available for calculating a change in minute volume or tidal volume. This one is presented because it takes into account more factors and can be used to calculate a change in tidal volume, rate, or mechanical dead space.

EXAMPLE

The patient is a 70-kg (154-lb) man who is being ventilated on the control mode (he is apneic). His ventilator settings are tidal volume of 1000 mL, rate of 12 times/min, fractional inspired oxygen concentration (Fio₂) of 0.3, and no added mechanical dead space. His ABG values are arterial oxygen pressure (Pao₂) of 90 torr, Paco₂ of 30 torr, pH of 7.48, Sao₂ of 95%, and base excess (BE) of 0. The clinical goal is to adjust the patient’s tidal volume as needed to produce a Paco₂ value of 40 torr. In summary,

• V_T = 1000 mL current tidal volume

• Vd_anat = 154 mL of anatomic dead space (This is calculated at 1 mL/lb or 2.2 mL/kg of ideal body weight.)

• Vd_mech = no added mechanical dead space

• f = 12 times/min for the ventilator rate

• Paco₂ = 30 torr actual patient Paco₂ value

• V_T′ = desired tidal volume

• Paco₂′ = 40 torr desired patient Paco₂ value

Placing the data and goal into the formula results in the following:

Simplifying produces the following:

The solution is to reduce the patient’s tidal volume from 1000 to 788 mL.

Exam Hint 15-8 (ELE, WRE)

Every available NBRC examination has had several problems that require the test taker to determine an original ventilator tidal volume or a new tidal volume to correct for overventilating (low carbon dioxide level) or underventilating (high carbon dioxide level) a patient. Start with a set tidal volume of 10 mL/kg of body weight. Increase or decrease the tidal volume, as needed, from this starting point. A patient with ARDS or COPD may have a smaller tidal volume. Often the patient’s height and weight are given. Be careful to not overventilate a patient who is obese.

5. Choose and adjust the rate for mechanical ventilation

See Table 1-2 for a listing of the normal resting respiratory frequencies based on age. If the patient is apneic and has a normal temperature and an appropriately set tidal volume, respiratory rates in the indicated ranges will produce a normal Paco₂ level. This must be confirmed by ABG measurements. If the tidal volume cannot be changed, adjusting the respiratory rate will modify alveolar ventilation. A higher respiratory rate, with everything else remaining the same, will result in a lower Paco₂ level. Conversely, a lower respiratory rate, with everything else remaining the same, will result in a higher Paco₂ level.

See Figure 15-18 for the Radford nomogram for use in predicting a normal respiratory rate and tidal volume based on weight. It can be used to establish an initial rate for most patients. As was mentioned earlier, chronically hypercapneic patients must be ventilated with some caution. Giving this type of patient a higher ventilator-delivered rate and a larger tidal volume may result in blowing off too much carbon dioxide and may cause a respiratory alkalosis. Adult patients with severe chronic restrictive lung disease or those who have had a pneumonectomy may need respiratory rates of 20 to 30 per minute or greater to meet their minute volume needs because their delivered tidal volume must be smaller than normal owing to their condition.

The same formula that was used to predict a tidal volume change can be used to help predict what respiratory rate will produce a desired Paco₂ value:

EXAMPLE

The patient is the same 70-kg (154-lb) man who is being ventilated on the control mode (he is apneic). His ventilator settings are tidal volume of 1000 mL, rate of 12 times/min, Fio₂ of 0.3, and no added mechanical dead space. His ABG values are arterial oxygen pressure (Pao₂) of 90 torr, Paco₂ of 30 torr, pH of 7.48, Sao₂ of 95%, and BE of 0. The clinical goal is to adjust the patient’s rate as needed to produce a Paco₂ value of 40 torr. In summary,

• V_T = 1000 mL current tidal volume

• Vd_anat = 154 mL of anatomic dead space (This is calculated at 1 mL/lb or 2.2 mL/kg of ideal body weight.)

• Vd_mech = no added mechanical dead space

• f = 12 times/min for the ventilator rate

• f′ = desired ventilator rate

• Paco₂ = 30 torr actual patient Paco₂ value

• Paco₂′ = 40 torr desired patient Paco₂ value

Placing the data and goal into the formula results in the following:

The solution is to reduce the patient’s respiratory rate from 12 to 9 breaths/min.

Exam Hint 15-9 (ELE, WRE)

Every available NBRC examination has had several problems that require the test taker to determine an original ventilator respiratory rate or a new respiratory rate to correct for overventilating (low carbon dioxide level) or underventilating (high carbon dioxide level) a patient. Typically start with a respiratory rate of 10 to 14 in an adult. Increase or decrease the rate as needed from this starting point.

6. Choose and adjust the minute ventilation for mechanical ventilation

The subjects of minute ventilation and alveolar minute ventilation were covered in Chapter 4. Review the calculations as needed. Blood gases must always be evaluated for the Paco₂ level to tell whether the patient’s minute ventilation is adequate. A high carbon dioxide level indicates a need to increase the tidal volume, respiratory rate, or both. A low carbon dioxide level indicates a need to decrease the tidal volume, respiratory rate, or both. In both cases, the key to modifying the carbon dioxide level is to modify the alveolar ventilation. This is best accomplished by changing the tidal volume rather than the rate. The following formula can be used to calculate a change in the minute volume:

in which

EXAMPLE

The patient is the same 70-kg (154-lb) man who is being ventilated on the control mode (he is apneic). His ventilator settings are tidal volume of 1000 mL, rate of 12 times/min, Fio₂ of 0.3, and no added mechanical dead space. His ABG values are Pao₂ of 90 torr, Paco₂ of 30 torr, pH of 7.48, Sao₂ of 95%, and BE of 0. The clinical goal is to adjust the patient’s minute volume as needed to produce a Paco₂ value of 40 torr. Placing the data and the goal into the formula results in the following:

The goal can be accomplished by reducing the minute volume from 12,000 to 9000 mL. As was mentioned earlier, this is best done by reducing the tidal volume. Remember that the tidal volume must be kept at no less than 10 mL/kg of ideal body weight to avoid the development of atelectasis. The respiratory rate may be decreased, if necessary, to provide this reduced minute volume.

7. Initiate and adjust positive end-expiratory pressure therapy (Code: IIID2d) [Difficulty: ELE: R, Ap; WRE: An]

Positive end-expiratory pressure (PEEP) is a mechanically elevated baseline pressure. In other words, the patient’s end-expiratory pressure is above atmospheric. PEEP is administered through a mechanical ventilator and is not a mode by itself. Rather, it is used in conjunction with any of the previously mentioned modes.

PEEP generally is indicated in any acute, bilateral, generalized pulmonary condition in which the FRC is decreased. When the FRC is decreased, shunt is increased, and the patient has refractory hypoxemia. Examples of small FRC conditions helped by PEEP include generalized atelectasis, pulmonary edema, ARDS, and infant respiratory distress syndrome (RDS). All of these patients show decreased lung compliance as measured by their static compliance (Cst). The higher the level of PEEP, the more progressively the patient’s FRC is increased. The therapeutic goal of this is to increase the patient’s arterial oxygen pressure (Pao₂). (Figure 15-13, F shows the pressure/time curve.) Patients with chronically small FRC, such as those with pulmonary fibrosis and kyphoscoliosis, are not helped by the application of PEEP.

Specific indications for PEEP include the following:

• Intrapulmonary shunt >15%

• Refractory hypoxemia (Pao₂ <60 mm Hg despite an Fio₂ of up to 0.8 to 1.0)

• The patient has had an Fio₂ >0.5 for 48 to 72 hours and shows no indication of a rapidly improving Pao₂. PEEP is added so that the Fio₂ can be lowered to a safer level.

Before PEEP is begun, the patient should be monitored carefully to establish the baseline condition. The same parameters should be monitored after each change in the PEEP level to determine how the patient is tolerating it. The best or optimal level of PEEP is the level that results in the best delivery of oxygen to the tissues (not necessarily the arterial blood). Often, a secondary goal is to reduce the inspired oxygen to a safe level. The patient is at risk for oxygen toxicity if more than 50% oxygen is inhaled for longer than 48 to 72 hours. See Box 15-3 for recommendations on what to monitor during the application of PEEP and how to evaluate the data.

The application of PEEP has risks. Clinically, these risks must be weighed against the potential benefit to the patient. In the profoundly hypoxic patient, PEEP can be lifesaving. Some clinicians use low levels of PEEP (up to 5 cm water) in patients with normal lungs or overly compliant lungs (emphysema) to maintain the baseline level of FRC. Hazards of PEEP include the following:

• Pulmonary barotrauma: pneumothorax, tension pneumothorax, mediastinal emphysema, pulmonary interstitial emphysema (PIE) in the neonate, subcutaneous emphysema

• Decreased venous return to the heart, causing decreased cardiac output and tachycardia, decreased blood pressure, decreased tissue perfusion as measured by a decreased mixed venous oxygen (Pv-o₂) level, decreased urine output

a. Increasing PEEP

PEEP therapy usually is begun at initial levels of 2 to 5 cm water. After the patient’s response has been determined, 2 to 5 cm more PEEP may be applied. The patient is reevaluated. This process goes on until the desired clinical benefit is reached. See Figure 15-19 for a number of physiologic parameters that can be measured and evaluated.

Figure 15-19 Optimal or best positive end-expiratory pressure (PEEP) level is determined by monitoring some or all of the following parameters: Pao₂, effective static compliance (Cst), pressure of mixed venous oxygen (Pv-o₂), cardiac output (CO), shunt (Q.s/Q.t), and pulmonary vascular resistance (PVR). Ideally, as functional residual capacity (FRC) is increased by PEEP, lung compliance is improved, and ventilation and perfusion are better matched. Cardiac output should remain stable. It can be measured by the use of a pulmonary artery (Swan-Ganz) catheter or indirectly followed by monitoring the patient’s heart rate and blood pressure. As can be seen, the optimal PEEP is found at 12 cm water pressure. Excessive PEEP is seen by the resulting decrease in static compliance and cardiac output.

Different approaches to the application of PEEP are used to find the best level. One approach could be called minimum PEEP. It involves the application of PEEP to the minimum level that allows the inspired oxygen percentage to be lowered to a safer level. A clinical goal is to minimize the risk of oxygen toxicity. In this approach, PEEP is raised until the Pao₂ is greater than 60 torr or the Spo₂ is greater than 90% on 60% oxygen or less. Usually no more than 10 to 15 cm water of PEEP is needed.

Another approach could be called best PEEP or optimum PEEP. This approach has the clinical goal of reducing the patient’s shunt fraction to less than 15%. Often this requires more pressure than the minimal PEEP approach. Because this higher pressure level is more likely to cause hemodynamic problems, the patient should have a pulmonary artery catheter inserted. With it, the patient’s cardiac output, mixed venous oxygen level, pulmonary capillary wedge pressure, and pulmonary vascular resistance can be measured (see Figure 15-19). In addition, the patient may need increased intravenous fluids, dopamine (Intropin), and digitalis (Digoxin) for cardiovascular support. The higher PEEP levels increase the risk of pulmonary barotrauma. Therefore the patient must be watched closely for signs of a pneumothorax.

b. Decreasing PEEP

As the patient begins to recover, the PEEP level may be reduced in steps of 2 to 5 cm water. Again, the patient is evaluated after every change in the PEEP level. If the patient’s cardiovascular status is normal, the following are recommendations for how to decrease PEEP and oxygen levels:

• Decrease PEEP first if the Pao₂ level is greater than 60 torr and the Fio₂ is less than 0.5.

• Decrease oxygen first if the Pao₂ level is greater than 60 torr and the Fio₂ is greater than 0.5.

If the patient is showing an adverse reaction to the PEEP level, such as decreased cardiac output or barotrauma, the PEEP level should be decreased before the oxygen percentage. Clinical judgment must be used to decide whether a high oxygen percentage or a high PEEP level is a greater danger to the patient. Minimize or remove the element that puts the patient at greater risk.

Exam Hint 15-10 (ELE, WRE)

Typically, several questions deal with the clinical application and modification of PEEP. Know to increase PEEP if the patient is hypoxic, is receiving a high oxygen percentage, and is hemodynamically stable. Know to decrease PEEP if the patient is well oxygenated and is receiving a moderate oxygen percentage, is not hemodynamically stable, or has a PEEP-related complication. A decrease in the patient’s cardiac output is a key indicator of excessive PEEP causing hemodynamic problems.

MODULE C

Provide unconventional mechanical ventilation to adequately oxygenate and ventilate the patient.

Unconventional mechanical ventilation is defined here as the use of specialized ventilators that can provide unique modes and options needed by special populations of patients. This ventilatory support may or may not be provided through an endotracheal tube or a tracheostomy tube.

1. Initiate high-frequency ventilation and select appropriate settings

High-frequency ventilation (HFV) is needed whenever the patient’s condition calls for a higher respiratory rate or a smaller tidal volume than usually is delivered on a conventional volume-cycled ventilator. A high-frequency ventilator can deliver a respiratory rate far higher than the limit of 150/min set by the U.S. Food and Drug Administration (FDA) on all conventional adult and neonatal/pediatric ventilators.

The FDA has approved HFV for use on adults during bronchoscopy and laryngoscopy procedures and when a patient with a bronchopleural fistula cannot be managed on a conventional ventilator. Although patients with ARDS have not been officially approved for HFV, the procedure has been used when a patient is hypoxic despite maximum settings on a conventional ventilator. (Box 16-5 lists clinical uses for HFV with infants and children.)

Currently HFV can be delivered in three different ways. The first involves the use of a conventional ventilator set at a rate of up to the FDA maximum of 150/min. This method is called high-frequency positive-pressure ventilation (HFPPV). With it, the set tidal volume is decreased to something less than standard (<10 mL/kg). The second involves the use of high-frequency jet ventilation (HFJV) and a special endotracheal tube with a standard lumen and an additional small-diameter air entrainment lumen (Figure 15-20). The HFJV unit is connected to the air entrainment lumen. Gas from the HFJV unit entrains other gas through the main lumen to create the patient’s tidal volume. HFJV units can deliver a small tidal volume several hundred times per minute. The third method involves a high-frequency oscillation (HFO) ventilator. HFO makes use of a conventional endotracheal tube (as does HFPPV). However, the delivered tidal volume is the smallest, and the respiratory rate can be the fastest of all three methods. Table 16-2 presents a comparison of all three methods. The equipment used for HFJV and for HFO is described later in this chapter. Table 15-1 lists considerations for the initial settings and for adjustment of HFV delivered to infants and adults by a jet ventilator or an oscillator ventilator.

Figure 15-20 A HiLo Jet endotracheal tube. This special purpose endotracheal tube features four lumina. A, Pilot line to the cuff. B, Jet catheter used to deliver small bursts of tidal volume gas from a high-frequency jet ventilator. It opens within the endotracheal tube above the cuff. C, Catheter for measuring proximal airway pressure within endotracheal tube near its tip. The main lumen of the endotracheal tube is used for suctioning and adding entrained, humidified gas from a conventional volume-cycled ventilator.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Cardinal Health Respiratory Care Products and Services, McGaw Park, IL.)

TABLE 15-1 High-Frequency Ventilation Operational Considerations

Current clinical experience is recommended for any HFV method. Although adult patients have been treated successfully with HFV, far greater use of these techniques has occurred with infants and children. Therefore, Chapter 16 provides further discussion.

2. Independent (differential) lung ventilation

Independent lung ventilation (ILV) involves the use of a separate mechanical ventilator for each lung. A double-lumen endotracheal tube must be placed into the patient to allow this procedure. (See Figures 12-36 and 12-37 and the related discussion.) Box 15-4 lists indications for double-lumen endotracheal tubes and ILV. In all cases, the patient has one normal lung and one abnormal lung. Overriding concerns with ILV are to ventilate the patient adequately through the normal lung and to allow the injured lung to heal.

BOX 15-4 Indications for Double-Lumen Endotracheal Tubes and Independent Lung Ventilation

From Tuxen D: Independent lung ventilation. In: Tobin MJ, editor: Principles and practice of mechanical ventilation, New York, 1994, McGraw-Hill, pp 571-588.

Thoracic surgery

Pneumonectomy

Some lobectomies

Thoracic aortic surgery

Thoracoscopy

Some esophageal surgery

Selective airway protection

Secretions (tuberculosis, bronchiectasis, or abscess)

Whole-lung lavage

Massive hemoptysis

Bronchopleural fistula

Unilateral lung disease

Unilateral parenchymal injury

Aspiration

Pulmonary contusion

Pneumonia

Massive pulmonary embolism

Reperfusion edema

Asymmetric ARDS

Asymmetric pulmonary edema

Atelectasis

Unilateral airflow obstruction

Single-lung transplant for chronic airflow obstruction

Unilateral bronchospasm

Severe bilateral lung disease

ARDS

Aspiration

Pneumonia

ARDS, Acute respiratory distress syndrome.

A common initial approach is to select two identical ventilators that allow synchronization of the patient’s respiratory rate. The Servo and Dräger ventilators are suited for this. They allow one unit to be designated the “primary” ventilator to set the respiratory rate for it and the “secondary” unit. Each unit then can have the same mode and I : E ratio. This synchronized ILV method still allows the independent setting of tidal volume, oxygen percentage, and PEEP for each lung.

For example, an 80-kg adult man normally receives an initial tidal volume of about 800 mL (10 mL × 80 kg). If both lungs functioned normally, each would receive 400 mL. However, with unilateral lung disease, the bad lung receives little tidal volume and the normal lung receives too much and becomes overdistended. It is therefore important to set the initial tidal volume to the good lung at half the normal volume for both lungs. For this patient example, the normal lung’s initial tidal volume should be set at 400 mL (5 mL × 80 kg). To avoid changing too many parameters at once, the same oxygen percentage and PEEP level as originally set are kept. The abnormal lung is ventilated on the basis of its pathologic condition or may be left unventilated.

After 15 minutes, check the patient’s ABG values. Depending on the results, adjust the ventilator settings for the good lung as would be done typically to remove carbon dioxide and maintain oxygenation. The ventilator settings for the diseased lung must be adjusted carefully to allow healing and prevent complications such as atelectasis and pneumonia.

When the patient has a bronchopulmonary fistula, the air leak through the bad lung can be so great that an HFV must be used rather than a conventional ventilator. In this situation, no synchronization of rate or any other parameters can be set. The good lung is ventilated conventionally to maintain the patient’s blood gas values. The HFV is set to provide some support with a small tidal volume and low ventilating pressures. The clinical goal is to prevent excessive lung pressure so that the lung tear heals.

The patient can be converted back to breathing through one conventional ventilator when normal functioning returns to the injured lung. This can be done when the peak pressure, plateau pressure, and mean airway pressure for both lungs are about the same. When these values match or are close, similar lung compliance and airway resistance values are indicated. The first step in converting from ILV to conventional ventilation involves combining the proximal ends of the double-lumen endotracheal tube with a Y adapter. When this is done, one ventilator can deliver tidal volume breaths to each lung. It is suggested that the PC mode be used so that excessive pressure is not applied to the healing lung. Set the peak pressure at or just below the previous peak pressure used with the healing lung. Set the PEEP level at the previous pressure used with the healing lung. Check the patient’s ABG values after 15 minutes. Be prepared to adjust the conventional ventilator as needed to get the desired blood gas values. Watch for problems with the healing lung, and be prepared to go back to independent lung ventilation if necessary. When it appears certain that the patient is tolerating conventional ventilation, the double-lumen endotracheal tube should be removed and replaced with an appropriate single-lumen tube. This allows better suctioning and results in less airway resistance through the tube. Wean and extubate the patient when appropriate.

3. External negative-pressure ventilation

These ventilators have proved useful in patients with the following characteristics: (1) normal, intact upper airway, (2) ability to swallow, (3) normal airway resistance, (4) normal lung/thoracic compliance, and (5) ability to ventilate until respiratory muscle fatigue becomes too great.

Patients with the following disease conditions have been ventilated successfully by a negative-pressure ventilator: (1) neuromuscular defects such as poliomyelitis, postpolio syndrome, muscular dystrophy, and high spinal cord injury; (2) kyphoscoliosis with resulting restrictive lung disease; and (3) COPD during an acute worsening.

Negative-pressure ventilators work by creating negative pressure around the patient’s whole body or over the anterior chest and abdomen. The negative pressure expands the thorax, and a tidal volume is inhaled. If the patient needs supplemental oxygen, it must be given by nasal cannula or face mask. Three basic types of external negative-pressure ventilators are available: Drinker body respirator (so-called “iron lung”), body wrap, and chest cuirass (Figure 15-21).

Figure 15-21 Patient in chest cuirass (LifeCare). Note control box on right that contains a vacuum motor. Hose connects vacuum motor to patient inside chest shell.

(From Hill NS: Clinical application of body ventilators, Chest 90[6]:897, 1986.)

The following steps are used to initiate ventilation:

1. Select a ventilator rate that meets the patient’s needs. This can be about 5 to 10 breaths less than the patient’s own rate in a patient with some breathing ability. In an apneic patient, the rate can be varied between 14 and 24 per minute.

2. Gradually increase the negative pressure until the patient cannot speak during the inspiratory phase. A pressure of −7 to −15 cm water is enough for most patients. A maximum pressure of −35 cm water often can be achieved. It is possible to create a positive pressure during exhalation, if needed, by closing a valve. This usually is limited to times when an assisted cough is called for.

3. Use a hand-held spirometer to measure the patient’s tidal volume.

4. After a few minutes, ask the patient, “Does the breath feel deep enough, too deep, or not enough? Do you have tingling fingers or feel dizzy (signs of hyperventilation)?”

5. Adjust the negativity or rate or both to meet the patient’s needs.

6. Draw an ABG sample after about 15 minutes. Speed is important because the unit must be opened for collection of the sample. Interpret the ABG results as usual.

7. If the carbon dioxide level is higher than desired, increase the rate or set a more negative pressure to increase the minute volume. Do the opposite if the patient is being hyperventilated. If the patient is hypoxic, provide supplemental oxygen by nasal cannula or face mask.

4. Initiate and adjust noninvasive ventilation (Code: IIID2c) [Difficulty: ELE: R, Ap; WRE: An]

Noninvasive ventilation (NIV) is also known as noninvasive positive-pressure ventilation (NPPV). These patients will be ventilated without the use of an endotracheal tube. Most of the time, patients receiving NIV are ventilated with the aid of a nasal mask similar to that used to deliver mask CPAP. A full face mask often is needed if the patient leaks through the mouth with a nasal mask.

Most patients who are candidates for NIV are fairly stable, are not intubated, can do some spontaneous breathing, and will need only short-term ventilator support. Examples include patients with the following conditions:

• COPD or asthma and an elevated carbon dioxide level

• Congestive heart failure with pulmonary edema and moderate hypoxemia despite supplemental oxygen

• Neuromuscular disease with chronic ventilatory muscle dysfunction

• Sleep apnea from upper airway obstruction

If the patient is critically ill, is unstable, must be intubated to secure the airway for secretion removal, needs a high level of therapeutic PEEP to maintain the FRC, or experiences a combination of these, he or she should be placed on a standard volume-cycled ventilator.

Often patients receiving NPPV are ventilated with two different levels of positive pressure. This is referred to as bilevel ventilation. The baseline pressure is greater than zero for setting a CPAP or PEEP level. The peak pressure is set to deliver a desired tidal volume (similar to PS ventilation). Both levels can be adjusted independently. If only the baseline pressure is elevated, the patient is receiving CPAP. If only the peak pressure is elevated, the patient is receiving PS ventilation. Respironics (Carlsbad, CA) has pioneered the development of ventilators for conventional ventilation or noninvasive bilevel ventilation.

As was stated above, the patient must have a properly fitting nasal or face mask to receive noninvasive ventilation. These ventilation masks are similar to CPAP masks and are referred to as such. CPAP masks are available in different sizes for children older than 3 years and for adults. Nasal masks are designed to cover only the nose. They allow the patient to eat, drink, speak, and use the mouth as a second airway for breathing in case a malfunction of the CPAP system occurs. The mouth also acts as a pressure relief route should the CPAP pressure become too great. Pressures of up to 10 to 15 cm water usually can be maintained (Figure 15-22). Usually, the mask is made of a transparent plastic. Face masks are designed to cover the nose and mouth. They are similar in design to the masks used during bag/mask ventilation and also are made of a transparent plastic. The face mask must be used if the patient has persistent mouth breathing and cannot use a nose mask. With a good seal, pressures of greater than 15 cm water can be maintained.

Figure 15-22 Effects of nasal continuous positive airway pressure (CPAP) mask. A, Normal upper airway remains patent during sleep. B, Abnormal upper airway of patient with obstructive apnea collapses on inspiration during sleep. C, Pressure from nasal CPAP mask keeps abnormal upper airway patent during sleep.

(Modified from Scanlan CL, Spearman CB, Sheldon RL, editors: Egan’s fundamentals of respiratory care, ed 5, St Louis, 1990, Mosby.)

In recent years, a wide variety of NIV mask systems have been developed (Figure 15-23). In any situation, the clinical goal is to find a CPAP/NIV mask with a soft, very compliant seal that closely fits the contours of the patient’s face. A strapping system is needed to hold the mask in place. Too large a mask will not seal and will allow gas to leak and pressure to decrease. The patient may show increased snoring or airway obstruction with periods of apnea. A mask that is too small or misfitting can cause an uneven distribution of pressure on the face. This can lead to abrasions or pressure sores and ulcers on the face.

Figure 15-23 Examples of types of masks used to deliver continuous positive airway pressure or noninvasive positive-pressure ventilation. A, Nasal mask designed to cover the entire nose. B, Full face mask (oronasal mask) designed to cover the nose and mouth. C, Nasal pillows that fit into the nostrils. D, Total face mask that covers the entire facial area. The key elements to consider in deciding which mask to use are patient fit without leaks and comfort.

(A, C, and D from Hess DR, Kacmarek RM: Essentials of mechanical ventilation, ed 2, New York, 2002, McGraw-Hill. B from Hill NS: Complications of noninvasive positive pressure ventilation, Respir Care 42(12):432-442, 1997.)

The bilevel settings must be determined at the bedside by asking for the patient’s subjective opinion, listening to breath sounds, checking vital signs, and evaluating ABG values. If supplemental oxygen is needed, it can be added at a port on the patient’s mask, at the humidifier, or at the outlet from the BiPAP unit. Up to 15 L/min can be added without affecting the performance of the BiPAP system. It is not possible to know the delivered oxygen percentage until after the bilevel ventilation settings have been determined. The oxygen flow then should be increased gradually while the patient’s Spo₂ value increases to the desired saturation level.

The function of the Respironics Esprit system is reviewed briefly. The operator can choose from two modes of operation and can select from the following:

• Expiratory positive airway pressure (EPAP) for an elevated baseline pressure. This is functionally similar to CPAP.

• Inspiratory positive airway pressure (IPAP) to deliver a tidal volume. This is functionally similar to setting the peak pressure on a pressure-cycled ventilator.

• Spontaneous ventilation mode (SPONT) requires the patient to initiate each assisted breath. The therapist can set IPAP and EPAP levels. This delivers bilevel ventilation.

• Spontaneous/timed ventilation mode (SPONT/T) allows the patient to breathe spontaneously while having a time-cycled minimum respiratory rate set by the therapist. IPAP and EPAP levels also are set to deliver bilevel ventilation.

When bilevel ventilation is started, set the EPAP level, if needed, to elevate the patient’s baseline pressure. The EPAP level establishes the patient’s functional residual capacity (FRC) to improve oxygenation. Increase or decrease EPAP as you would adjust PEEP or CPAP. Review Box 15-3 for patient monitoring with EPAP. Next, set the IPAP level to achieve the desired tidal volume.

With bilevel ventilation, the difference between IPAP and EPAP is called pressure boost and delivers the tidal volume. If a larger tidal volume is needed, increase the IPAP level. Conversely, decrease the IPAP level to obtain a smaller tidal volume. It is important to remember that the delivered tidal volume varies depending on changes in the patient’s airway resistance and lung/thoracic compliance, as well as in the machine settings.

5. Initiate and adjust continuous positive airway pressure (Code: IIID2d) [Difficulty: ELE: R, Ap; WRE: An]

CPAP is a pressure above atmospheric that is maintained at the airway opening throughout the respiratory cycle during spontaneous breathing. CPAP is similar to PEEP in purpose and effect. Remember that with CPAP, the patient does not receive any ventilator-delivered tidal volume breaths. The patient must be capable of providing all ventilation for carbon dioxide removal. Although CPAP usually is delivered through the mechanical ventilator, the respiratory rate is turned off. However, alarm systems still are functioning for patient safety. Some hospitals will make use of a free-standing system for delivering CPAP. This system is discussed below (Figure 15-13, G shows the pressure/time curve for CPAP).

Before a patient receives CPAP therapy, the practitioner and the physician must be assured that the patient has the ability to breathe adequately to eliminate carbon dioxide. CPAP is contraindicated in an apneic patient or in one who may become apneic. (This patient must be fully supported on the ventilator.) The patient must have an adequate respiratory rate, tidal volume, and minute volume. The heart rate and blood pressure should be stable. Maximum inspiratory pressure and vital capacity values may be acceptable or low. Blood gas analysis typically shows refractory hypoxemia but a normal or low Paco₂ level. This shows that the patient would benefit from an elevated baseline pressure to increase the FRC but is capable of ventilating.

CPAP involves the same indications, hazards, and patient evaluation processes as were discussed earlier in PEEP therapy. One possible physiologic benefit of CPAP over PEEP is that less reduction in venous return to the heart is seen. This occurs because with CPAP, the patient is breathing spontaneously. Therefore patients treated with CPAP may be able to tolerate higher pressure levels than those being ventilated with PEEP therapy.

CPAP usually is increased and decreased in steps with 2 to 5 cm water. As with PEEP, the patient is evaluated before CPAP is begun and again after each pressure change is made. See Box 15-3 for recommendations on what to monitor during the application of CPAP and how to evaluate the data.

The patient must be monitored carefully for fatigue because the patient is providing all of the minute ventilation. Signs of fatigue include increasing respiratory rate, decreasing tidal volume or vital capacity, decreasing maximum inspiratory pressure, and tachycardia. Blood gas measurement may show a stable or decreasing Pao₂ value. An increasing Paco₂ level is a definite sign of fatigue. The patient may complain of dyspnea. The practitioner may notice that the patient is working harder than normal to breathe, as shown by increased use of the accessory muscles of ventilation and heavy perspiration. When these signs occur, CPAP therapy should be discontinued and mechanical ventilation instituted in a mode that best fits the patient’s needs.

Exam Hint 15-11 (WRE)

Recent exams have focused on delivery of mechanical ventilation through conventional modes or combinations of modes. Typically only one or two questions deal with rarely used modes such as high-frequency ventilation, independent lung ventilation, or negative-pressure ventilation.

Exam Hint 15-12 (ELE, WRE)

Know that CPAP can be used and increased if the patient’s carbon dioxide level is normal and hypoxemia is present. Increase or decrease CPAP by following the same parameters used to evaluate a change in PEEP. Know to switch to mechanical ventilation when (1) the maximum level of CPAP is being used and the patient is still hypoxemic, or (2) the patient is hypoventilating.

MODULE D

Mechanical ventilation equipment

Note: The literature produced by the manufacturers and the descriptions used in many standard texts break down the various ventilators into more categories than are used by the NBRC. To avoid confusion, this text uses the more simplified terminology of the NBRC.

A pneumatically powered ventilator is defined here as a ventilator powered by compressed gas. Older units operate without any electrically powered control systems (electrically powered alarm systems may or may not be added). More recent units will also have electrically powered controls and alarm systems. An electrically powered ventilator is defined here as a ventilator that is electrically powered or controlled. Most volume-cycled ventilators fall into this category. Microprocessor ventilators are electrically powered but are controlled by one or more microprocessors (computers). Many of the most current volume-cycled ventilators have microprocessors to control their functions. Fluidic ventilators typically make use of electrical circuits with flip-flops to respond to changes in gas flow and pressure throughout the system. Fluidic ventilators are powered by compressed gas. Noninvasive ventilators are designed for home use or short-term hospital use and are electrically powered and controlled. They have fewer controls and alarms than hospital-based critical care ventilators. A nasal or full face mask, rather than an endotracheal tube, is used to attach the ventilator to the patient. High-frequency ventilators are used in a limited population of critically ill patients who are doing poorly despite all attempts at conventional mechanical ventilation. These units are designed to deliver very rapid respiratory rates and very small tidal volumes.

1. Manipulate pneumatic ventilators by order or protocol (Code: IIA6a) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

Historically, the most commonly used pneumatically powered ventilators include the Bird series and the Bennett PR-2. A control or backup rate can be set on these units in case the patient is apneic. All other controls and functions are the same as those discussed in Chapter 14. The following equipment and procedures are necessary:

• Bennett PR-2 or Bird ventilator with an air/oxygen blender and hoses

• Bennett or Bird breathing circuit

• Bacteria filters

• Proper humidification system: a passover- or cascade-type humidifier or an HME (discussed later). Put sterile, distilled water into the passover- or cascade-type humidifier.

• One or two water traps for condensation from the circuit

• Add an alarm system(s) such as a low-volume bellows spirometer or a low-pressure/disconnection alarm.

• If a low-volume bellows spirometer alarm is used, a length of large bore tubing is needed to connect the exhalation valve to the bellows.

Most modern pneumatically powered ventilators require electricity by battery or wall outlet to operate properly. They are used for patient transportation within or between hospitals or at the scene of a disaster. The Impact Uni-Vent 750 is an example. It has been included in the Strategic National Stockpile for rapid shipment to the site of a natural disaster or terrorist attack.

b. Put the equipment together and make sure that it works properly

Refer to Figures 14-10 and 14-11 in Chapter 14 for the IPPB circuits. The ventilator and circuits are similar to those used in IPPB therapy with the following exceptions:

• Set the ordered oxygen percentage on the blender; set the Bird air mix and Bennett air dilution selection controls to pure source gas from the blender. Analyze the Fio₂ through a port in the inspiratory limb of the circuit.

• If a passover or cascade humidifier is used, a short length of large bore tubing is connected between the outlet of the mainstream in-line bacteria filter and the inlet of the humidifier. The inspiratory limb of the circuit is connected to the outlet of the humidifier. If a heat-moisture exchanger (HME) is used, it is added between the circuit and the tracheostomy/endotracheal tube adapter (Figure 15-24).

• A tracheostomy/endotracheal tube adapter is always used to connect the circuit to the patient.

• Add at least one disconnection alarm to the circuit. It could be a low-pressure or disconnection alarm added into the inspiratory limb of the circuit with a Briggs adapter/T-piece.

• Water traps are placed in the lowest part of the inspiratory and expiratory limbs of the circuit to hold any condensed water vapor.

Figure 15-24 The basic components of the two main types of patient breathing circuits for continuous mechanical ventilation. A, This circuit contains an external exhalation valve and is used on a limited number of pressure-type ventilators. Note that when gas is sent through the expiratory valve line, the exhalation valve balloon inflates to seal the circuit. B, This circuit is used on most current ventilators and does not contain its own exhalation valve. Instead, the exhalation valve is part of the ventilator. This is called an internal exhalation valve.

(From Cairo JM, Pilbeam SP: Mosby’s respiratory care equipment, ed 8, St Louis, 2010, Mosby.)

c. Troubleshoot any problems with the equipment

Both of these types of ventilators send gas through the circuit during an inspiration and do not cycle off until the preset pressure is reached. Test the tightness of the circuit, backup rate, and delivered tidal volume by placing a test lung on the patient connection of the circuit. Set the controls to deliver the prescribed order. Be prepared to make final adjustments once either unit has been placed on the patient. The patient’s airway resistance and lung compliance greatly affect the functioning of both units.

Troubleshooting problems with these units were discussed in Chapter 14. Make sure that all connections are tight; more are available now with the addition of the humidification system and expiratory limb to the spirometer.

A defective exhalation valve will allow gas to pass through the circuit rather than go to the patient. A pressure-cycled ventilator will not cycle to exhalation.

2. Manipulate electrical ventilators by order or protocol (Code: IIA6a) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

Many mechanical ventilators are powered or controlled electrically. They function primarily as volume-cycled units, meaning that a preset volume is delivered from the ventilator with each breath regardless of the patient’s condition. Each ventilator is unique in its abilities, modes, and so forth. It is beyond the scope of this book to discuss each and every volume-cycled ventilator. They are presented in a generic manner. The learner should become familiar with the function of the Maquet Servo-i (Maquet Inc., Wayne, NJ) and other widely used machines.

b. Put the equipment together and make sure that it works properly (WRE code: IIA6a) [Difficulty: An]

Each ventilator must be learned for its specifics. Generally speaking, the following steps should be taken to ensure that the ventilator is functioning properly:

1. Select the proper ventilator for the physician’s orders and the patient’s needs.

2. Attach the circuit properly, and make sure that all connections are tight (see Figure 15-24).

3. Select the appropriate humidification device: a passover type, cascade type, or HME. Put sterile, distilled water into the passover or cascade unit.

4. Preset all of the physician-ordered parameters and any other settings that are needed to make the ventilator fully functional.

5. Place a test lung on the circuit at the patient connection.

6. Make sure that the ventilator delivers the preset rate, volume, oxygen percentage, I : E ratio, and so forth.

c. Troubleshoot any problems with the equipment

A low volume can be caused by a leak; check all connections, and tighten them as needed. The volume can be measured directly as it leaves the ventilator and at the exhalation valve to help determine the source of the wrong volume. If the unit shows a volume entering the exhalation valve and spirometer instead of the test lung during inspiration, the exhalation valve is broken. All alarms must be working properly. Batteries must be replaced when discharged.

3. Manipulate microprocessor ventilators by order or protocol (Code: IIA6a) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

The microprocessor ventilators (e.g., Maquet Servo-i [Maquet Inc.], Hamilton GALILEO [Hamilton Medical AG, Reno, NV], Dräger E-4 [Draeger Medical Inc., Telford, PA]) are the most advanced generation of mechanical ventilators. They are powered electrically but are controlled by one or more microprocessors (computers). They offer all commonly found modes of ventilation. In addition, many of these machines offer computer software for measuring bedside spirometry for weaning, work of breathing, and other parameters that give the clinician much valuable information. Airway resistance and static and dynamic lung compliance can be calculated automatically. Flow, volume, and pressure tracings are displayed graphically on the computer screen. Auto-PEEP can be documented and measured. These units offer the greatest quantity of patient data and the best clinical flexibility of all currently available ventilators. The most challenging patients probably can be best cared for on one of these machines.

b. Put the equipment together and make sure that it works properly

c. Troubleshoot any problems with the equipment

All of the previous general discussion on electrically powered ventilators applies to the microprocessor ventilators as well. An additional advantage of these units is that they self-diagnose most problems and display the problem for you. If a microprocessor should fail, the unit should be removed from the patient. The biomedical department or manufacturer has to replace the computer chip.

4. Manipulate fluidic ventilators by order or protocol (Code: IIA6a) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

An example of a commonly available fluidic ventilator is the Bio-Med MVP-10 (Bio-Med Devices, Guilford, CT). It is a neonatal/pediatric unit that can be used in the hospital or for patient transport.

b. Put the equipment together and make sure that it works properly

Make sure that high-pressure gas hoses between the unit and wall outlets are connected tightly to prevent leaks. The patient circuit and humidification system must be installed properly and must be operating.

c. Troubleshoot any problems with the equipment

Typically, fluidic ventilators are powered pneumatically and have fluidic controls. Make sure that the oxygen and air sources are up to the required pressure (usually 50 psig). Fluidic controls are very sensitive to any obstruction and to changes in other settings. Make sure that gas inlet and outlet filters are kept clear of obstructions.

5. Manipulate noninvasive positive-pressure ventilators by order or protocol (Code: IIA6b) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

A noninvasive ventilator is intended for an adult patient who is capable of some spontaneous breathing for a limited period. The unit typically is used for a short period with a patient who is having breathing difficulty but does not require intubation and full ventilatory support. Current ventilators include the Respironics Esprit (Respironics, Philips Healthcare, Carlsbad, CA) and the Puritan-Bennett GoodKnight 425 (Covidien-Puritan Bennett, Boulder, CO).

b. Put the equipment together and make sure that it works properly

c. Troubleshoot any problems with the equipment

Follow the manufacturer’s guidelines for putting the circuit on the unit and adding a humidifier, if needed. A nasal or full face mask of proper size is needed to attach the circuit to the patient. Check carefully for any leakage between the mask and the patient’s face.

6. Manipulate high-frequency ventilators by order or protocol (WRE code: IIA6c) [Difficulty: WRE: R, Ap]

a. High-frequency jet ventilators (HFJVs)

1. Get the necessary equipment for the procedure

The Bunnell Life Pulse High-Frequency Jet Ventilator (Bunnell Inc., Salt Lake City, UT) is designed for use with neonatal patients with infant respiratory distress syndrome (RDS) who have failed with conventional ventilation. The ventilator is unique in that it must be used in conjunction with a conventional ventilator that is placed in the CPAP or SIMV mode during use of the jet.

In addition, a special endotracheal tube must be placed into the patient before HFJV can be initiated. Ideally, the patient is intubated with a triple-lumen Hi-Lo Jet tube (see Figure 15-20). If the neonate has already had a standard endotracheal tube inserted, the Bunnell LifePort Endotracheal Tube Adapter (Bunnell Inc.) can be substituted for the standard adapter (Figure 15-25). Both tubes now allow the jetted gas from the Bunnell ventilator to go to the patient. The traditional ventilator is attached to the main lumen of the endotracheal tube. Based on the physical principles that govern jets, additional gas is entrained through the main lumen. This entrained gas should be humidified if the jet gas is dry. All exhaled gas passes out through this main lumen, where it can be measured through the traditional ventilator’s spirometry system. This ventilator’s alarm systems also can be used, and SIMV breaths and PEEP can be added if needed.

Figure 15-25 Special airways needed for high-frequency ventilation. A, A multiple-channel endotracheal tube needed for high-frequency jet ventilation. The jet gas enters the patient through the jet line. The pressure-sensing line is used to monitor pressures at the end of the endotracheal tube. The main channel is used to entrain additional tidal volume gas and to perform suctioning. B, If the patient is already intubated, the standard endotracheal tube adapter is replaced by the jet cannula adapter. Jet gas enters through the jet connection. The main channel is used to entrain additional tidal volume gas and to perform suctioning.

(From Cairo JM, Pilbeam SP: Mosby’s respiratory care equipment, ed 8, St Louis, 2010, Mosby.)

2. Put the equipment together and make sure that it works properly

Figure 15-26 shows a schematic drawing of an HFJV. Also select a companion neonatal/pediatric ventilator and make sure it is operating properly. The following are general considerations with setting up the HFJV system:

• 50 psig source gas(es) of oxygen or oxygen and air are needed to generate a driving pressure.

• A patient circuit is specifically suited to the jet ventilator.

• A drive pressure control lets the operator set the peak pressure of the jet.

• An inspiratory time control sets the I : E ratio.

• Rate can be varied within the limits set by the unit.

• The inspired oxygen percentage can be dialed on the unit itself or on an external air/oxygen blender before going into the unit.

• Humidification must be provided by the Bunnell ventilator and the conventional ventilator.

• Both ventilators must be attached to the special endotracheal tube.

Figure 15-26 Schematic diagram of a high-frequency jet ventilator concept.

(From MacIntyre NR: Jet ventilation in the adult with breathing rates up to 150 bpm, Riverside, CA, 1985, Bear Medical Systems. Courtesy of Viasys Respiratory Care Inc.)

Experience with the equipment is recommended for assembly of the specific circuit, as needed. Any humidification system can be added. Both ventilators’ circuits are designed to combine two separate flows of gas for the patient’s tidal volume. As with any circuit, make sure that all connections are tight.

7. Manipulate continuous mechanical ventilation and noninvasive ventilation breathing circuits by order or protocol (ELE code: IIA11a and IIA11d) [Difficulty: Ap, An]

a. Get the necessary equipment for the procedure

A permanent or a disposable circuit may be selected, based on the type of ventilator on which it must be placed. A circuit with an external exhalation valve must be used with older ventilators such as the Bennett MA-1 (Covidien-Puritan Bennett) and the Bear 2 (Cardinal Health, Viasys Bear Medical Systems, Palm Springs, CA). In addition, if a Bird-series unit is used as a ventilator, its circuit has an external exhalation valve (see Figure 15-24, A). All modern electrical and microprocessor ventilators feature an internal exhalation valve and do not need one included in the circuit (see Figure 15-24, B). If the patient must receive aerosolized medications, the circuit should include a nebulizer or should be able to accept one. If not included, the nebulizer or metered-dose inhaler adapter must be added into the inspiratory limb of the circuit (Figure 15-27).

Figure 15-27 The CircuVent™ adapter allows the medication aerosol from a small-volume nebulizer to bypass a heat-moisture exchanger (HME). After treatment, remember to reset the adapter so that the patient’s tidal volume passes though the HME normally. The HME is located properly between the Y of the ventilator circuit and the patient’s endotracheal tube. Try to select an HME that has the smallest internal volume to minimize mechanical dead space and rebreathed carbon dioxide. Mechanical dead space in this system includes all tubing between the Y of the circuit and the endotracheal tube. This includes the CircuVent adapter and its bypass tubing when used, or the HME and its large-bore tubing during normal breathing.

(From DHD Healthcare, Wampsville, NY.)

Also consider whether it is better to use an unheated or heated circuit. Usually the circuit is unheated. With these, a cascade-type humidifier or an HME is used to warm and humidify the inspired gas. However, some practitioners prefer to use a heated circuit for the care of neonates. These circuits may have heated wires loosely running through the lumen of the tubing or may have a wire embedded within the tubing itself. A heated-wire circuit offers finer control over the temperature of the inspired gas and minimizes condensation. Follow the manufacturer’s guidelines to make sure that the system can adequately humidify the minute volume that is being used.

Noninvasive ventilators, such as the Respironics series, have specific circuits designed only for the unit. Traditional ventilator circuits cannot be placed on a noninvasive ventilator.

b. Put the equipment together and make sure that it works properly

Assemble the circuit with the features needed to manage the patient. Common, but not universal, features of the inspiratory limb of the circuit include a water trap, a humidification system, a nebulizer, a thermometer or temperature probe, a pressure-monitoring port, and an oxygen-monitoring port. Common, but not universal, features of the expiratory limb of the circuit include an exhalation valve and a water trap.

The heated-wire circuits must be used only with the humidifier with which they are specifically designed to work. The humidifier has a thermostat that regulates warming of the humidifier water and heated wires to the same temperature. Never cover a heated-wire circuit with a patient’s sheets or blanket or any other material. Do not rest the circuit on anything such as the bed rail, patient’s body, or medical equipment. These circuits should always be supported on a boom arm or tube-tree.

Noninvasive ventilator circuits must be set up as specified for the ventilator. Depending on the unit, an HME or cascade-type humidifier is added to add moisture to the patient’s tidal volume gas.

All circuits use a Y-connector to tie the inspiratory and expiratory limbs together and attach the circuit to the patient. See Figure 15-25 for examples of a generic ventilator circuit. Make sure that the water level is maintained properly in the humidifier.

c. Troubleshoot any problems with the continuous ventilation circuit

Check the circuit and connections for leaks if the volume returned from the patient is less than what was set or delivered. The set volume should be measured with a hand-held spirometer as it exits the ventilator, at connection points through the circuit, and at the exhalation valve or ventilator spirometer or both. The volume control or ventilator spirometer may be out of calibration. If the unit shows a volume entering the spirometer instead of the test lung during inspiration, the exhalation valve is broken. Replace a circuit with a leak or a defective exhalation valve that cannot be fixed.

d. Independently change the patient’s ventilator circuit as needed (ELE code: IIIF2i10) [Difficulty: R, Ap, An]

A circuit must be replaced if it is damaged in a way that prevents the patient from being ventilated. This is seen most commonly in circuits with external exhalation valves (see Figure 15-24A). If the balloon-type valve is damaged and will not close, the tidal volume will leak out rather than enter the patient. The circuit must be replaced. If the expiratory valve line is pulled off at the ventilator or exhalation valve, it must be replaced or the valve will not close.

The American Association for Respiratory Care (AARC) Clinical Practice Guideline on ventilator circuit changes (1994) included the following recommendations:

• The circuit should be changed every 24 hours if a nebulizer is used for humidification.

• The circuit may be used for up to 5 days if a cascade- or wick-type humidifier is used for humidification.

• If an HME is used, it should be changed every 24 hours.

More recently, the AARC Clinical Practice Guideline on care of the ventilator circuit and its relation to ventilator-associated pneumonia (2003) included the following guidelines:

• The circuit should not be changed routinely for infection control purposes.

• Change the circuit when it is obviously soiled (e.g., blood, sputum).

• Passive humidifiers (HMEs) do not need to be changed daily.

• An HME can be used for at least 48 hours, and possibly for up to a week.

• Change an HME when it is obviously soiled (e.g., blood, sputum).

It is reasonable to expect the 2003 guidelines to be tested by the NRBC.

8. Manipulate ventilator breathing circuits: PEEP valve assembly, by order or protocol (ELE code: IIA11c) [Difficulty: ELE: R, Ap, An]

a. Get the necessary equipment for the procedure

A variety of PEEP systems can be added to a ventilator or CPAP circuit. Consult an equipment book for the details of their operation. Many of the newer ventilators have internal exhalation valves and PEEP-generating Venturi systems with nothing to assemble at the bedside. Failure to generate PEEP indicates that the exhalation valve or the PEEP-generating Venturi system has failed in some manner (see Figure 15-24, B).

Most older ventilators (Bird-series, Bennett MA-1, and Bear 2) use a balloon-like exhalation valve. A direct relationship exists between the volume of gas that is kept in the balloon, the pressure and resistance that it creates, and the PEEP level that is generated (see Figure 15-24, A).

b. Put the equipment together and make sure that it works properly

The key thing to check with any PEEP-generating system is that the proper level of PEEP is generated and maintained. Once the ordered PEEP level is set, it should be seen as stable on the pressure manometer throughout the respiratory cycle. Sensitivity should be set at no more than −1 to −2 cm water. That way, the PEEP level is maintained at close to the ordered level even during an assisted breath. For example, PEEP is set at 10 cm, and the sensitivity is set at −1 cm. When the patient triggers a breath, it will occur at 9 cm PEEP.

c. Troubleshoot any problems with the equipment

Malfunctioning internal exhalation valves or PEEP-generation Venturi systems cannot be repaired easily at the bedside. The ventilator must be replaced. Balloon-type exhalation valves (see Figure 15-24, A) are prone to the following two problems:

1. The small-bore tube carrying gas from the ventilator to the balloon valve is pulled off. Reconnect the tubing to the ventilator nipple connection or to the exhalation valve nipple connection.

2. The balloon is torn, and the gas leaks out. This can be confirmed by disassembling the exhalation valve assembly. If possible, replace the balloon, and reassemble the exhalation valve. If the balloon cannot be repaired, replace the entire circuit with a new exhalation valve.

Both of these problems are exhibited when the inspiratory tidal volume flows past the patient and directly into the exhaled tidal volume spirometer. Little or no airway pressure is generated. The patient is poorly ventilated, if at all. This problem must be corrected immediately while the patient is ventilated manually.

9. Manipulate continuous positive airway pressure (CPAP) systems: breathing circuits, by order or protocol (ELE code: IIA11c) [Difficulty: ELE: R, Ap, An]

a. Get the necessary equipment for the procedure

Most current generation ventilators have a built-in CPAP mode. No additional circuitry is needed. After switching to the CPAP mode, set the desired level by adjusting the PEEP/CPAP dial and watching the reading on the pressure manometer.

Several manufacturers make CPAP systems for home use in the treatment of patients with obstructive sleep apnea. These CPAP systems typically include an air pump to generate flow to the patient, a CPAP generating device, a circuit designed to work with the system, patient mask(s), and an alarm system.

Free-standing CPAP breathing circuits used in hospitals vary considerably. No manufacturer has developed a system that dominates the marketplace. Most commonly, each respiratory care department develops its own breathing circuit to meet its own needs.

b. Put the equipment together and make sure that it works properly

Figure 15-28 shows the typical components used in a free-standing CPAP breathing circuit. Components include the following:

• Air/oxygen blender

• Pediatric or adult flowmeter on the blender

• Cascade-type humidifier

• Inspiratory circuit of large-bore or aerosol tubing with the following additions: water trap, one-way valve, and thermometer

• Y connector to connect the inspiratory and expiratory limbs of the circuit

• Patient connector (elbow adapter) to endotracheal or tracheostomy tube, CPAP prongs, or CPAP mask

• Expiratory circuit of large bore or aerosol tubing with the following additions: water trap, high-pressure pop-off valve (not shown), pressure manometer for measuring the CPAP level, low-pressure or disconnection audible alarm, anesthesia bag as a reservoir, variable resistance clamp on the tail of the anesthesia bag, Briggs adapters/T-pieces for connecting the various features, emergency pop-in valve in case gas flow is stopped, and CPAP device.

Figure 15-28 Sample free-standing patient breathing circuit for CPAP. Note the components that are commonly added into the circuit.

The CPAP level is adjusted by means of a variety of devices collectively called threshold resistors, which include the following:

• A column of water with a length of expiratory tubing inserted the needed depth below the surface (so-called “bubble CPAP”).

• A vertically mounted ball bearing. It creates a resistance as gas flows up past it. These ball-bearing resistors come in weights of 2.5, 5, and 10 cm water.

• A spring-loaded resistor that is adjusted to apply the desired CPAP level against the airway.

• A free-standing Venturi PEEP system. Gas from the Venturi jet creates backpressure against the escaping patient tidal volume.

Air exits through each of these devices when it is working properly. All CPAP systems must be adjusted by checking the pressure level on the manometer. Set the low pressure or disconnection audible alarm to sound at a few centimeters below the CPAP level. For example, if 10 cm CPAP is ordered, set the alarm to sound if the pressure drops below 8 cm of CPAP.

c. Troubleshoot any problems with the equipment

Flow through the CPAP breathing circuit must be sufficient to meet the patient’s needs. Adjust the flowmeter setting and clamp on the anesthesia bag so that it is somewhat inflated, with excess air escaping out past the clamp. With all of the devices, gas escapes through the path of least resistance. All or some may escape through the anesthesia bag, the CPAP device, or both. The bag should collapse somewhat during the patient’s inspiration and expand somewhat during the expiration. The CPAP level should not decrease by more than 1 or 2 cm from baseline during an inspiration.

Make sure that the water level is maintained properly in the humidifier. Fill it with sterile, distilled water as often as necessary.

A sudden decrease in the CPAP level to zero indicates a disconnection at the patient or somewhere in the breathing circuit. Check all connections, and reassemble the break. The patient may have to be ventilated manually while the problem is corrected.

If the CPAP level decreases by more than 2 cm water during an inspiration, the flow is inadequate and should be increased. Flow also is inadequate if the patient shows increased use of accessory muscles of respiration or complains of increased WOB. Too high a flow is seen by an inadvertently high level of CPAP or the patient complaining of difficulty exhaling.

Water column systems must be monitored frequently because of water loss caused by evaporation. The actual CPAP level is progressively less than desired as the water is lost gradually. This system must have water added to it regularly or must have the expiratory tubing inserted deeper to keep the desired CPAP level.

The ball-bearing resistor system must be mounted vertically for gravity to keep the desired weight against the circuit. If it falls over and is horizontal, the CPAP pressure will be lost.

10. Manipulate CPAP systems: masks, nasal, and bilevel, by order or protocol (Code: IIA2) [Difficulty: ELE: R, Ap; WRE: An]

a. Get the necessary equipment for the procedure

A CPAP mask and breathing circuit are used primarily for patients who have obstructive sleep apnea. CPAP, by means of the mask, forces soft tissues open to the point that the airway is never obstructed (see Figure 15-22). The patient now is able to sleep normally and remain oxygenated. The patient should have the CPAP mask, breathing circuit, and proper CPAP level determined by a sleep study in the hospital. The patient can use the system at home once it is set up properly and he or she has been trained in its use.

In recent years, a CPAP mask has been used with a noninvasive mechanical ventilator to assist temporarily in the breathing of a patient with respiratory distress. The hope is to support the patient’s breathing long enough to treat the underlying problem(s). If this is successful, the patient does not need to be intubated. These patients require careful assessment and monitoring.

The two main categories of CPAP masks come in different sizes for children older than 3 years to adults. (See Figure 15-23 for examples.) Nasal mask and pillow systems are designed to cover only the nose. These allow the patient to speak and offer the mouth as a second airway for breathing in case a malfunction of the CPAP system occurs. The mouth also acts as a pressure relief route if the CPAP pressure should become too great. Pressures of up to 15 cm water usually can be maintained in an adult. Pressures of up to 10 cm water usually can be maintained in a child.

Full face mask and total face mask systems are designed to cover the nose and mouth. These are transparent and are similar to the mask used during bag/mask ventilation. The face mask must be used if the patient has persistent mouth breathing and cannot use a nose mask. With a good seal, pressures of greater than 15 cm water can be maintained.

All types of CPAP masks have a soft, very compliant seal to closely fit the contours of the face. Straps are needed to hold the mask in place. It is imperative that the mask properly fit the patient’s face.