a. Review the patient’s chart for information on airway graphics (Code: IA7c) [Difficulty: ELE: R; WRE: Ap]

A patient who has been intubated and placed on a modern mechanical ventilator with a microprocessor and graphics software can have ventilator flow, volume, and pressure waveforms visualized on the monitor, stored in memory, or printed out. Look for this information and compare it with the patient’s current situation. See Figure 15-1 for examples of pressure, volume, and flow tracings. See Figure 15-2 for key points of information available from a flow vs. time graph.

Figure 15-1 Examples of waveforms for pressure, flow, and volume that can be seen on the graphics monitor of a microprocessor-type ventilator. The practitioner can select from these and other options to display needed graphs to help guide patient care. See examples throughout the chapter.

(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.

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.

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:

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.

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.

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.)

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:

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:

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.

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.

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.

c. I : E ratio

The I : E ratio is adjusted to ensure that the patient can inhale in as physiologically appropriate a manner as possible and completely exhale the inspired tidal volume. Typically, the I : E ratio should be 1 : 2 or more. Incomplete exhalation will cause air trapping and auto-PEEP (see Figure 15-3).

d. Alarms

Alarm systems are different for each type of ventilator. Generally speaking, they are set with a safety margin of ±10% from the patient’s normal ventilator settings. A variation of greater than 10% results in an audible or visual alarm condition. Most ventilators alarm if the I : E ratio is 1 : 1 or less.

e. Gas temperature

The goal for most patients is to minimize their humidity deficit by using gas that is humidified and warmed to near body temperature. This can be accomplished by a cascade- or wick-type humidifier or by a heat-moisture exchanger (HME). It is common to see the gas warmed to 90° F to 95° F (31° C to 35° C). This decreases the patient’s humidity deficit to a minuscule level and reduces the “rain out” of water vapor condensing in the ventilator circuit. A temperature probe should be placed in the inspiratory tubing as close to the patient as possible to monitor the inspired gas temperature.

f. Modes of ventilation

The following modes of ventilation are delivered through most types of electrically powered and microprocessor-type volume-cycled ventilators.

1. Control

Control (C) is the simplest method of providing ventilatory support and is used on an apneic patient. The ventilator is set with a mandatory respiratory rate and tidal volume. The machine is incapable of allowing any patient interaction. For example, the ventilator might be set to deliver a tidal volume of 700 mL at a rate of 14 times/min. Because of this limitation, it is rarely, if ever, used in modern medicine except when the patient must be kept sedated or pharmacologically paralyzed (Figure 15-13 shows the pressure/time curve).

Figure 15-13 Pressure vs. time waveforms for various modes of mechanical ventilation. A, Control (C) mode shows no patient effort and consistent inspiration/expiration ratios. B, Assist/control (A/C) mode shows that patient’s initial effort triggers machine tidal volume breath. C, Intermittent mandatory ventilation (IMV) mode shows spontaneous tidal volume breaths occurring between predetermined machine tidal volume breaths. Note “stacked” breaths that happen when patient takes in a breath that then is supplemented by a machine breath. D, Synchronous intermittent mandatory ventilation (SIMV) mode shows that a patient effort within a time window results in delivery of machine tidal volume. Any other patient efforts within the time window result in a spontaneous tidal volume. If no patient efforts occur within the time window, machine tidal volume will be delivered automatically. E, Pressure support ventilation (PSV) mode shows how patient must initiate all breaths that then are supported to the predetermined airway pressure. Stable tidal volumes are seen if the patient inhales passively. Variably larger tidal volumes result if the patient inhales more actively. F, Positive end-expiratory pressure (PEEP) therapy can be added to A/C mode (as shown) or any other mode. The elevated baseline pressure prevents alveolar collapse. The sensitivity control must be set at −1 to −2 cm water so that the patient is able to trigger a breath without undue effort. G, Continuous positive airway pressure (CPAP) shows that the patient takes spontaneous tidal volumes while exhaling against an elevated baseline pressure.