VENTILATOR MANAGEMENT

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CHAPTER 11 VENTILATOR MANAGEMENT

PRETEST QUESTIONS

Answer the pretest questions before studying the chapter. This will help you determine your strong and weak areas in the material covered.

1. The following information has been obtained from a ventilator patient.

Peak inspiratory pressure 48 cm H2O
Plateau pressure 27 cm H2O
VT 850 mL
PEEP 4 cm H2O

On the basis of these data, the patient’s static lung compliance is approximately which of the following?

A. 18 mL/cm H2O
B. 20 mL/cm H2O
C. 31 mL/cm H2O
D. 37 mL/cm H2O

2. A volume-cycled ventilator is in the control mode and the I:E inspiratory/expiratory ratio alarm is sounding. Which control adjustment would correct this problem?

A. Decrease the flow rate.
B. Increase the VT.
C. Increase the respiratory rate.
D. Increase the flow rate.

3. Mechanical ventilation can lead to which of the following complications?

1. Increased renal output
2. Barotrauma
3. Increased cardiac output
A. 1 only
B. 2 only
C. 1 and 2 only
D. 2 and 3 only

4. Static lung compliance will decrease as a result of which of the following?

A. Bronchospasm
B. Mucosal edema
C. Atelectasis
D. Bronchial secretions

5. These data have been collected from a patient whose ventilator was in the control mode.

image

To increase this patient’s PaCO2 to 40 mm Hg, the ventilator rate should be adjusted to what level?

A. 10/min
B. 11/min
C. 12/min
D. 13/min

6. The following data have been collected from a patient using a volume ventilator in the control mode.

image

Based on this information, the respiratory therapist should recommend which of the following ventilator changes?

A. Increase FiO2 to 0.60.
B. Increase VT to 800 mL.
C. Add 5 cm H2O of PEEP.
D. Initiate CPAP with 4 cm H2O and an FiO2 of 0.50.

See answers and rationales at the back of the text.

REVIEW

I. MECHANICAL VENTILATORS

CRT Exam Content Matrix: IIA6a-b, IIA11a, d, IIID2c, IIID10, IIIF2i6,9, IIIG3f,i,k, l

RRT Exam Content Matrix: IIA2a-b, IIID2c, IIID8, IIIF2e6, IIIG3f,h,k

A. Positive Pressure Ventilation
1. Types of positive pressure ventilators
a. Preset volume ventilators (volume-limited or volume-cycled)
b. Preset pressure ventilators (pressure-limited or pressure-cycled)
2. Volume ventilators
a. A preset VT is delivered to the patient in each machine breath, and once it is delivered, inspiration ends.
b. The volume-limited ventilator can develop an inspiratory pressure that maintains the preset VT when changes in airway resistance and compliance occur (i.e., volume is constant, pressure is variable).
c. Used for mechanical ventilation of the lungs of adult patients.
3. Pressure-cycled ventilators
a. A preset inspiratory pressure is delivered to the patient, and once it is reached, inspiration ends.
b. The delivered VT is unknown but varies with changes in airway resistance and lung compliance (i.e., volume varies, pressure is constant).
c. When lung compliance decreases, delivered VT decreases. In other words, as the patient’s lungs become stiffer and harder to ventilate, the delivered VT decreases.
d. The pressure control is used like the volume control. When inspiratory pressure is increased, delivered VT is increased and vice versa.
e. Used to ventilate the lungs of infants and postoperative patients and to administer IPPB treatments. Inspiration ends on most infant ventilators when a preset time is reached. The set inspiratory pressure is reached during that time.
f. Recently, pressure control ventilation (PCV) has been used as a mode of ventilation, especially for adult patients with ARDS in whom conventional volume ventilation with PEEP has not improved ventilation or oxygenation.
g. Specific indications for PCV vary, but generally the patient should be used to the following ventilator variables before PCV is instituted
(1) FiO2, 1.0
(2) PEEP >15 cm H2O
(3) PIP >50 cm H2O
(4) Assist/control rate >16/min
h. Initial settings for PCV should include
(1) Whatever the FiO2 was set on prior to PCV

image Exam Note

PIP should be set to obtain a specified exhaled VT. In other words, if the “target” exhaled VT is 600 mL and the actual exhaled VT is 500 mL, then PIP should be increased.

(2) VT, 6 to 10 mL/kg of ideal body weight (IBW)
(3) PEEP of about 50% of the volume ventilator setting
(4) I:E, 1 : 2

image Exam Note

PCV is often combined with inverse I:E ratio ventilation. Because this type of ventilation may be uncomfortable, the patient should be sedated and paralyzed.

i. Studies have shown that PCV improves gas exchange, increases oxygenation, reduces PIP, increases mean airway pressure, reduces required PEEP levels, and decreases minute ventilation, especially when it is combined with an inverse I:E ratio. It has also been shown to reduce cardiovascular side effects and barotrauma compared with volume ventilation with PEEP.
j. It is important that exhaled VT is monitored during PCV. Because inspiration is pressure-limited, the volume varies with changes in lung compliance and airway resistance.

image Exam Note

When a ventilator scenario is given with a patient receiving PCV and the exhaled VT decreases with no change in ventilator settings, either compliance has decreased or airway resistance has increased. Remember, in PCV, inspiratory pressure is limited and cannot increase if compliance decreases or resistance increases. Therefore, delivered tidal volume will decrease.

k. If an inverse I:E ratio of greater than 2 : 1 is used, intrinsic PEEP, also referred to as auto-PEEP, may occur. Auto-PEEP may be detected by monitoring expiratory waveform curves, which reveal the expiratory flow not returning to zero before the next breath. Auto-PEEP may result in barotrauma, decreased venous return and cardiac output, and increased patient effort to initiate a breath if the patient is assisting.
l. Extensive monitoring of the patient’s cardiovascular and respiratory variables is vital for patients receiving PCV.
m. The ARDS network has developed guidelines and lung protective strategies to help better ventilate the lungs of patients with ARDS. Below is a summary of those strategies
(1) Target VT of 6 mL/kg of IBW
(2) Maintenance of alveolar (plateau) pressure ≤30 cm H2O
(3) Use of relatively high PEEP levels (up to 24 cm H2O)
(4) Permissive hypercapnia: With the use of lower tidal volumes and therefore lower peak pressures, CO2 levels may begin to rise, resulting in respiratory acidosis. Studies indicate that a higher percentage of patients recover with minimal side effects from the acidosis if the pH does not drop below 7.20. In fact, acidosis may produce some positive effects. As CO2 increases, the patient will become sleepy and easier to control while using the ventilator. Also, an acidotic environment shifts the oxyhemoglobin curve to the right, which means that hemoglobin releases oxygen to the tissues more easily and increases the level of tissue oxygenation.
(5) Oxygenation target: PaO2, 55 to 80 torr; SpO2, 88% to 95%; PEEP/FiO2 adjustments should be assessed at least every 4 h.
(6) Target pH of 7.30 to 7.45
(7) Avoidance of excessively high FiO2 levels (try to maintain below 0.60)
n. Another type of pressure ventilation is bilevel positive airway pressure, or Bi-PAP. Bi-PAP may be used on intubated or nonintubated patients. Because patients with COPD have difficulty being weaned from mechanical ventilation, Bi-PAP is used to ventilate the lungs of these patients through a nasal mask, which avoids intubation and conventional volume ventilation. Bi-PAP may buy time for the patient to get past the initial ventilatory crisis and avoid intubation and ventilation. Although it is not always successful, it is becoming a first step in ventilating the lungs of many patients with COPD.
o. Bi-PAP is also used in home ventilation for patients with neuromuscular dysfunction, obstructive sleep apnea, and other conditions that result in hypoventilation.
p. Bi-PAP may be time-triggered or patient-triggered. Once inspiration begins, a preset inspiratory positive airway pressure (IPAP) is reached. Expiratory positive airway pressure (EPAP) should be preset to avoid CO2 buildup in the nasal mask. EPAP is the equivalent of PEEP.
q. The initial IPAP setting is usually 10 to 15 cm H2O, and the EPAP setting is 4 to 5 cm H2O. The difference between the IPAP and EPAP settings is the pressure support. In this example, pressure support is 6 cm H2O. IPAP settings range from 2 to 25 cm H2O, and EPAP ranges from 2 to 20 cm H2O.
r. A frequency control determines the timed breath rate and is adjustable from 6 to 30 cycles/min. The % IPAP control is set to determine the time spent in inspiration and functions as the I:E ratio control.
s. Bi-PAP is not a life-support ventilator. The practitioner should use an IPAP that results in an exhaled VT of 8 to 10 mL/kg IBW.
t. Criteria for Bi-PAP mask ventilation include stable hemodynamics, a cooperative patient, minimal airway secretions, and no need for airway protection.

II. VENTILATOR CONTROLS

CRT Exam Content Matrix: IIID2b,d, IIIF2i1-5,11, IIIG3a-e

RRT Exam Content Matrix: IIID2b,d, IIIF2e1,2,3,4,5,9, IIIG3a-e

A. Ventilator Modes (Volume Ventilators)
1. Control mode
a. Patient is not able to trigger a ventilator breath.
b. Inspiration is strictly time-triggered.

EXAMPLE:

Control rate of 10: The ventilator delivers a breath every 6 s (60 ÷ 10). The minute volume remains constant because the rate cannot be altered.

c. The patient should be heavily sedated or paralyzed.
2. Assist-control mode
a. In this mode, each breath is either patient-triggered or time-triggered.
b. This is a commonly used mode of ventilation.
c. The patient may initiate as many ventilator breaths as required above the set rate; therefore, the patient’s minute volume is not consistent.
3. Synchronized intermittent mandatory ventilation
a. Allows for spontaneous breathing along with positive pressure ventilator breaths. It is built into the ventilator and senses when the patient is breathing spontaneously; therefore, no “breath stacking” occurs.
b. It is used as both a weaning technique and for ventilation before weaning.
c. The sensitivity is left on because the patient must open a demand valve to obtain gas flow for spontaneous breathing.
4. Pressure support ventilation
a. This mode of ventilation is found on the new generation of ventilators that aid in the weaning process from the ventilator.
b. It is a patient-assisted, pressure-generated, flow-cycled breath, which may be augmented with SIMV or used by itself.
c. It was designed to make spontaneous breathing through the ET tube during weaning more comfortable by overcoming the high resistance and increased inspiratory work caused by the ET tube (5 to 10 cm H2O is all that is required to overcome tubing resistance).
d. An inspiratory pressure is set (usually 5 to 10 cm H2O for weaning purposes). As the patient initiates inspiration, the preset pressure is reached and held constant until a specific inspiratory flow is reached. Then the pressure is terminated.
e. The inspiratory pressure level may be set to achieve a specific VT.
f. Pressure support ventilation (PSV) may be used for patients who are ventilating well but are intubated to protect their airway or for patients using CPAP but who have oxygenation deficiencies.
5. Continuous positive airway pressure
a. May be achieved with the use of a CPAP mask, nasal prongs, or intubation and a ventilator.
b. A preset pressure is maintained in the airways as the patient breathes totally on his or her own. No positive pressure breaths are delivered.
c. Patients whose PaO2 level cannot be maintained within normal limits using a 60% or more O2 mask and who have normal or low PaCO2 levels should begin using CPAP. CPAP is also indicated for patients with obstructive sleep apnea who gain benefit from the positive airway pressure, which relieves the obstruction in the upper airway.
d. CPAP setups should always have a low-pressure alarm so that leaks in the system are detected.
e. CPAP separate from the ventilator, such as a CPAP mask, may be accomplished.
(1) In this type of setup, the patient’s exhaled gas flow enters the expiratory limb of the circuit, where it meets an opposing gas flow entering through a Venturi tube.
(2) This opposing gas flow causes a resistance to exhalation that the patient must overcome.
(3) The pressure the patient must generate to overcome this resistance results in a positive pressure that is read on a manometer placed in the setup line. This pressure is the CPAP level.
(4) The CPAP level may be increased or decreased by adjustment of the opposing gas flow that is passing through the Venturi tube in the expiratory limb.
f. The indications and hazards of CPAP are the same as those listed for PEEP later in this chapter.
B. VT Control
1. Determines the delivered VT to the patient in milliliters or liters.
2. Should be set at 8 to 12 mL/kg IBW. May use as low as 5 to 6 mL/kg IBW for patients with ARDS. For patients with COPD, 8 to 10 mL/kg is generally adequate.

image Exam Note

Use tidal volumes that do not cause overdistention of alveoli. It is best to maintain alveolar pressure (static pressure) less than 35 cm H2O. Studies indicate that alveolar pressures greater than 35 cm H2O result in lung damage.

3. For children, use 5 to 7 mL/kg IBW.
a. Calculating IBW (in lb)

For males

106 + [6 × (height in inches − 60 in)]

For females

105 + [5 × (height in inches − 60 in)]

b. This is very important when you are asked to determine the ventilator VT for an obese patient.

EXAMPLE:

The physician wants your recommendation for the ventilator VT setting for a 5 ft 3 in female patient who weighs 150 kg (330 lb).

image

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image Exam Note

To save time, after determining IBW, simply divide the weight by 2 and add a zero to the end of the number. This will represent an adequate initial VT setting that is close to 10 mL/kg. For example, the patient’s IBW is 140 lbs. Divide 140 by 2 and you get 70. Adding a zero makes the number 700. Select the choice that is closest to a 700-mL setting.

4. Increasing the VT increases alveolar ventilation while also increasing the minute volume. This decreases the PaCO2.

image Exam Note

The most effective way to improve alveolar ventilation and decrease PaCO2 is by increasing the tidal volume, not by increasing the ventilator rate.

image Exam Note

A ventilator scenario may be given indicating the patient has a high PaCO2. Increasing the rate and increasing the VT are both given as choices to correct the hypercapnia. Never assume you always select one over the other in a situation like this. You must review the scenario. If the question states the patient has atelectasis, you must increase the VT, not the rate. Atelectasis indicates an inadequate VT; therefore, increasing the rate is not as beneficial.

5. Decreasing the VT decreases alveolar ventilation while also decreasing the minute volume. This increases the PaCO2.

EXAMPLE:

A patient breathing 10/min with a VT of 600 mL has a minute volume of 6 L. (Minute volume is calculated as respiratory rate × VT.) If the patient begins breathing at a respiratory rate of 20/min with a VT of 300 mL, the minute ventilation remains 6 L, but the alveolar ventilation has decreased as a result of the decreased VT.

6. On volume ventilators
a. Increasing the VT increases the inspiratory time.
b. Decreasing the VT decreases the inspiratory time.
7. The preset machine VT is not the actual volume reaching the patient’s lungs.
a. Volume is “lost” in the ventilator circuit because of airway resistance from gas flow.
b. Tubing compliance may be calculated to determine how much volume is being lost in the circuit.
c. With the ventilator set on a specific VT and the high-pressure limit turned up completely, the machine is cycled into inspiration with the ventilator Y adapter occluded. Observe the manometer pressure reading.
d. Compliance formula

image

EXAMPLE:

Set VT at 200 mL (0.2 L). Peak pressure reached is 40 cm H2O.

image

This means that once the patient has started using the ventilator, 5 mL of the set tidal volume will be lost in the tubing for every 1 cm H2O registering on the manometer.

EXAMPLE:

Tubing compliance, 5 mL/cm H2O

VT, 800 mL

Peak inspiratory pressure, 20 cm H2O

Lost volume = tubing compliance × peak inspiratory pressure

image

Set VT is 800 mL

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e. Lost volume is affected by the water level in the humidifier. Lower water levels allow more compressed volume into the walls of the humidifier; hence, more volume is lost or less volume is delivered to the patient.
f. Volume is also lost in the patient’s conducting airways (e.g., trachea and bronchi) as a result of resistance to gas flow. This part of the patient’s airway is called the anatomic dead space. It is the part of the airway where no gas exchange occurs, and it is often called “wasted air.”

image Exam Note

Anatomic dead space is equal to 1 mL/lb of the patient’s IBW, but it is reduced by 50% in the intubated patient. Thus, anatomic dead space is equal to about 1 mL/kg.

EXAMPLE:

An intubated 75-kg (165-lb) patient has an anatomic dead space of approximately 75 mL. This means that 75 mL of the patient’s VT does not reach the alveoli to take part in gas exchange. This may also be subtracted from the ventilator VT setting to obtain a corrected VT.

image When the formula for selecting ventilator VT as 10 to 12 mL/kg of body weight is used, this lost volume is taken into account, and this is usually an adequate VT. Some ventilators compensate for volume lost in the tubing by delivering a higher VT automatically.

8. Exhaled VT is measured by an exhaled volume digital display. Exhaled VT is most accurately measured with a respirometer placed between the ET tube and ventilator Y adapter or at the exhalation valve. To most accurately measure the VT delivered by the ventilator, place a respirometer directly on the ventilator outlet.
9. Volume may be lost from other causes.
a. Loose humidifier jar or tubing connections cause low exhaled VT readings.
b. Leakage around the ET tube cuff results in low exhaled VT readings.
C. Respiratory Rate Control
1. Normal initial setup is 8 to 12 breaths/min.
2. Adjusting the rate control alters the expiratory time, therefore altering the I:E ratio.
a. Increasing the rate decreases expiratory time.
b. Decreasing the rate increases expiratory time.
3. Adjusting the rate alters the minute volume.
a. Increasing the rate increases minute volume.
b. Decreasing the rate decreases minute volume.
4. Adjusting the rate affects the PaCO2 level.
a. Increasing the rate decreases PaCO2.
b. Decreasing the rate increases PaCO2.

image Exam Note

Adjusting the rate to alter the patient’s PaCO2 level is more beneficial for patients receiving controlled ventilation or SIMV. When the ventilator is on assist/control, the patient may obtain as many machine breaths as needed, no matter what rate is set, so the PaCO2 is most effectively altered by adjustment of the VT control.

D. Inspiratory Flow Control
1. Normal setting: 40 to 60 L/min.
2. Adjusting the flow rate alters the inspiratory time, therefore altering the I:E ratio.
a. Increasing the flow rate decreases inspiratory time.
b. Decreasing the flow rate increases inspiratory time.
E. I:E ratio
1. A comparison of the inspiratory time with the expiratory time
2. Normal I:E ratio for the adult is 1 : 2. This means that expiration is twice as long as inspiration.
3. Normal I:E ratio for the infant is 1 : 1.
4. The I:E ratio is established by the use of three ventilator controls on the volume ventilator.
a. VT control
(1) Increasing the VT increases inspiratory time (makes inspiratory time longer).
(2) Decreasing the VT decreases inspiratory time (makes inspiratory time shorter).
b. Flow rate control
(1) Increasing the flow rate decreases inspiratory time.
(2) Decreasing the flow rate increases inspiratory time.
c. Respiratory rate control
(1) Increasing the respiratory rate decreases expiratory time.
(2) Decreasing the respiratory rate increases expiratory time.
5. Calculate the I:E ratio with the following formula

image

EXAMPLE:

VT 800 mL (0.8 L)
Rate 12/min
Flow rate 40 L/min

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6. Calculation of inspiratory time

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EXAMPLE:

Calculate the inspiratory time if the I:E ratio is 1 : 2 and the ventilator rate is 10/min.

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7. Inspiratory time should not exceed expiratory time, except in specific situations. This is referred to as an inverse I:E ratio. Inverse ratio ventilation is sometimes utilized to increase the PaO2 when FiO2 and PEEP levels are already high. It may greatly compromise venous blood return to the heart and increase intrathoracic pressure.

image Exam Note

When given a scenario on the exam where a ventilator patient has a normal PaCO2 with hypoxemia and the PEEP and FiO2 levels are high, increase the inspiratory time to increase the PaO2. This allows for a longer time for oxygen to diffuse across the alveolar capillary membrane, thereby increasing the PaO2.

a. If the I:E ratio alarm is sounding on the ventilator indicating an inverse I : E ratio, three controls may be altered to correct it.
(1) Rate: decrease to lengthen expiratory time.
(2) Volume: decrease to shorten inspiratory time.
(3) Flow: increase to shorten inspiratory time.

image Exam Note

Increasing flow is the most common adjustment to correct for an inverse I:E ratio.

F. O2 Percentage Control
1. Adjustable from 21% to 100% to maintain normal PaO2 levels
2. O2 percentage should be increased to a maximum of 60% to maintain normal PaO2 levels. Once 60% is reached, PEEP should be added or increased.
3. O2 percentage should be reduced first to a level of 60% before decreasing PEEP levels in hyperoxygenated patients.
4. The initial setting should be what FiO2 level the patient was receiving before the initiation of mechanical ventilation.

image Exam Note

The major ventilator controls have been discussed thus far in the chapter, so let’s summarize how to set the variables to initiate mechanical ventilation.

Mode: A/C or SIMV
VT: 8 to 12 mL/kg IBW
Ventilator rate: 8 to 12/min
FiO2: whatever the FiO2 the patient was receiving before the initiation of ventilation

G. Sensitivity Control
1. This determines the amount of patient effort required to cycle the ventilator into inspiration.
2. Should be set so that the patient generates −0.5 to −2.0 cm H2O pressure.
3. If the ventilator self-cycles, the sensitivity is too high. Decrease the sensitivity.
4. If it takes more than −2.0 cm H2O pressure to cycle the ventilator into inspiration, increase the sensitivity.
5. In the control mode of ventilation, the sensitivity is turned off and does not allow the patient to trigger a machine breath.
H. Sigh controls
1. The sigh rate should be set at 6 to 12 sighs/h.
2. The sigh volume should be set 1.5 to 2.0 times the VT.
3. Sighs aid in preventing atelectasis.
4. Usually not functional in the SIMV mode

image Exam Note

Although sighs are not commonly used in practice in many areas, the exam may still offer questions on the subject.

I. Inflation Hold Control
1. Adjustable from 0 to 2 s
2. The mechanism keeps the exhalation valve closed, causing the ventilator VT to be held in the lungs for a preset time.
3. Used to improve oxygenation by reducing atelectasis and shunting and increasing the diffusion of gases.
4. Using an inspiratory hold causes an increase in intrathoracic and mean airway pressure, which may result in reduced venous return and cardiac output.
5. Used to obtain a plateau pressure to calculate static lung compliance.
J. Expiratory Retard (Expiratory Resistance)
1. Used to prevent premature airway collapse during expiration.
2. Increases the expiratory time, therefore altering the I:E ratio and increasing intrathoracic pressure.
3. Expiratory retard causes an increase in intrathoracic and mean airway pressure, which may result in reduced venous return and cardiac output.

image Exam Note

Although expiratory resistance is rarely an option on ventilators, exam questions may appear on the subject.

K. Positive End-Expiratory Pressure
1. Used to maintain positive pressure in the airway after a ventilator breath.
image

FIGURE 11-1 Positive end-expiratory pressure (PEEP) curves compared with other curves.

2. Indications for PEEP
a. Atelectasis
b. Hypoxemia with the use of 60% or more O2
c. Decreased functional residual capacity (FRC)
d. Lowering O2 percentage to safe levels (less than 60%)
e. Decreased lung compliance
f. Pulmonary edema
3. Hazards of PEEP
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