Positive End-Expiratory Pressure
A Positive end-expiratory pressure (PEEP): The establishment and maintenance of a preset airway pressure greater than ambient at end-exhalation.
B Continuous positive airway pressure (CPAP): The application of PEEP to the spontaneously breathing patient. Inspiratory and expiratory airway pressures are supraatmospheric, but no inspiratory assistance is provided.
C Continuous positive pressure ventilation (CPPV): The application of PEEP to a patient receiving positive pressure ventilation.
II Physiologic Effects of PEEP
A Effects on intrapulmonary pressures
1. When only the end-expiratory pressure is maintained above atmospheric in a spontaneously breathing patient, the shape of the intrapulmonary pressure curve is not altered; only the baseline pressure from which the patient ventilates changes. Therefore, the dynamics of air movement are not directly affected.
2. As illustrated in Chapter 5, intrapulmonary pressure decreases approximately 3 cm H2O during inspiration and increases approximately 3 cm H2O during expiration from the set CPAP level.
3. During ventilatory support, regardless of mode, PEEP simply increases the baseline about which mechanical ventilation is initiated.
B Effects on intrapleural (intrathoracic) pressures
1. PEEP increases intrapleural pressures.
2. The extent of the increase is determined by
b. The stiffness of the individual’s lung
(1) The greater the pulmonary compliance, the greater the transmission of PEEP to the intrapleural space and the greater the increase in intrapleural pressure.
(2) In patients with normal lungs and chest wall approximately 50% of the PEEP applied is transmitted to the intrathoracic space, increasing the intrapleural pressure.
(3) In patients with acute respiratory distress syndrome (ARDS; stiff lungs) only approximately 25% of the applied PEEP is transmitted to the intrathoracic space, increasing intrapleural pressure.
(4) Patients with localized pulmonary disease (e.g., pneumonia, atelectasis) demonstrate an increase in overall intrapleural pressure similar to patients with normal pulmonary compliance; however, transmission of pressure may be reduced in the area of concern.
(5) The effects of PEEP on intrapleural pressure are most marked in patients with chronic obstructive pulmonary disease (COPD) because of their increased pulmonary compliance.
c. Changes in thoracic compliance
(1) If thoracic compliance is decreased more pressure than normal will be transmitted to the intrapleural space because overall expansion of the lung-thorax system is inhibited. It is in this setting that hemodynamic compromise is most likely.
(2) An increase in thoracic compliance will allow the system to expand and usually results in less of an increase in intrapleural pressure when compared with normal. Hemodynamic compromise is minimal in this setting.
C Effect on functional residual capacity (FRC) (Figure 40-1)
1. Regardless of the condition of the lung at the time of application, PEEP increases FRC.
2. FRC is increased by two primary mechanisms.
a. Because the lungs are elastic any increase in end-expiratory pressure increases overall lung volume. The diameter of conducting airways can increase 1 to 2 mm as PEEP is applied.
b. In patients with a decreased FRC as a result of alveolar collapse caused by surfactant instability, PEEP maintains alveoli inflated after they are recruited by the peak airway pressure.
(1) This is accomplished by PEEP maintaining a backpressure exceeding the force of surface tension and lung elastance, which tend to collapse alveoli.
(2) The actual reexpansion of alveoli is accomplished by the force of normal inspiration or the application of positive inspiratory airway pressure. PEEP simply maintains the alveoli open once they are reexpanded.
D Effect on pulmonary compliance
1. Because PEEP increases FRC, it alters pulmonary compliance.
2. The compliance curve of the normal total respiratory system is depicted in Figure 40-2. Note that the curve is only linear above the FRC level and again becomes alinear at some pressure and volume well beyond FRC.
3. In the normal lung the increased FRC caused by excessive PEEP levels may move alveoli from the steep portion to the flat portion of the compliance curve, thus decreasing compliance.
4. In patients with acute lung injury (ALI) or ARDS, the application of PEEP increases compliance (see Figure 40-2).
a. As ARDS develops the compliance curve shifts to the right and downward.
b. As PEEP is applied the compliance curve shifts upward and to the left.
c. With appropriate application of PEEP compliance in ALI/ARDS normally improves.
5. The monitoring of effective static compliance (see Chapter 41) can be used to help to determine the “optimal” or most appropriate PEEP level.
a. The best compliance is thought to coincide with the most appropriate PEEP level; however, changes in tidal volume (Vt) will change the PEEP level considered appropriate. Vt should always be constant during PEEP titration.
b. The major problem associated with the use of compliance as a means to determine optimal PEEP is the difficulty in determining compliance in patients ventilated in anything but the control mode. With other modes a reliable measurement of effective static compliance is often difficult because of active movement of the chest wall, preventing correct determination of the static end-inspiratory plateau pressure.
1. Because PEEP increases FRC by distending alveoli, deadspace is usually increased in
2. Because of stabilization of recruited alveoli, appropriate PEEP levels usually decrease deadspace in patients with ALI/ARDS. Some have proposed monitoring deadspace or change in CO2 at a constant minute ventilation as an indication of appropriate PEEP level.
F Effect of PEEP on the cardiovascular system (Table 40-1)
TABLE 40-1
Potential Physiologic Effects of Appropriately and Excessively Applied PEEP
| Appropriate Level | Excessive Level | |
| Intrapulmonary pressure | Increased | Increased |
| Intrathoracic pressure | Increased | Increased |
| FRC | Increased | Increased |
| Respiratory system compliance | Increased | Increased or decreased |
| Closing volume | Decreased | Decreased |
| Pao2 | Increased | Increased or decreased |
| Sao2 | Increased | Increased or decreased |
| Paco2 | No change or decreased | Increased |
 s/ t |
Decreased | Decreased or increased |
| P(A−a)O2 | Decreased | Decreased or increased |
C(a- ) O2 |
Decreased | Decreased or increased |
P o2 |
Increased | Increased or decreased |
| Paco2-PETCO2 | Decreased | Increased |
| VD/VT | Decreased | Increased |
| Work of breathing | Decreased | Increased |
| Extravascular lung water | No change or increased | No change or increased |
| Pulmonary vascular resistance | Increased | Increased |
| Total pulmonary perfusion | No change or decreased | Decreased |
| Cardiac output | No change or decreased | Decreased |
| Pulmonary artery pressure | No change or increased or decreased | Decreased |
| Pulmonary capillary wedge pressure | No change or increased or decreased | Decreased |
| Central venous pressure | No change or increased or decreased | Decreased |
| Arterial pressure | No change or increased or decreased | Decreased |
| Intracranial pressure | No change or increased | Increased |
| Urinary output | No change or decreased | Decreased |
S/
T, shunt fraction; P(A − a)O2, alveolar-arterial O2 pressure difference; C(a-
) O2, arterial-mixed venous O2 content difference; P
O2, mixed venous O2 pressure; PETCO2, end-tidal CO2 pressure; VD/VT, dead space/tidal volume ratio.1. The primary effect of PEEP on the cardiovascular system is a reduction in cardiac output (CO) as a result of increased impedance to venous return by an increase in intrathoracic pressure.
2. This increase in pressure decreases cardiac transmural pressure, potentially decreasing the end-diastolic volume and stroke volume of both ventricles.
3. Low-level PEEP reduces right ventricular end-diastolic volume, but right ventricular ejection fraction normally remains constant, provided no previous right ventricular dysfunction exists.
4. Higher levels of PEEP markedly increase right ventricular afterload, increasing end-diastolic volume and decreasing ejection fraction.
5. Increased right ventricular end-diastolic volume with high levels of PEEP causes right ventricular distention and a leftward shift of the interventricular septa.
a. This reduces left ventricular distensibility.
b. This results in a decrease in left ventricular end-diastolic volume and stroke volume.
6. Actual changes in pulmonary hemodynamics after the application of PEEP depend on many factors.
7. Provided that pulmonary blood flow is not markedly reduced, PEEP generally results in
a. An increase in right ventricular preload (central venous pressure [CVP])
b. An increase in right ventricular afterload (pulmonary artery pressure [PAP])
c. An increase in left ventricular preload (pulmonary capillary wedge pressure [PWP])
d. Generally the higher the PEEP, the less likely the wedge pressure will reflect left atrial pressure.
e. Because CO is usually decreased with PEEP, left ventricular afterload may also decrease as PEEP is applied.
8. If pulmonary blood flow is markedly reduced by the application of PEEP, the preload and afterload of left and right ventricles decrease. As a result it is difficult to predict the precise effect that PEEP will have on hemodynamics.
9. Any of the hemodynamic pressures measured may increase, decrease, or stay the same, depending on the maintenance of pulmonary blood flow.
10. When the effect of PEEP on CO is evaluated, it is important to place the decreased CO into proper perspective. The following are two examples of the effect of PEEP on CO. In example A the patient is young and has excellent cardiovascular reserves, whereas in example B the patient is older and has limited cardiovascular reserves.
| Pao2 | 48 mm Hg | Pulse | 130 beats/min |
| pH | 7.53 | Blood pressure (BP) | 160/100 mm Hg |
| Paco2 | 27 mm Hg | CO | 10.5 L/min |
| HCO3− | 22 mEq/L | CI | 5.7 L/min/m2 |
| Spontaneous respiration rate (RR) | 35 breaths/min | FIO2 | 0.8 |
| VT | 350 ml | No mechanical ventilatory support | |
With the application of PEEP the following data are obtained.
| Pao2 | 75 mm Hg | Pulse | 85 beats/min |
| pH | 7.43 | BP | 130/80 mm Hg |
| Paco2 | 38 mm Hg | CO | 6.6 L/min |
| HCO3− | 24 mEq/L | CI | 3.7 L/min/m2 |
| Spontaneous RR | 20 breaths/min | FIO2 | 0.5 |
| VT | 350 ml | CPAP at 10 cm H2O by mask | |
| Pao2 | 48 mm Hg | Pulse | 130 beats/min |
| pH | 7.48 | BP | 140/90 mm Hg |
| Paco2 | 32 mm Hg | CO | 5.5 L/min |
| HCO3− | 23 mEq/L | CI | 3.6 L/min/m2 |
| Spontaneous RR | 35 breaths/min | FIO2 | 0.6 |
| VT | 300 ml | No mechanical ventilation support | |
With the application of PEEP the following data are obtained.
| Pao2 | 68 mm Hg | Pulse | 150 beats/min |
| pH | 7.47 | BP | 90/60 mm Hg |
| Paco2 | 33 mm Hg | CO | 3.5 L/min |
| HCO3− | 23 mEq/L | CI | 2.3 L/min/m2 |
| Spontaneous RR | 28 breaths/min | FIO2 | 0.6 |
| VT | 300 ml | CPAP at 10 cm H2O by mask | |
11. The following example is designed to illustrate the effect of PEEP on hemodynamic values.
| No PEEP | |||
| Pulse | 160 beats/min | CVP | 12 cm H2O |
| BP | 150/100 mm Hg | PAP | 26 mm Hg |
| PWP | 10 mm Hg | ||
| 5 cm H2O PEEP | |||
| Pulse | 158 beats/min | CVP | 13 cm H2O |
| BP | 148/92 mm Hg | PAP | 27 mm Hg |
| PWP | 11 mm Hg | ||
| 10 cm H2O PEEP | |||
| Pulse | 140 beats/min | CVP | 15 cm H2O |
| BP | 142/96 mm Hg | PAP | 29 mm Hg |
| PWP | 13 mm Hg | ||
| 12 cm H2O PEEP | |||
| Pulse | 126 beats/min | CVP | 16 cm H2O |
| BP | 130/84 mm Hg | PAP | 30 mm Hg |
| PWP | 14 mm Hg | ||
| 15 cm H2O PEEP | |||
| Pulse | 154 beats/min | CVP | 6 cm H2O |
| BP | 90/60 mm Hg | PAP | 22 mm Hg |
| PWP | 5 mm Hg | ||
| 15 cm H2 O PEEP | |||
| Pulse | 124 beats/min | CVP | 18 cm H2O |
| BP | 120/84 mm Hg | PAP | 32 mm Hg |
| PWP | 16 mm Hg | ||
G Effects of PEEP on lung water (Figure 40-3)
1. PEEP does not decrease overall pulmonary vascular volume.
2. Normally PEEP causes a redistribution of lung water and may increase overall lung water.
3. Fluid generally moves from the intraalveolar to the perivascular interstitial space; extraalveolar and corner vessels are expanded with PEEP.
4. This movement assists in improving oxygenation, increasing compliance, and decreasing shunting.
