What is the effect of Positive End-Expiratory Pressure (PEEP) therapy on extravascular lung water?

Effects of Positive End-Expiratory Pressure (PEEP) Therapy

Positive end-expiratory pressure (PEEP) therapy results in a decrease in extravascular lung water. 1

Primary Effects of PEEP

• PEEP increases end-expiratory lung volume and functional residual capacity (FRC) by preventing alveolar collapse at end-expiration, which reduces atelectasis 2
• PEEP improves oxygenation by reducing intrapulmonary shunt and increasing the number of alveolar units participating in gas exchange 2
• PEEP decreases nonaerated lung volume, which is closely correlated with decreases in extravascular lung water (EVLW) 1

Effects on Extravascular Lung Water

• PEEP effectively decreases extravascular lung water (EVLW) in acute lung injury models 1
• The decrease in EVLW correlates with PEEP-induced decrease in nonaerated lung volume, making EVLW a valuable parameter indicating alveolar recruitment 1
• Early application of 10 cm H2O of PEEP reduces EVLW in permeability pulmonary edema models 3
• When combined with low tidal volume ventilation (6 ml/kg), PEEP further reduces EVLW compared to standard tidal volumes (12 ml/kg) 3

Hemodynamic Effects

• PEEP decreases cardiac preload by increasing pleural pressure, which decreases the gradient for venous return to the right ventricle 2
• PEEP reduces the pressure gradient from the upstream venous reservoir to the heart, further contributing to decreased preload 2
• PEEP may have beneficial effects on left ventricular function by decreasing LV afterload, provided there are no deleterious effects on the right ventricle 2

Mechanism of Action on Lung Water

• PEEP works through multiple mechanisms to improve pulmonary function: increasing functional residual capacity, promoting alveolar recruitment, redistributing extravascular lung water, and improving ventilation-perfusion matching 4
• PEEP can increase compliance by recruiting previously collapsed alveoli, shifting the pressure-volume curve upward and to the left 4
• The reduction in EVLW with PEEP appears to be related to decreased nonaerated lung volume and improved alveolar recruitment 1

Clinical Considerations and Caveats

• While PEEP generally decreases EVLW, some studies have shown that PEEP may actually increase EVLW in certain conditions due to decreased lung lymph flow 5
• Excessive PEEP levels can lead to hemodynamic compromise, barotrauma, and increased pulmonary vascular resistance 2
• The optimal method of applying PEEP remains controversial and should be titrated carefully to avoid overdistension of already open alveoli 2
• PEEP must be carefully applied to prevent ventilator-induced lung injury from overdistension 2

Practical Application

• Higher PEEP levels (15.1 ± 3.6 cm H₂O) are recommended for moderate to severe acute respiratory distress syndrome (ARDS) 2
• PEEP helps prevent atelectrauma by reducing cyclic opening and closing of alveoli during mechanical ventilation 2
• Zero end-expiratory pressure (ZEEP) should be avoided as it contributes to decreased compliance and increased risk of volutrauma 2
• In patients with intrinsic PEEP (auto-PEEP), application of external PEEP can improve patient-ventilator interaction and reduce the magnitude of inspiratory effort during assisted ventilation 6