What is the primary effect of Positive End-Expiratory Pressure (PEEP) therapy on the lungs?

Effects of Positive End-Expiratory Pressure (PEEP) Therapy

Positive end-expiratory pressure (PEEP) therapy results in an increase in functional residual capacity (FRC), not a decrease. PEEP therapy is a cornerstone of mechanical ventilation that maintains positive pressure in the airways during the expiratory phase, preventing alveolar collapse and improving oxygenation.

Primary Effects of PEEP on Pulmonary Physiology

• PEEP increases end-expiratory lung volume (EELV) and functional residual capacity (FRC) by preventing alveolar collapse at end-expiration, which reduces atelectasis 1
• PEEP improves oxygenation by reducing intrapulmonary shunt and increasing the number of alveolar units participating in gas exchange 1
• PEEP improves respiratory system compliance (CRS) in de-recruited areas and enhances ventilation to dependent lung regions 1

Effects of PEEP on Extravascular Lung Water

• Contrary to option (a), PEEP can actually increase extravascular lung water (EVLW) due to decreased pulmonary lymph flow 2
• The increased pleural pressure from PEEP can impede lymphatic drainage from the lungs, leading to accumulation of fluid in the extravascular space 2
• This effect is generally reversible when PEEP is discontinued 2

Hemodynamic Effects of PEEP

• PEEP decreases cardiac preload (not increases as suggested in option b) by:
  • Increasing pleural pressure which decreases the gradient for venous return to the right ventricle 1
  • Reducing the pressure gradient from the upstream venous reservoir to the heart 1
• PEEP can decrease cardiac output by reducing venous return, though this effect varies depending on volume status and cardiovascular function 3
• PEEP may have beneficial effects on left ventricular function by decreasing LV afterload, provided there are no deleterious effects on the right ventricle 1, 3

Effects on Neurohormonal Regulation

• PEEP therapy is associated with increased (not decreased) atrial natriuretic peptide (ANP) levels, contrary to option (c) 1
• The increase in ANP is related to increased right atrial pressure and atrial stretch caused by the altered hemodynamics from PEEP 1

Clinical Applications and Considerations

• PEEP is essential in treating acute respiratory distress syndrome (ARDS), with higher PEEP levels (15.1 ± 3.6 cm H₂O) recommended for moderate to severe ARDS 1
• PEEP helps prevent atelectrauma by reducing cyclic opening and closing of alveoli during mechanical ventilation 1
• Zero end-expiratory pressure (ZEEP) is not recommended as it contributes to decreased compliance and increased risk of volutrauma 1
• PEEP must be carefully titrated as excessive levels can lead to:
  • Hemodynamic compromise 1
  • Barotrauma 1
  • Increased pulmonary vascular resistance 1
  • Potential adverse effects on cerebral perfusion 4

In conclusion, among the given options, PEEP therapy primarily results in an increase in functional residual capacity, while it decreases cardiac preload, increases (not decreases) atrial natriuretic peptide, and increases (not decreases) extravascular lung water.