What is the main physiological purpose of PEEP in mechanical ventilation?

• A. To prevent alveolar collapse at end expiration, improve oxygenation, and recruit collapsed alveoli
• B. To increase respiratory rate
• C. To decrease tidal volume
• D. To lower PaCO2

Answer: (A)

PEEP's main purpose is to keep alveoli open at the end of expiration, preventing collapse of dependent airways and alveolar units. By maintaining a positive pressure at end‑expiration, more alveoli stay recruited, which raises functional residual capacity and reduces atelectasis. This expanded open lung tissue improves the surface area for gas exchange and lowers intrapulmonary shunt, leading to better arterial oxygenation. PEEP can also help reopen collapsed alveoli over time, further enhancing oxygen delivery. While changes in respiratory rate, tidal volume, or aiming to lower PaCO2 involve other aspects of ventilation, the primary physiologic effect of PEEP is preventing end-expiratory collapse and promoting recruitment to improve oxygenation.