What effect does applying PEEP have on functional residual capacity?

• A. Increases
• B. Decreases
• C. No Change
• D. Varies with Lung Compliance

Answer: (A)

PEEP raises the pressure at the end of expiration, keeping alveoli open and preventing collapse. With airways and alveoli held open, the end-expiratory lung volume increases, so the functional residual capacity goes up. The amount of increase depends on how recruitable the lung is and its compliance: more recruitment and higher compliance typically produce a larger rise, while very stiff lungs or excessive PEEP can blunt the added volume or risk overdistension. Overall, the direction of change is an increase in FRC.