## **Core Concept**
Positive end-expiratory pressure (PEEP) is a technique used in mechanical ventilation that maintains a positive pressure in the lungs at the end of exhalation. This method is primarily used to improve oxygenation by increasing the functional residual capacity (FRC) of the lungs.

## **Why the Correct Answer is Right**
The application of PEEP increases the functional residual capacity (FRC) of the lungs. FRC is the volume of air present in the lungs at the end of passive expiration. By maintaining a positive pressure at the end of exhalation, PEEP prevents the alveoli from collapsing, thereby increasing the FRC. This increase in FRC can improve oxygenation by allowing more alveoli to participate in gas exchange.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While PEEP can indirectly affect tidal volume by changing lung compliance, it does not directly cause an increase in tidal volume. Tidal volume is the volume of air inhaled or exhaled during normal breathing.
- **Option B:** PEEP does not directly decrease residual volume. Residual volume is the volume of air remaining in the lungs after maximum exhalation and is not directly affected by PEEP.
- **Option C:** PEEP can actually decrease peak inspiratory pressure if lung compliance improves due to the recruitment of alveoli, but this is not the parameter that PEEP directly increases.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that PEEP is used not only to improve oxygenation but also to prevent atelectasis (alveolar collapse). The optimal level of PEEP is titrated to achieve the best oxygenation while minimizing the risk of barotrauma.

## **Correct Answer:** .