What effect does positive end-expiratory pressure (PEEP) have on functional residual capacity (FRC)?

Positive end-expiratory pressure (PEEP) increases functional residual capacity (FRC) primarily by preventing the complete collapse of the alveoli at the end of expiration. This means that with the application of PEEP, the pressure at the end of expiration is elevated, which keeps more air in the lungs.

By maintaining a certain level of pressure in the airways, PEEP helps to reduce or eliminate the closing of small airways, thereby allowing more air to remain in the lungs after expiration. As a result, the volume of air that remains in the lungs (FRC) is increased. This is particularly beneficial for patients with conditions such as acute respiratory distress syndrome (ARDS) or other forms of pulmonary compromise, where keeping alveoli open can improve oxygenation and reduce the work of breathing.

In contrast, without PEEP, the functional residual capacity can decrease due to the collapse of alveoli, which diminishes the lung's ability to maintain adequate ventilation and gas exchange.