What is the expected physiological effect of increasing PEEP?

Increasing Positive End-Expiratory Pressure (PEEP) primarily enhances oxygenation at lower tidal volumes. By raising PEEP, the pressure within the airways is maintained at the end of expiration, which helps to keep the alveoli open and increases the functional residual capacity. This recruitment of collapsed or under-inflated alveoli improves the overall surface area available for gas exchange, thus enhancing the ability of the lungs to pick up oxygen.

Moreover, the reduction of shunting (where blood passes through the lungs without participating in gas exchange) effectively improves the ventilation-perfusion ratio, leading to better oxygenation. Importantly, this can allow for the use of lower tidal volumes during mechanical ventilation, which is particularly beneficial in patients with conditions like acute respiratory distress syndrome (ARDS). Therefore, increasing PEEP can strategically improve oxygenation while minimizing potential lung injury associated with high tidal volumes.