In severe ARDS, what is the intended effect of prone positioning?

• A. Decrease oxygenation.
• B. Improve oxygenation by enhancing ventilation-perfusion matching and distributing ventilation more evenly.
• C. Increase plateau pressures.
• D. Eliminate the need for PEEP.

Answer: (B)

Prone positioning in severe ARDS aims to improve oxygenation by improving ventilation-perfusion matching and distributing ventilation more evenly. When a patient lies on the stomach, the dependent posterior lung regions—often better perfused but prone to collapse in the supine position—become better ventilated, while the ventral regions are less overdistended. This rebalancing reduces shunt and spreads tidal volume more uniformly across the lung, enhancing gas exchange. It’s about making the lung’s ventilation follow its perfusion more closely, rather than trying to decrease oxygenation or mechanically raise pressures. PEEP remains important to keep alveoli open, and prone positioning is used specifically to improve oxygenation in the context of severe ARDS.