Prone positioning is not just a way to improve the oxygen saturation. In severe ARDS, proning changes the distribution of ventilation, perfusion, lung stress, and regional overdistension.
ARDS produces a heterogeneous lung. Some regions are collapsed or fluid-filled. Others remain aerated and may receive a disproportionate share of tidal volume. When a patient lies supine, dependent dorsal lung regions are compressed by edema, heart weight, abdominal pressure, and gravity. Ventilation shifts toward more ventral regions, while perfusion often remains greater dorsally.
This worsens V/Q mismatch. Proning changes that relationship. It can recruit dorsal lung units, improve ventilation-perfusion matching, reduce regional overdistension, and make transpulmonary pressure more evenly distributed across the lung.