Which statement best explains why prone positioning can improve oxygenation in ARDS?

• A. It promotes dorsal lung recruitment and better V/Q matching
• B. It increases cardiac preload
• C. It reduces systemic blood pressure
• D. It substitutes for mechanical ventilation

Answer: (A)

Prone positioning improves oxygenation in ARDS mainly by improving ventilation-perfusion matching through better recruitment of the dorsal lung. When the patient is supine, the posterior (dorsal) lung tends to collapse under the weight of the heart and abdominal contents, which makes that well-perfused tissue poorly ventilated (a shunt). Turning prone relieves this posterior compression, allowing those dorsal regions to re-expand and participate in gas exchange. Because these posterior areas are still the ones receiving most of the blood flow, their renewed aeration leads to much better V/Q matching and higher arterial oxygen levels. It also promotes a more uniform distribution of ventilation, which can reduce ventilator-induced lung injury. This effect is about improving oxygenation alongside ventilation, not about increasing preload, lowering systemic blood pressure, or replacing mechanical ventilation.