Which positioning strategy improves oxygenation in ARDS?

• A. Prone positioning
• B. Lateral decubitus with unaffected lung down
• C. Supine with head-down tilt
• D. Upright only

Answer: (A)

Prone positioning improves oxygenation in ARDS by enhancing ventilation-perfusion matching and recruiting the dorsal lung regions that tend to collapse when a patient is on their back. In ARDS, edema and atelectasis preferentially affect the dependent (posterior) areas in the supine position, while perfusion remains relatively constant there, creating a large shunt and poor oxygenation. When the patient is turned onto the abdomen, the dependent and nondependent regions swap, allowing previously collapsed dorsal units to re-expand and participate in gas exchange. This change leads to a more even distribution of transpulmonary pressures, less compression of the posterior lung by the heart and mediastinal structures, and better alveolar recruitment overall. The net effect is improved oxygenation, often with a reduced need for high FiO2. In practice, prone positioning is used for extended periods (often many hours per day) in moderate to severe ARDS because it has been shown to improve outcomes. The other positions don't address the dorsal collapse as effectively; lateral decubitus may help in unilateral disease but is not as globally beneficial in diffuse ARDS, while head-down tilt or upright-only positioning do not optimize dorsal lung recruitment and can worsen ventilation–perfusion mismatch.