In ARDS, prone positioning improves oxygenation by what mechanism?

• A. Has no effect on gas exchange
• B. Improves oxygenation by increasing dorsal recruitment and ventilation-perfusion matching
• C. Reduces cardiac output
• D. Causes hypercapnia

Answer: (B)

The key idea is that turning a patient with ARDS onto their stomach improves oxygenation by enhancing how air and blood are matched in the lungs. In ARDS, the posterior (dorsal) parts of the lung tend to be atelectatic and compressed when lying on the back, while the front can be overdistended. When you are prone, those dorsal regions are less compressed and recruit better, so they receive more airflow. Meanwhile, perfusion tends to stay relatively blood-flow–driven to the dependent areas, so more of the lung’s blood flow is now matched to well-ventilated tissue. This improved ventilation-perfusion matching reduces shunt and increases oxygenation.

The other options don’t fit because prone positioning doesn’t primarily work by changing cardiac output, nor does it drive hypercapnia; it actually tends to improve CO2 clearance as ventilation becomes more uniform. And it certainly has a meaningful effect on gas exchange, not “no effect.”