On the static pressure-volume curve for ARDS, the pressure at which large numbers of alveoli become overinflated is located at which point?

• A. Lower inflection point
• B. Upper inflection point
• C. Midpoint
• D. Peak point

Answer: (B)

The key idea is that the upper inflection point on the static pressure-volume curve marks when pressure rises enough to cause many alveoli to overinflate. In ARDS, the curve is sigmoidal: at low pressures, collapsed units begin to open (lower inflection point) and the curve rises steeply as recruitment occurs. As pressure continues to increase, compliance eventually drops because more units become overdistended, causing the curve to flatten. The pressure at which this overdistension begins—many alveoli distending together—is the upper inflection point. Clinically, this point represents the risk of volutrauma if pressures rise beyond it, so ventilation is often set to stay below it to recruit without overdistending. The other points don’t specifically indicate the onset of widespread overinflation.