Which condition can cause the PV loop to extend farther to the right and flatten out?

• A. Pneumonia
• B. Asthma
• C. ARDS
• D. Fibrosis

Answer: (C)

Lung pressure-volume behavior shows how much air the lungs can hold at a given pressure and how easily they recruit unrecruited units. In acute respiratory distress syndrome, the lung is highly heterogeneous: many units are collapsed at baseline but can be opened (recruited) with pressure. This recruitment lets you achieve a larger volume for the same pressure, so the PV loop shifts farther to the right. However, as volume increases further, opened and overdistended regions begin to stiffen, so additional volume requires disproportionately more pressure and the loop’s slope flattens. This combination—a rightward extension with a plateau at higher volumes—is characteristic of ARDS. Pneumonia, fibrosis, or asthma tend to produce different patterns: overall reduced compliance tends to pull the loop leftward or alter its shape in other ways, and dynamic hyperinflation in asthma changes but doesn’t produce the same rightward-then-flattening pattern seen with ARDS.