Obstruction to pulmonary blood flow results in which of the following?

• A. Increased perfusion
• B. Normal V/Q ratio
• C. Decreased perfusion and a high V/Q ratio
• D. Increased alveolar ventilation

Answer: (C)

When blood flow to a portion of the lung is blocked, that region continues to be ventilated but receives less (or no) blood for exchange. This creates a mismatch where ventilation is high relative to perfusion, so the ventilation-to-perfusion ratio rises. In other words, there’s decreased perfusion and a high V/Q ratio, because the air reaching the alveoli isn’t matched by blood flow to pick up or drop off gases. The other options describe scenarios that don’t fit the effect of blocking blood flow: perfusion isn’t increased; the ratio isn’t kept normal; and alveolar ventilation need not be increased by the obstruction.