In acute pulmonary embolism, which ventilation-perfusion change is most likely?

In acute pulmonary embolism, which ventilation-perfusion change is most likely?

• A. Severe dead-space ventilation with high V/Q and a drop in ETCO2
• B. Low V/Q mismatch
• C. Increased ETCO2
• D. Decreased dead space

Answer: (A)

When a pulmonary embolus blocks blood flow to parts of the lung, ventilation to those regions continues while perfusion drops. This creates ventilated-but-not-perfused areas, meaning very high V/Q units and an overall increase in physiologic dead space. Because these dead-space regions don’t participate in gas exchange, the amount of CO2 that reaches the alveoli to be exhaled drops, so end-tidal CO2 falls. The described option—severe dead-space ventilation with high V/Q and a drop in ETCO2—captures this pattern best. The other ideas don’t fit as well: a low V/Q mismatch implies more perfusion relative to ventilation in some regions, which isn’t the main issue in PE; increased ETCO2 would suggest more CO2 in exhaled gas, which isn’t typical with the rise in dead space; and decreased dead space is the opposite of what PE does, since dead space actually increases when perfusion is blocked.

When a pulmonary embolus blocks blood flow to parts of the lung, ventilation to those regions continues while perfusion drops. This creates ventilated-but-not-perfused areas, meaning very high V/Q units and an overall increase in physiologic dead space. Because these dead-space regions don’t participate in gas exchange, the amount of CO2 that reaches the alveoli to be exhaled drops, so end-tidal CO2 falls. The described option—severe dead-space ventilation with high V/Q and a drop in ETCO2—captures this pattern best.

The other ideas don’t fit as well: a low V/Q mismatch implies more perfusion relative to ventilation in some regions, which isn’t the main issue in PE; increased ETCO2 would suggest more CO2 in exhaled gas, which isn’t typical with the rise in dead space; and decreased dead space is the opposite of what PE does, since dead space actually increases when perfusion is blocked.