A patient has an end-tidal CO2 of 35 mm Hg while an arterial blood gas drawn at the same moment shows a PaCO2 of 45 mm Hg. Which of the following is the least plausible explanation for this discrepancy?

A patient has an end-tidal CO2 of 35 mm Hg while an arterial blood gas drawn at the same moment shows a PaCO2 of 45 mm Hg. Which of the following is the least plausible explanation for this discrepancy?

• A. Cystic fibrosis
• B. Pulmonary embolism
• C. Intrapulmonary shunt
• D. Morbid obesity

Answer: (C)

End-tidal CO2 reflects the CO2 content of the alveolar gas that is being exhaled from ventilated regions of the lung. When PaCO2 is higher than the end-tidal value, it points to ventilation–perfusion mismatch with increased dead-space ventilation, where portions of the lung are ventilated but not adequately perfused, so CO2 from those regions doesn’t fully contribute to the exhaled gas. Cystic fibrosis can create mucus plugging and regional V/Q mismatch, increasing physiologic dead space, which lowers the end-tidal CO2 relative to the arterial CO2. Pulmonary embolism directly increases dead space by occluding perfused alveoli, producing the same pattern. Morbid obesity causes hypoventilation and CO2 retention, so both arterial and end-tidal CO2 rise, and the difference between them is not as pronounced, but this still can occur with overall elevated CO2. Intrapulmonary shunt, where blood bypasses ventilated alveoli, tends to spare CO2 exchange in the ventilated regions and does not usually produce a large gradient between PaCO2 and end-tidal CO2; the CO2 in exhaled air remains more closely tied to the CO2 in the ventilated units, so a marked discrepancy of this kind is less expected. Therefore, intrapulmonary shunt is the least plausible explanation for the observed gap.

End-tidal CO2 reflects the CO2 content of the alveolar gas that is being exhaled from ventilated regions of the lung. When PaCO2 is higher than the end-tidal value, it points to ventilation–perfusion mismatch with increased dead-space ventilation, where portions of the lung are ventilated but not adequately perfused, so CO2 from those regions doesn’t fully contribute to the exhaled gas.

Cystic fibrosis can create mucus plugging and regional V/Q mismatch, increasing physiologic dead space, which lowers the end-tidal CO2 relative to the arterial CO2. Pulmonary embolism directly increases dead space by occluding perfused alveoli, producing the same pattern. Morbid obesity causes hypoventilation and CO2 retention, so both arterial and end-tidal CO2 rise, and the difference between them is not as pronounced, but this still can occur with overall elevated CO2.

Intrapulmonary shunt, where blood bypasses ventilated alveoli, tends to spare CO2 exchange in the ventilated regions and does not usually produce a large gradient between PaCO2 and end-tidal CO2; the CO2 in exhaled air remains more closely tied to the CO2 in the ventilated units, so a marked discrepancy of this kind is less expected. Therefore, intrapulmonary shunt is the least plausible explanation for the observed gap.