A 55-year-old man with severe emphysema is mechanically ventilated for a laparotomy. Arterial blood gas shows PaCO2 of 56 mmHg while the capnograph displays ETCO2 of 33 mmHg. Assuming correct calibration and sampling, what best explains the widened PaCO2 minus ETCO2 gradient of 23 mmHg (normal 2 to 5 mmHg)?
- A Increased alveolar dead space and ventilation-perfusion mismatch preventing complete equilibration of end-tidal gas with arterial blood ✓
- B Excessive fresh gas flow diluting the sampled carbon dioxide concentration
- C Increased cardiac output improving carbon dioxide delivery to the lungs
- D Hyperventilation lowering alveolar carbon dioxide below arterial levels
Correct answer: A. Increased alveolar dead space and ventilation-perfusion mismatch preventing complete equilibration of end-tidal gas with arterial blood
Explanation
End-tidal gas reflects alveoli that are both perfused and ventilated. In emphysema, enlarged anatomical and alveolar dead space plus regional V/Q mismatch add carbon dioxide-poor gas to the expired breath, diluting end-tidal CO2 well below the mixed arterial value. The gradient widens similarly in pulmonary embolism and low cardiac output states. Hyperventilation lowers both PaCO2 and ETCO2 together and does not widen the gradient, and raised cardiac output narrows it, excluding C and D.
Reference: Miller's Anesthesia, 9th ed.
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