In a patient with pulmonary embolism causing dead space ventilation, which parameter would most directly reflect the increased dead space fraction (Vd/Vt)?

• A Increased alveolar-arterial O₂ gradient (A-a gradient) alone
• B Decreased FEV₁/FVC ratio below 0.70
• C Increased DLCO (diffusing capacity) beyond normal
• D Widened arterial-end-tidal CO₂ gradient (PaCO₂ – PetCO₂ > 5 mmHg) ✓

Explanation

In normal individuals, PetCO₂ approximates PaCO₂ (gradient < 2–5 mmHg) because alveolar CO₂ equals arterial CO₂ in perfused alveoli. Increased dead space (as in pulmonary embolism) means some exhaled gas comes from unperfused alveoli with near-zero CO₂, diluting the expired CO₂ and lowering PetCO₂ relative to PaCO₂. A widened PaCO₂ – PetCO₂ gradient >5 mmHg specifically indicates dead space physiology. The A-a gradient increases but reflects V/Q mismatch or shunt more broadly. FEV₁/FVC measures airflow obstruction.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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