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Alveolar ventilation and hypercapnia — FRCA Primary MCQ

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HardPhysiologyAlveolar ventilation and hypercapniaFRCA Primary

A ventilated patient has PaCO2 8.0 kPa, pH 7.21 and minute ventilation 6 L min-1. The ventilator tubing shows a large compressible volume and measured expired tidal volume at the airway is lower than the set tidal volume. What is the most likely explanation for the hypercapnia?

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Correct answer: B — Increased oxygen consumption from fever

Alveolar ventilation determines PaCO2; a low delivered tidal volume reduces effective alveolar ventilation and causes hypercapnia. Fever can increase CO2 production, but the direct data point is reduced expired tidal volume at the airway. Intubation reduces upper airway dead space and would tend to improve alveolar ventilation for a given minute volume. The pearl is that the ventilator set tidal volume may not equal the patient-delivered tidal volume.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology