According to the alveolar ventilation equation, if metabolic CO2 production is constant and alveolar ventilation doubles from 5 L/min to 10 L/min, arterial PCO2 will:

• A Double, to approximately 80 mmHg
• B Fall slightly, to approximately 35 mmHg
• C Remain unchanged because metabolism sets PaCO2
• D Fall by half, to approximately 20 mmHg ✓

Explanation

The alveolar ventilation equation states PaCO2 = k x VCO2 / VA, where k is 0.863. With VCO2 constant, PaCO2 is inversely proportional to alveolar ventilation, so doubling VA halves PaCO2 from 40 to about 20 mmHg. This inverse hyperbolic relationship is the basis for interpreting hyperventilation and hypoventilation on arterial blood gases.

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

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