A resting subject has a steady metabolic CO2 production. If her alveolar ventilation suddenly doubles while VCO2 remains constant, her arterial PCO2 will become approximately:

• A Halved, to about 20 mmHg ✓
• B Unchanged, because PaCO2 is set by metabolism alone
• C Doubled, to about 80 mmHg
• D Reduced to one quarter, to about 10 mmHg

Explanation

The alveolar ventilation equation states that PaCO2 equals K times VCO2 divided by VA, where K is 863 divided by the respiratory exchange quotient. With VCO2 fixed, PaCO2 varies inversely with alveolar ventilation, so doubling VA halves PaCO2 from 40 to about 20 mmHg. This inverse proportionality, not a linear or exponential relation, is the tested concept and underlies the interpretation of every arterial blood gas.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.