A patient with stable chronic respiratory failure has his PaCO2 rise from 40 mmHg to 80 mmHg while his body's CO2 production remains unchanged. According to the alveolar ventilation equation, his alveolar ventilation has:
- A Doubled
- B Halved ✓
- C Remained unchanged
- D Increased fourfold
Correct answer: B. Halved
Explanation
The alveolar ventilation equation states that PaCO2 = k x VCO2 / VA, so PaCO2 is inversely proportional to alveolar ventilation when production is fixed. Doubling PaCO2 from 40 to 80 mmHg therefore means alveolar ventilation has fallen to half its previous value. This inverse relationship is the reason alveolar ventilation, not total minute ventilation, determines arterial CO2, since increases in dead space ventilation do not lower PaCO2.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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