When given high concentrations of O2, a patient with chronic hypercapnia may develop a more serious respiratory acidosis. Which of the following might be contributing to the patient's in-creased PCO2?

1. Worsening ventilation-perfusion (VV/QQ) balance
2. Desensitization of the carotid bodies
3. Removal of the hypoxic stimulus
a. 1 and 2 only
b. 1 only
c. 1 and 3 only
d. 1, 2, and 3

ANS: C
Nevertheless, the reduction in minute ventilation following O2 breathing in advanced COPD is not always severe enough to account for the increased PaCO2. Some investigators suggest that O2 breathing worsens the VV/QQ relationships in the lungs and is responsible for the increase in PaCO2. Other investigators have suggested that O2-induced hypercapnia is caused by the com-bined effects of hypoxic stimulus removal and redistribution of relationships in the lungs.