Evidence for hypoxic depression of CO2-ventilation response in carotid body-resected humans

Steady-state CO2-ventilation response curves with hyperoxia (end-tidal PO2 greater than 200 Torr) and mild hypoxia (end-tidal PO2 approximately equal to 60 Torr) were compared in five carotid body-resected (BR) patients and five control patients. The data were analyzed by fitting a linear equation, V = S(PETCO2-B), where V is minute ventilation S is the response curve slope. PETCO2 is end-tidal PCO2, and B is the response curve threshold. S slightly increased from hyperoxia to hypoxia in both BR and control groups. On the other hand, B moderately increased with hypoxia in BR patients, whereas it slightly decreased in controls. These changes were all not significant. However, in accordance with the change in B, the response curve to hypoxia at V of 10 1/min was significantly shifted in opposite directions in the two groups, i.e., rightward and leftward shift in BR and control groups, respectively. Thus the average magnitude of V calculated at PETCO2 of 40 Torr in hypoxia was significantly lower in BR patients than in controls (P less than 0.01). We conclude that this hypoxic depression of the CO2-ventilation response found in BR patients may have resulted, at least in part, from modulation of the brain stem neural mechanisms that were elicited by loss of afferent discharges from the carotid body.