Effect of hypoxia and hyperoxia on cardiorespiratory responses during exercise in man.

To clarify the role of the carotid body in the mechanism governing exercise hyperpnea, the effect of hypoxia and hyperoxia on ventilation and cardiac output was studied in four healthy men. The VE increased 10.7% in hypoxia and decreased 10.1% in hyperoxia from normoxia as judged from the steady-state values during exercise. On the contrary, Q showed only a slight reduction of -3.2% in hyperoxia. The hypoxic hyperpnea and hyperoxic hypopnea led to a concomitant alteration in PETCO2. An overshoot following the onset of exercise was observed during the first 30s of VE response in hypoxia, which damped progressively in normoxia and hyperoxia. No remarkable difference was observed in the early transient responses of Q between hypoxia and hyperoxia. The discrepancy in the dynamics between VE and Q led to a phasic deviation in PETCO2; an isocapnic transition from the control to stimulus period in normoxia, hypocapnic in hypoxia and hypercapnic in hyperoxia. The time constant representing the kinetics of VE and that for VCO2 prolonged significantly in hyperoxia. These results support the cardiodynamic consequence of exercise hyperpnea, i.e., the carotid body is the first to respond to the increase in CO2 flow into the lungs.