VENTILATORY RESPONSE TO STEADY-STATE EXERCISE IN HYPOXIA IN HUMANS

The linear relationship between minute ventilation (VE) and CO2 output (V(CO2)) was steeper in 9 of 10 healthy subjects, when treadmill walking was carried out breathing 14% oxygen than when breathing air. This confirmed that the ventilatory response to modest exercise was usually potentiated by mild hypoxia. Arterial oxygen saturation did not significantly correlate with VE in 7 healthy subjects, walking, breathing air or 14% oxygen; whereas there was a significant correlation between VE and calculated mixed venous saturation in 5 of these subjects. Transient relief of hypoxia, when breathing 14% oxygen, by 5 breaths of 30% oxygen both at rest and during walking, reduced ventilation more at higher levels of exercise. This indicated that the peripheral chemoreceptor response to a given level of arterial desaturation was enhanced by exercise. Directly measured femoral venous saturation was correlated with VE in another 3 subjects, and there was also a close, but curvilinear, relationship between VE and femoral venous lactate concentrations during similar exercise, when breathing 21 or 14% oxygen. The authors suggest that receptors in working muscle could be sensitized by muscular hypoxia during exercise when breathing 14% oxygen, and thus contribute to the potentiation of the peripheral chemoreceptor stimulation of exercise ventilation by hypoxia.