The relation between hypoxia and CO2-induced reflex alternation of breathing in man

Four healthy young volunteers, selected for the responsiveness and steadiness of their breathing, were studied in rest and mild exercise while receiving alternate inspirates of low and high PCO2 (0 and 8.6 kPa). PACO2, oscillated between ca. 6 and 7.5 kPa (45-55 torr). PAO2 was held steady at 4-7 levels between 6 and 28 kPa (45-210 torr). Thirteen separate inspiratory and expiratory variables (volumes, times, flows) were recorded and tested for reflex alternation. Matched controls were performed. Responses were generally small in relation to the scatter. Reflex alternation of any one variable was not always evident. The incidences of the responses were, in descending order, inspiratory flows and volumes, expiratory flows and volumes, expiratory duration; inspiratory duration alternated seldom, and then with only small amplitude. Reflex alternation was more likely to be observed in hypoxia than in euoxia or hyperoxia. A tendency for the incidences to be greater in exercise than at rest was not significant, but the amplitudes of alternation showed a significant difference in favour of exercise. In a substantial minority of experiments the amplitude of reflex alternation was significantly and positively correlated with hypoxia (1/(PAO2--C)). Alternation also occurred frequently in another substantial minority of experiments in which, however, there was no significant amplitude-hypoxia correlation. It was concluded that these two groups probably differed not so much in the form of the amplitude-hypoxia relation as in respect of the extent of the scatter in the observations. The results are consistent with interaction of non-steady-state with steady-state signals at the arterial chemoreceptors.