Interactions among metabolic rate, hypoxia, and control of breathing.

This review attempts to emphasize the fact that the interpretation of the ventilatory response to hypoxia may be complicated by the reduction in metabolic rate that is often associated with hypoxia. The hypoxic hypometabolism is more apparent when oxygen consumption is relatively high, either in small or young mammals at subnormal ambient temperatures or in larger mammals exposed to cold. This hypometabolism is not mediated by an activation of the arterial chemoreceptors and, furthermore, may result from a decrease in arterial oxygen content independent of the arterial PO2. Substantial experimental evidence supports the hypothesis of a lowering of the thermoregulatory set point during hypoxia through a direct action on central neural structures. The ventilatory response to hypoxia, which may appear blunted or depressed, especially in small animals exposed to cold, should in fact be reevaluated by taking into account the hypometabolism and hypothermia associated with hypoxia. Finally, it is emphasized that the mechanisms involved in control of body temperature and those that account for the interactions with hypoxia are located in the hypothalamus. This suggests that common integrative structures are probably involved in the metabolic and ventilatory responses to hypoxia.