Ventilatory and metabolic responses to hypoxia during moderate hypothermia in anesthetized rats

In resting euthermic mammals, hypoxia elicits a hyperventilation that results from a combination of hyperpnea and hypometabolism. Often accompanying the hypoxia-induced hypometabolism is a drop in body temperature. To separate the synergic effects of hypothermia per se from the direct effects of hypoxia on metabolic rate, ventilation (V̇E), and O 2 consumption (V̇O 2 ) were measured in anesthetized rats fitted with abdominal heat exchangers and maintained at either normothermic (37.5°C) or hypothermic (35°C) body temperatures while exposed to either normoxia or hypoxia (7% O 2 ). Hypothermia induced parallel decreases in V̇E and V̇O 2 , thereby maintaining V̇E/V̇O 2 . Hypoxia resulted in a hyperventilation achieved with the same relative decrease in V̇O 2 and increase in V̇E in both normothermic and hypothermic rats. The results suggest that 1) the changes in metabolic rate and V̇E during hypothermia reflect a direct effect of cold and, 2) because of similar levels of hypoxic hyperventilation in the hypothermic and normothermic rats, relative to metabolic rate, respiratory gain has not been depressed in hypothermic rats.