Tolerance to acute hypoxia maximally increases in case of joint effect of normobaric hypoxia and permissive hypercapnia in rats

IntroductionWe studied the comparative efficacy of independent and combined effects of normobaric hypoxia (90 mmHg) and permissive hypercapnia (50 mmHg) in increasing the tolerance of rats to acute hypobaric hypoxia.MethodsWe determined the time to loss of pose and life duration as a measure to assess the degree of tolerance of animals to hypobaric hypoxia by exposing them to an altitude of 11,500 m (barometric = 180 mmHg).ResultsExposure to hypercapnic hypoxia increased the tolerance to acute hypobaric hypoxia compared to exposure to normobaric hypoxia or permissive hypercapnia alone.DiscussionThe positive effects of hypercapnia and hypercapnic hypoxia occurred after one exposure, and increasing the number of exposures proportionally increased the tolerance to acute hypobaric hypoxia. The effect of permissive hypercapnia on increasing the tolerance to acute hypobaric hypoxia was found to be significantly greater than that of exposure to normobaric hypoxia. Therefore, we propose that hypercapnia is the dominant factor in increasing tolerance to acute hypobaric hypoxia.ConclusionTolerance to acute hypoxia maximally increases in case of joint effect of normobaric hypoxia and permissive hypercapnia.