The present study demonstrated that the existence of radiation-induced impairment of cardiovascular function is disguised by a large hemodynamic reserve in nonhuman primates. These findings were revealed when the cardiovascular function of gamma-irradiated baboons was challenged with three transient bleeding episodes. Thirty baboons in five groups were treated as follows: one group was sham irradiated and the remaining four exposed to either 250, 500, 750, or 1000 R of single, whole-body gamma radiation (35 R/min). The cardiovascular system of each group was challenged at 2, 48, and 168 hr postirradiation with a bleeding episode which lasted approximately 10 min. Each challenge consisted of the rapid withdrawal and replacement of an estimated 43% (25 ml blood/kg body weight) of the baboon's blood volume. Hemodynamic function was assessed before and 30 min after each bleeding episode. The hemodynamic response to the first challenge appeared to be independent of radiation exposure. The greater variability in the response of gamma-irradiated groups to the second and third challenges, however, suggests evidence of a compensating but deteriorating cardiovascular system.

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