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152 CARDIOVASCULAR EFFECTS OF HYPOTHERMIA AND REWARMING IN NEWBORN DOGS

The cardiovascular effects of acute hypothermia and rapid rewarming were studied in 11 newborn dogs, 2-5 days of age, anesthetized lightly with penthothal, 20 mg/kg. Cardiac output and organ blood flows were measured by the radioactive microsphere reference organ technique. Animals were made hypothermic by placing them in an incubator without heat, resulting in rectal and skin temperatures of 33.0±.25° C and 33.7±.28° C, respectively (mean±SE). Animals were rapidly rewarmed by setting the incubator heater to maximum, resulting in a rise of rectal and skin temperatures to 35.1±.49° C and 35.5±.40° C, respectively, within 60 minutes. Rapid rewarming resulted in a slight rise in BP from 45.0 to 49.4 mmHg (p<.025), a slight rise in renal blood flow from 2.17 to 2.81 ml/min/g (p<.05) and a large (65%) increase in cerebral blood flow (CBF) from .31 to .51 ml/min/g (p<.05). There were no significant changes in cardiac output or blood flow to the GI tract. Measurements of the effects of cooling were made in 4 animals and revealed a marked drop in CBF from .51 to .20 (p<.025). During the rewarming procedure, 4 animals became apneic and bradycardic, responding to tactile stimulation. These studies demonstrate that acute changes in body temperature result in altered function of the cardiovascular system, particularly flow to the brain, and such changes may contribute to the clinical disorders found in infants under similar thermal conditions.