PHYSIOLOGY 970. Printed in U.S.A. Increased recovery of rats ’ respiration following profound hypothermia

1970.-Rats were anesthetized with pentobarbitone, cooled to 2 C, held in cardiac arrest for 30 min, and rewarmed with artificial respiration. On rewarming 86 % of the rats restarted breathing if they had been artificially respired during cooling, whereas only 9 % restarted if they had not been respired during cooling. Blood 02 saturation, CO2 content, pH, Na+, Kf, phosphate, and lactate measurements indicated that the latter rats were hypoxic relative to the former. The hearts of the rats artificially respired during cooling stopped before-or at the same time as-a sharp fall in their carotid arterial pressures; in those not respired during cool-ing, cardiac arrest occurred about 29 set after the fall in blood pressure. It was inferred that hypoxia in the presence of hypo-thermia led to a massive vasodilatation, related to the inability of the rats, which had not been respired during cooling, to recover on rewarming with artificial respiration. When rats which ha