Reversible Cessation of Blood Circulation in Deep Hypothermia

¹ From the Department of Physiology, the University of Rochester, Rochester, New York

In rats at body temperatures below 15°C, the arterial pressure reflected immediately the temperature of the heart. Pressure fell precipitously between 17°C and 8°C and depended on low pulse frequency. Pressure and pulse frequency were the same at each heart temperature during warming as during cooling. Below 8°C the heart could remain stopped for 1 hour, with subsequent recovery. Anoxia, produced by ventilating the lungs with purified nitrogen at a temperature of 16°–10°C, suddenly decreased the arterial pressure and pulse frequency, but did not stop the heart entirely. The vagus nerves were unnecessary for this effect, which could be reproduced also in isolated hearts. The oxygen content of blood in nonbreathing rats at 10°C gradually diminished, but an arteriovenous difference of oxygen concentration persisted as long as the heart was beating, which indicated that oxygen was being consumed. Neither circulation of blood nor delivery of oxygen to tissues during hypothermia directly limited survival of the rat after it was rewarmed.