Vagal inhibition of the heart in deep hypothermia

¹ Department of Physiology, The University of Rochester, Rochester, New York

Rats were cooled to 16°C esophageal temperature; the vagi were exposed. As the rats were further cooled one vagus was stimulated to produce bursts of impulses of varying frequencies. At low temperatures a lesser frequency of vagal stimulation and a greater intensity were required to produce inhibitory vagal impulses than in euthermia. Electrocardiographic tracings recorded the heart beats. Vagal impulses sharply decreased the heart frequency at all temperatures down to 12°C. Between 12° and 8°C the effect disappeared reversibly. Atropine blocked the vagal effect at the low body temperatures (16°C) as well as at high ones. Evidently vagal impulses may affect the heart rate at temperatures nearly as low as those at which the heart ceases to beat.