THE CARDIOVASCULAR EFFECTS OF ACUTELY INDUCED HYPOTHERMIA 12

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Abstract
Circulatory changes and cardiac output were studied in 12 dogs during cooling to 27[degree] C by ice packs and during rewarming, and in 3 control dogs. During the early cooling period, thermogenic reflexes cause shivering, and produce increases in O2 consumption, respiratory and heart rates, blood pressure and cardiac output. These reflexes may be inhibited to some extent by deep anesthesia. As cooling continues, and the combined depressing effects of hypothermia and anesthesia act, hemoconcentration and progressive falls in O2 consumption, in respiratory and heart rates, in blood pressure and in cardiac output occur. Marked prolongation of electrical systole, with the occurrence of intraventricular and A-V blocks, and changes in the S-T-T contour, are seen. The changes in these values are related to the degree of hypothermia obtained. During rewarming these effects are reversed with return of the blood pressures, respiratory rates, and pulse rates to normal, and with return toward normal in the hemoconcentration and cardiac output. Electrocardiographc changes were also reversed. When it occurred, death in hypothermia was attributable to failure of the vasomotor or respiratory centers, rather than to failure of the heart. No consistent correlated changes in right auricular pressure, on the one hand, and pulmonary arterial pressures, cardiac output, or the work of the right heart, on the other, were observed during either the cooling or rewarming periods.