Role of Bradycardia and Cold per se in Increasing Mechanical Efficiency of Hypothermic Heart

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Abstract
The relative influence of bradycardia and of cold per se on the oxygen consumption and mechanical efficiency of the dog heart was investigated in the modified heart-lung preparation (11 experiments). Myocardial oxygen uptake was determined under constant arterial pressure and cardiac output in a) normothermia, b) normothermia with bradycardia induced by a cold thermode on the pacemaker, and c) hypothermia producing the same bradycardia as in (b). At 36.8°C with a rate of 153 beats/min. the efficiency was 8.5% ± 0.3(S.E.), whereas with a rate of 110/min. efficiency was 9.1% ± 0.4(S.E.), a change that is statistically not significant. In hypothermia of 31.5°C with a rate of 110/min. the efficiency was 10.8% ± 0.3(S.E.), an increase that is statistically significant. Performing the same stroke work the hypothermic myocardium consumed less oxygen than the normothermic. It is concluded that the metabolic effect of cold per se is the chief factor responsible for increasing the mechanical efficiency of the hypothermic heart when pressure-volume work is kept constant.

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