Oxygen Consumption and Mechanical Efficiency of the Hypothermic Heart

Cardiac oxygen consumption in canine heart-lung preparations was compared at 37°C and 27°C in relation to equal cardiac work per unit of time and per beat. Per unit of time the hypothermic heart used less oxygen than the normothermic heart at equal work performance, and the absolute difference in oxygen consumption remained nearly constant over a wide range of work loads. A pronounced positive correlation between mechanical efficiency and increasing output loads of the hypothermic heart was found and provides an explanation of the observed relative high cardiac oxygen demands in the intact hypothermic animal. At equal work per beat the hypothermic heart used slightly less oxygen per beat than the normothermic, in spite of its greater diastolic volume. The lower oxygen consumption of the hypothermic heart is attributed to slower rate and depressed resting cardiac metabolism. Submitted on May 17, 1956