1. 1.

    Myocardial oxygen consumption (VOO2) was measured using an in situ, perfused heart preparation at 10°C. VOO2 increased in a linear fashion with power output when cardiac output (Vb) was elevated (volume loading). The increased VOO2 was possible through improved O2 delivery (increased Vb), but Δ POO2 (input POO2 - output POO2) was reduced. The mechanical efficiency of the heart was improved.

  2. 2.

    VOO2 also increased in a linear fashion with power output when output pressure was increased with Vb constant (pressure loading). The increased VOO2 was supported by increased O2 removal from the perfusate since oxygen delivery (Vb and input POO2) was constant. Once more, improved mechanical efficiency was observed.

  3. 3.

    VOO2 decreased as O2 delivery was reduced with progressive hypoxia. Even so, power output was maintained at a perfusate input POO2 of 81 Torr. Five of 11 hearts survived a 30-Torr POO2 exposure, but with a 29% decrease in power output and a 5-fold reduction in VOO2. The increase in the apparent aerobic efficiency which enabled this is discussed.

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