Constancy and Variability of Oxygen Costs of Mechanical Energy (PVA) and Contractility (Emax)

We have found in canine left ventricles that myocardial oxygen consumption (Vo2) is an increasing function of ventricular mechanical energy (PVA) and contractility (Emax). Sensitivities of changes in Vo2to changes in PVA and Emax are characterized by oxygen costs of PVA and Emax. We have found that these costs are usually constant under a variety of loading, heart rate and contractile conditions in normal canine hearts. However, oxygen cost of PVA increases in hyperthyroid rabbit hearts and decreases in acute failing hearts. Oxygen cost of Emax also increases in abnormal hearts such as postischemic stunned, hypercapnic acidotic, and post-acidotic stunned hearts. To obtain insights into the mechanisms of the constancy and variability of these costs, we carried out computer simulation studies. The results showed that changes in Ca transient, and Ca and crossbridge kinetic parameters affect oxygen cost of PVA in a complex manner and that increases in Ca sensitivity in the subnormal range decrease oxygen cost of Emax. These results help us speculate about the mechanisms underlying the experimentally observed constancy and variability of the oxygen costs of PVA and Emax in normal and diseased hearts.