Optimal Afterload that Maximizes External Work and Optimal Heart that Minimizes O₂ Consumption

Summary

We examined the optimality of ventriculoarterial coupling from the point of view of energy efficiency. We defined the optimal afterload as that which maximizes external work. A theoretical analysis indicated, for a given end-diastolic volume, that external work becomes maximal when effective arterial elastance (E _a ) equals end-systolic elastance (E _es ). The ratio of external work to its maximum value was used to indicate the optimality of the afterload, Q _load . We defined the optimal heart as that which minimizes myocardial oxygen consumption for a given mean arterial pressure and cardiac output under a constant end-diastolic volume. This is accomplished when E _a /E _es is about 0.4. The ratio of minimum oxygen consumption, which is derived theoretically, to actual oxygen consumption is used to indicate the optimality of the heart, Q _heart . In chronically instrumented dogs, E _a /E _es ranged from 0.39 to 1.00 (0.69 ± 0.26 [SDI]. Q _load was 0.93 ± 0.08, suggesting that the external work of the left ventricle was nearly maximum at this condition. Similarly, Q _heart was 0.98 ± 0.01, indicating that myocardial oxygen consumption was almost minimum. When dogs ran on a treadmill, their heart rate and cardiac output doubled. E _es and E _a increased moderately; E _a / E _es , however, remained unchanged. Thus, despite dramatic changes in hemodynamic conditions, external work remained nearly maximum and oxygen consumption was minimum. In contrast, experimental left heart dysfunction increased E _a /E _PS , which, in turn, decreased Q _heart . We concluded that the ventriculoarterial coupling is nearly optimal under physiological stress. It is, however, no longer optimal for pathological conditions.