LEFT VENTRICULAR-ARTERIAL COUPLING RELATIONS IN THE NORMAL HUMAN HEART

This investigation was undertaken to assess left ventricular-arterial coupling relations in the normal human heart under varying loading conditions and inotropic states and thereby to establish whether the working point of the normal human heart is at optimal output or mechanical efficiency under basal hemodynamic conditions. In 22 patients with an atypical chest pain syndrome who had normal coronary arteriograms, left ventricular (LV) pressures, volumes, ejection fractions, and masses at cardiac catheterization, we acquired radionuclide angiograms in duplicate simultaneously with micromanometer LV pressures. These values were derived under control conditions and during methoxamine and nitroprusside infusions with heart rate held constant by right atrial pacing. Seven other patients underwent the same protocol but, in addition, we acquired these parameters during a steady-state, intravenous infusion of dobutamine (5 mug/kg/min). The interaction of LV chamber elastance (E(es)) and effective arterial elastance (E(a)) revealed that the normal human heart was operating at an E(es)/E(a) ratio of 1.62, a stroke work of 76 +/- 31 gm-m, and a mechanical efficiency (stroke work to pressure-volume area ratio [SW/PVA]) of 0.65 +/- 0.10. With an increase in LV load, the E(es)/E(a) ratio approached 1 (p < 0.01), LV stroke work increased (p < 0.01), and mechanical efficiency declined (p < 0.01). In contrast, during vasodilation, the E(es)/E(a), ratio increased to slightly above 2.0 (p < 0.01), LV stroke work decreased (p < 0.001), and mechanical efficiency improved (p < 0.01). During the dobutamine infusion, similar observations were made for E(es)/E(a), LV stroke work, and SW/PVA over a similar range of LV loading conditions, but enhanced inotropy improved the energy transfer from the left ventricle to the arterial system at comparable E(es)/E(a) ratios without affecting mechanical efficiency. In conclusion, these data indicate that the normal human heart operates at neither optimal output nor efficiency. The working point, however, more closely approximates maximal mechanical efficiency than maximal LV output, but the normal human heart operates over a narrow range of LV SW values.