In an in situ dog heart model, increasing contractility with dobutamine improves the efficiency of converting pressure-volume area to external work, but decreases overall oxygen consumption efficiencies.
Dobutamine's differential effects on cardiac efficiencies in dogs are hypothesis-generating; human studies needed to assess clinical relevance.
We studied the efficiencies from oxygen consumption (VO2) to external mechanical work (EW), from VO2 to the systolic pressure-volume area (PVA), and from PVA to EW, and the effects of cardiac output and contractility index (Emax) on these efficiencies in the left ventricle of open-chest, right-heart-bypassed dogs. PVA is an intermediate form of energy between VO2 and EW. PVA, EW, and Emax were determined by an abrupt occlusion of the ascending aorta. The right-heart bypass allowed us to collect all coronary venous return for VO2 measurement. The EW/VO2 efficiency ranged between 4 and 21%, the PVA/VO2 efficiency ranged between 5 and 27%, and the EW/PVA efficiency ranged between 60 and 95%. At a given Emax, EW/PVA efficiency was independent of cardiac output, but PVA/VO2 and EW/VO2 efficiencies increased with cardiac output. An increase in Emax by dobutamine increased EW/PVA efficiency, but decreased PVA/VO2 and EW/VO2 efficiencies. We could theoretically account for these changes in EW/PVA, PVA/VO2, and EW/VO2 efficiencies of the in situ heart by the VO2-PVA relation and its dependence on Emax that we had observed.
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Nozawa et al. (1988) studied this question.
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