Oxygen consumption for constant work is minimal at lowest working contractility in normal dog hearts

We tested whether minimal myocardial oxygen consumption (MVO2) for a given external work would exist in the middle of a normal contractility range as previously predicted theoretically. The left ventricle of the excised cross-circulated dog heart preparation was connected to a volume servo pump. Myocardial contractility in terms of ventricular end-systolic elastance (Emax) was gradually increased from control 8.9 +/- 3.4 (mean +/- SD) to 30.0 mmHg/(ml/100 g) by epinephrine and decreased to 1.8 mmHg/(ml/100 g) by propranolol while heart rate, end-systolic pressure and stroke work were kept constant. MVO2 was determined as the product of total coronary flow and coronary arteriovenous oxygen content difference in each contractile state. We plotted MVO2 values against E(max) values in each heart. The MVO2-E(max) relation for a constant cardiac work showed that MVO2 was minimal at the low end of the covered E(max) range. We conclude that minimal MVO2 for a given cardiac work is generally obtained at the lowest working contractility in normal dog hearts. This conclusion might pose some problems in the previous theoretical prediction as to the contractility that achieves the minimal MVO2 in a given external work.