INCREASED OXYGEN COST OF CONTRACTILITY IN STUNNED MYOCARDIUM OF DOG

Recent studies have shown that myocardial oxygen consumption does not proportionally decrease with the deterioration of contractile function in stunned myocardium. To investigate this disproportion, we studied the end-systolic pressure-volume relation and the relation between oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA, a measure of total mechanical energy) in stunned hearts. In the VO2-PVA relation, VO2 can be divided into PVA-dependent and PVA-independent fractions. In excised cross-circulated dog left ventricles, a 15-minute normothermic global ischemia followed by 60-120 minutes of reperfusion significantly decreased the ventricular contractility index (E(max)) by approximately 40%, but the PVA-independent VO2 did not significantly decrease. Oxygen cost of PVA, defined as the slope of the VO2-PVA relation, was slightly decreased in stunned hearts. Restoration of the depressed E(max) to the preischemic control level by calcium infusion increased the PVA-independent VO2 to 137 +/- 27% of control level (p < 0.01). Oxygen cost of contractility, defined as the slope of the relation between PVA-independent VO2 and E(max), increased from 0.0011 +/- 0.0003 to 0.0023 +/- 0.0005 ml O2.ml.mm Hg-1.beat-1 per 100 g myocardium in control and stunned hearts, respectively (p < 0.01). From these new findings, we conclude that the unchanged VO2, despite the depressed contractility in stunned myocardium, is mainly due to the increased oxygen cost of contractility.