Myocardial energetics after thermally graded hyperkalemic crystalloid cardioplegic arrest

Previous studies assessing the efficacy of myoprotective regimens have compared preischemic and postischemic myocardial oxygen consumption within a limited range of cardiac performance. However, recent data suggest that ischemia-induced perturbations in myocardial energetics may occur only when the left ventricle develops physiologic pressures. Therefore, in canine hearts supported by cardiopulmonary bypass, myocardial oxygen consumption (ml oxygen × 10 2/beat/100 gm left ventricular weight) was determined during incremental isovolumic pressure-volume loading before and 30 minutes after 2 hours of cardioplegic arrest The ischemic insult was graded by maintaining myocardial temperature at 12° C (Group I, n = 6), 20° C (Group II, n = 7), or 28° C (Group III, n = 6). Postischemic Starling curves were unchanged in Groups I and II but depressed 53% in Group III hearts (p < 0.005). In Group I, postischemic myocardial oxygen consumption at specific peak developed pressures was similar to preischemic oxygen consumption. In contrast, postischemic Group II and III hearts consumed 39% more oxygen than preischemically when peak developed pressure exceeded 75 mm Hg (p < 0.01). Postischemic hearts demonstrated reciprocal changes in arteriovenous oxygen content difference (24%, 30%, and 34% lower than preischemic values for Groups I, II, and III, respectively) and coronary blood flow (156%, 195%, and 192% higher than preischemic values for Groups I, II, and III, respectively). Only in Group II and III hearts did the increased coronary blood flow offset the defect in oxygen extraction such that myocardial oxygen consumption was increased. These data suggest that inefficient utilization of oxygen when the heart is developing physiologic pressures is a sensitive marker for myocardial injury after crystalloid cardioplegic arrest.