MYOCARDIAL ANAEROBIC METABOLISM OCCURS AT A CRITICAL CORONARY VENOUS PO-2 IN PIGS

We tested the hypothesis that the onset of myocardial anaerobic metabo lism is fundamentally different from the whole body and other organs, where the onset of anaerobic metabolism occurs at a critical oxygen ex traction ratio-not at a critical venous Po-2. We measured oxygen satur ation and Po-2 of arterial and coronary venous blood at the onset of g lobal myocardial anaerobic metabolism during progressive hypoxic: hypo xia (n = 7) compared with carbon monoxide hypoxia (n = 7), which left- shifted the oxygen-hemoglobin dissociation curve. The onset of global myocardial anaerobic metabolism was defined by decreased myocardial la ctate consumption and left ventricular contractility. Coronary venous Po-2 was no different during hypoxic hypoxia and carbon monoxide hypox ia at equivalent arterial oxygen saturations, particularly at the onse t of myocardial anaerobic metabolism (Po-2 77.0 +/- 1.7 torr versus 15 .9 +/- 2.2 torr, p = NS). However, the myocardia( oxygen extraction ra tio was significantly greater during hypoxic hypoxia than during carbo n monoxide hypoxia at the onset of myocardial anaerobic metabolism (0. 88 +/- 0.02 versus 0.65 +/- 0.04, p < 0.01). Thus, in contrast to the whole body where the onset of anaerobic metabolism occurs at a critica l oxygen extraction ratio, the onset of myocardial anaerobic metabolis m occurs at a critical coronary venous Po-2.