Myocardial anaerobic metabolism occurs at a critical coronary venous PO₂ in pigs

We tested the hypothesis that the onset of myocardial anaerobic metabolism is fundamentally different from the whole body and other organs, where the onset of anaerobic metabolism occurs at a critical oxygen extraction ratio-not at a critical venous PO₂. We measured oxygen saturation and PO₂ of arterial and coronary venous blood at the onset of global myocardial anaerobic metabolism during progressive hypoxic hypoxia (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 lactate consumption and left ventricular contractility. Coronary venous PO₂ was no different during hypoxic hypoxia and carbon monoxide hypoxia at equivalent arterial oxygen saturations, particularly at the onset of myocardial anaerobic metabolism (PO₂ 17.0 ± 1.7 tort versus 15.9 ± 2.2 torr, p = NS). However, the myocardial oxygen extraction ratio was significantly greater during hypoxic hypoxia than during carbon 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 critical oxygen extraction ratio, the onset of myocardial anaerobic metabolism occurs at a critical coronary venous PO₂.