KR Walley, RM Collins, DJ Cooper and CB Warriner
Pulmonary Research Laboratory, St. Paul's Hospital, University of British Columbia, Vancouver, Canada.

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 torr 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.