Patterns of myocardial oxygen and lactate extraction in patients undergoing cardiopulmonary bypass

Abstract

Forty-nine patients with acquired mitral or aortic valve disease had myocardial oxygen metabolic studies performed during cardiopulmonary bypass and moderate hypothermia. Twenty-two of these also had lactate determinations performed; lactate production was considered indicative of myocardial anaerobiasis. Metabolic data were obtained during specific intraoperative events such as crossclamping of the aorta, unclamping of the aorta with restoration of coronary autoperfusion, initiation of continuous direct coronary perfusion, arrhythmias, electrocardiographically demonstrable ischemia, and ventricular fibrillation and defibrillation. In all cases ECG evidence of ischemia was accompanied by alterations in O 2 extraction patterns and lactate production, but metabolic derangement often continued to be present after the ECG had returned to normal. Ventricular fibrillation occurred more commonly during coronary artery perfusion than during ischemic cardioplegia and was associated with pronounced increases in calculated myocardial oxygen consumption, as well as lactate production. Seventy-five per cent of patients having ischemic cardioplegia induced by intermittent aortic crossclamping had diminished myocardial extraction of O 2 throughout the operative procedure, with associated production of lactate. Seventy-two per cent of patients having continuous direct coronary perfusion showed an increase in myocardial O 2 extraction with concomitant lactate production during bypass. The pattern of either decreased or increased O 2 extraction persisted in any given patient and was felt to be due to operative technique (ischemic cardioplegia versus direct coronary perfusion) rather than to the underlying lesion. Possible mechanisms involved in the two patterns of O 2 extraction, both associated with anaerobiasis, are discussed. It is concluded that, under the operative conditions prevailing during this study, (1) the surface ECG gives a poor indication of myocardial metabolic dysfunction, (2) spontaneous ventricular fibrillation may be detrimental to cardiac function as the increase in myocardial oxygen consumption is not met by an increase in coronary blood flow, (3) intermittent aortic crossclamping might produce derangement in cellular oxidative metabolic processes, and (4) continuous direct coronary artery perfusion does not meet, and is inadequate to meet, myocardial metabolic demands.