Monitoring arrhythmias and signs of intraoperative ischemia by electrocardiogram
Electrocardiographic monitoring serves two critical roles during anesthesia and operation in patients with coronary artery disease. It is a qualitative source of information regarding myocardial oxygen supply/demand relationships by means of changes in the ST segment, and it is the means by which arrhythmias of any etiology can be immediately recognized and treated if necessary. Although ST-segment deviations are commonly appreciated to be accompaniments of acute myocardial ischemia or injury, it is not so commonly understood that arrhythmias also may be manifestations of acute ischemia, injury, or infarction. In the coronary care unit ventricular arrhythmias are well known to be expressions of underlying electrical instability invoked and sustained by myocardial ischemia or infarction. Thus the cardiac anesthesiologist must be aware that intraoperative myocardial ischemia, injury, or infarction may become evident by deviations in the ST segment in the appropriate lead or by the acute onset of ventricular ectopy,1 or by both signs of underlying electrical changes precipitated by acute ischemia. The purpose of this paper is to review the current understanding of the electro-physiologic mechanisms involved in the evolution of ST-segment deviations and ischemia-induced ventricular arrhythmias. The emphasis will be on electrophysiologic concepts rather than on recognition and treatment of the disorders themselves. This may provide the clinical cardiac anesthesiologist with further insight into mechanisms that generate the observed electrocardiographic abnormalities.
Electrophysiologic events underlying ST-segment displacements
During acute myocardial ischemia the transmembrane potential of ischemic cells is altered as compared with normal cells.2–5 During diastole (phase 4 of the . . .