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SUMMARY The role of intracardiac volume in controlling electrocardiographic R-wave amplitude changes during acute myocardial ischemia was studied in 24 open-chest dogs. The R-wave amplitude in surface ECG leads 2, V. and Frank X, Y and Z leads were correlated with hemodynamic, echocardiographic and angiographic changes in a 5-minute circumflex coronary artery ligation and reperfusion model. After coronary ligation, left ventricular end-diastolic diameter and volume increased progressively above control, reached a peak and plateau at 120-130 seconds after ligation and did not return to control levels until more than 5 minutes after release of the occlusion. In contrast, the R-wave amplitude showed a biphasic response to acute ischemia, reaching a nadir (IR = 18.2% below control) at 30 seconds after coronary ligation and only subsequently increased to reach a peak ( =R 52% above control) at 150 seconds after ligation. In addition, R-wave amplitude returned immediately to control levels within 10 seconds after reperfusion. In six other dogs, both venae cavae were occluded for a 30-second period, beginning 180 seconds after coronary ligation. Although intracardiac volume decreased markedly, R-wave amplitudes increased even more. Thus, the demonstration of discordance between alterations in intracardiac volume and R-wave amplitude in these studies suggests that factors other than intracardiac volume determine R-wave amplitude changes in the course of acute myocardial ischemia.THE IMPORTANCE of intracardiac blood volume to the magnitude of scaler ECG potentials was first postulated by Brody.1 Brody suggested that an increase in intracardiac blood volume decreases tangentially oriented electrical vectors, whereas the radially oriented vectors are increased. The initial QRS vectors that generate the R wave on the surface ECG are radially oriented, so increases in intracardiac volume could thereby result in increases in R-wave amplitude -the Brody effect.' These theoretical considerations were later corroborated by a series of experiments in laboratory animals with normal myocardium2 8 and in other studies using biophysical and computer models.9'-1 Clinical studies have shown an increase in R-wave amplitude during maximal exercise in patients with proved coronary artery disease.12"17 Assuming that the increases in R-wave amplitude in these patients are directly related to ischemia-induced increases in intracardiac volume by the Brody effect, some authors have advocated using R-wave changes as a sensitive and specific indicator of coronary artery disease.12 Moreover, exercise-induced increases in R-wave amplitude have been suggested as a diagnostic marker to identify The present study was designed to determine, under controlled experimental conditions, the relationship between intracardiac volume changes and surface electrocardiographic R-wave amplitude variations during the immediate phase of acute myocardial ischemia. A canine model of acute coronary ligation was used. Materials and Methods Twenty

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