EFFECT OF MAGNESIUM ON THE MONOPHASIC ACTION-POTENTIAL DURING EARLY ISCHEMIA IN THE IN-VIVO HUMAN HEART

Objectives. This study sought to examine the effects of magnesium on e picardial action potential duration in patients during early myocardia l ischemia. Background. Magnesium has been shown to reduce arrhythmias in experimental models of myocardial ischemia. Experimental and clini cal observations suggest an effect on repolarization. Methods. Patient s undergoing elective coronary artery bypass surgery were randomized ( double blind) to receive intravenous magnesium (n = 10) or placebo (n = 10). Patients were placed on cardiopulmonary bypass and paced at 600 ms, and stable monophasic action potentials were obtained. Ischemia w as achieved by aortic cross-clamping for 2 min while normothermia was maintained. Results. Serum magnesium levels increased from 0.60 +/- 0. 03 to 1.69 +/- 0.07 mmol/liter (mean +/- SEM) in the magnesium group, with no change in the placebo group. Epicardial temperature was identi cal in the two groups and did not alter during ischemia. At 90% repola rization, initial action potential prolongation was observed in the pl acebo group over the first minute of ischemia (282.0 +/- 6.0 to 294.0 +/- 4.8 ms) but not in the magnesium group (278.3 +/- 5.9 to 274.5 +/- 7.4 ms), At 2 min of ischemia, action potential duration was shorter in the magnesium group than in the placebo group (258.1 +/- 5.5 vs. 28 1.3 +/- 5.9 ms, respectively, p < 0.05). Conclusions. Intravenous magn esium infusion altered the epicardial action potential response to isc hemia in patients. These findings may have important implications in t he pathogenesis of arrhythmias in ischemic myocardium. (C) 1996 by the American College of Cardiology