Effect of Magnesium on the Monophasic Action Potential During Early Ischemi in the In Vivo Human Heart

Objectives. This study sought to examine the effects of magnesium on epicardial action potential duration in patients during early myocardial ischemia. Background. Magnesium has been shown to reduce arrhythmias in experimental models of myocardial ischemia. Experimental and clinical observations suggest an effect on repolarization. Methods. Patients 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. Ischemi was achieved by aortic cross-clamping for 2 min while normothermi 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 identical in the two groups and did not alter during ischemia. At 90% repolarization, initial action potential prolongation was observed in the placebo group over the first minute of ischemi (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. 281.3 ± 5.9 ms, respectively, p < 0.05). Conclusions. Intravenous magnesium infusion altered the epicardial action potential response to ischemi in patients. These findings may have important implications in the pathogenesis of arrhythmias in ischemic myocardium.