Effect of magnesium on ischemic and reperfusion arrhythmias in a canine model with diminished collateral blood flow

To assess the effects of elevated serum magnesium on ischemic and reperfusion arrhythmias, the left anterior descending coronary artery was cannulated and perfused by a shunt from a carotid artery in 20 open-chest anesthetized dogs. Ischemia was caused for 30 minutes by shunt occlusion and retrograde diversion of collateral blood flow. Dogs (10/group) were treated prior to occlusion with either saline or MgSO4 (100 mg/kg IV). Plasma magnesium rose from 0.72 +/- 0.05 mM to 3.89 +/- 0.29 mM before occlusion (p < 0.01) and fell to 3.28 +/- 0.21 mM just before reperfusion (p < 0.01). Compared to saline, magnesium significantly slowed heart rate (113 +/- 4 beats/min vs. 124 +/- 3 beats/min, p < 0.05), lowered arterial blood pressure (90 +/- 2 mmHg vs. 111 +/- 4 mmHg, p < 0.05), and reduced myocardial blood flow to the ischemic zone before the occlusion (59 +/- 7 ml/min/100 g vs. 83 +/- 5 ml/min/100 g, p < 0.01). The incidence of ventricular tachycardia during occlusion was 80% in the saline group and 70% in the magnesium group (p = 1.0). The time required for a monophasic complex to develop in an electrogram over the ischemic zone was 4.5 +/- 0.24 minutes in the saline group and was not altered by magnesium (4.6 +/- 0.18 minutes). The incidence of reperfusion-induced ventricular fibrillation was 100% in both groups. The results suggest that acute infusion of magnesium offers little protection against ventricular tachyarrhythmias evoked by occlusion or reperfusion in a canine model of myocardial ischemia with diminished collateral blood flow.