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.