Influence of magnesium ion on human ventricular defibrillation after aortocoronary bypass surgery

The administration of magnesium ion (Mg⁺⁺) has been reported to defibrillate the ventricles and to decrease the incidence of arrhythmias after cardiopulmonary bypass. In a prospective study of 76 randomly selected patients undergoing coronary artery bypass grafting, patients received either no Mg⁺⁺, 0.25 mEq/kg of Mg⁺⁺ during cardiopulmonary bypass with the aorta clamped, or 0.375 mEq/kg of Mg⁺⁺ before cardiopulmonary bypass. Spontaneous resumption of a cardiac rhythm or spontaneous defibrillation during reperfusion was not significantly affected by Mg⁺⁺ administration. However, the number of shocks to initial and to sustained defibrillation and the energy required for the last direct-current shock was greatest in patients who received Mg⁺⁺ before bypass and in those whose plasma Mg⁺⁺ was greater than 2.26 mg/dl. Thus, the administration of Mg⁺⁺ may have adverse effects on the heart if intraoperative plasma Mg⁺⁺ exceeds 2.26 mg/dl.