Effects of magnesium sulfate on cardiac conduction and refractoriness in humans

Magnesium has been used empirically for several decades in the treatment of atrial and ventricular arrhythmias in patients with normal and decreased serum magnesium levels. However, a systematic evaluation of the effects of magnesium on cardiac conduction and refractoriness in humans has not been described. In this study, the electrocardiographic and electrophysiologic effects of magnesium were determined in 10 patients with normal baseline serum magnesium and other electrolyte levels. Six grams of magnesium sulfate was administered intravenously over 6 minutes followed by a continuous infusion of 1 additional gram over 1 hour. Serum magnesium levels rose significantly from a baseline of 2.0 ± 0.2 to 5.4 ± 0.4 mg/dl (p < 0.001). No significant change occurred in heart rate at rest, or in duration of the QRS complex or QT or QTc intervals during sinus rhythm. There were significant increases in sinus node recovery time (1,000 ± 211 to 1,106 ± 223 ms, p < 0.01) and corrected sinus node recovery time (279 ± 87 to 336 ± 104 ms, p < 0.05). Significant increases occurred in atrioventricular (AV) node conduction time during sinus rhythm (82 ± 22 to 97 ± 17 ms, p < 0.02), in the atrial paced cycle length at which AV node Wenckebach block occurred (350 ± 46 to 419 ± 65 ms, p < 0.01) and in the AV node relative refractory period (397 ± 27 to 422 ± 18 ms, p < 0.05), functional refractory period (395 ± 41 to 415 ± 33 ms, p < 0.05) and effective refractory period (306 ± 67 to 338 ± 38 ms, p < 0.05). During rapid ventricular pacing at 500 and 250 ms, QRS duration increased significantly (154 ± 15 to 164 ± 20 ms, p < 0.01) but was not rate dependent. There was no significant change in atrial or ventricular refractory periods. Acute intravenous administration of magnesium has several electrophysiologic effects that may be beneficial in the treatment of atrial and ventricular arrhythmias.