Effect of the infusion of magnesium sulfate during atrial pacing on ECG intervals, serum electrolytes, and blood pressure.

Magnesium salts have been used for decades for the empiric treatment of arrhythmias, particularly torsades de pointes, associated with long QT syndrome. The mechanism underlying this antiarrhythmic effect is still not clear. Therefore the effect of intravenous MgSO4 on serum electrolytes, blood pressure, and ECG variables was evaluated in nine patients with sick sinus syndrome, equipped with a DDD pulse generator, programmed in the atrial asynchronous mode. A total dose of 10 gm MgSO4 was given intravenously over 6 hours at a constant rate. Blood pressure and serum electrolytes were determined before (t0), 3 hours after (t3), and at the end of the magnesium infusion (t6). ECG variables were measured at t0, t3, and t6 at different pacing frequencies (60, 80, and 100 beats/min). Serum magnesium levels rose significantly from 0.88 mmol.l-1 at t0 to 1.91 mmol.l-1 at t6 (p less than 0.05). Magnesium infusion did not affect blood pressure, pulse rate, PR or QRS or QT interval. Increasing the pacing frequency resulted in a statistically significant QT shortening at each serum magnesium level. We conclude that intravenous magnesium administration does not influence the QT interval. Increasing atrial pacing rate shortens the QT interval and this QT shortening is not affected by magnesium. Sustained serum magnesium levels between 1.5 and 2 mmol.l-1 are hemodynamically well tolerated and do not give rise to the development of higher degree atrioventricular block.