Effect of intravenous magnesium sulfate on cardiac arrhythmias in critically ill patients with low serum ionized magnesium.

Magnesium affects cardiac function, although until the recent development of a new ion-selective electrode no method existed for measuring the physiologically active form of magnesium, free ions (iMg2+), in the blood. We investigated the antiarrhythmic effect of magnesium sulfate administered to critically ill patients with cardiac arrhythmias and reduced iMg2+ as determined using the ion-selective electrode. Eight patients with a low iMg2+ level (less than 0.40 mmol/L) were given intravenous magnesium sulfate (group L). Magnesium sulfate was also administered to patients with a normal iMg2+ level (more than 0.40 mmol/L) but who did not respond to conventional antiarrhythmic drugs (group N). Intravenous magnesium sulfate significantly increased the iMg2+ level in patients in group L from 0.35 +/- 0.06 mmol/L (mean +/- SD) to 0.54 +/- 0.09 mmol/L (p < 0.01), and had an antiarrhythmic effect in 7 of the 8 patients (88%). However, in group N patients, intravenous magnesium sulfate had an antiarrhythmic effect in only 1 of the 6 patients (17%) (p < 0.05 vs group L). These results suggest that intravenous magnesium sulfate may be effective in the acute management of cardiac arrhythmias in patients with a low serum iMg2+ level.