Usefulness of magnesium sulfate in stabilizing cardiac repolarization in heart failure secondary to ischemic cardiomyopathy

Experimental heart failure is associated with cardiac magnesium loss, causing increased beat-to-beat variability in the action potential. Unstable repolarization contributes to sudden death, but no therapy has been shown to reduce repolarization variability safely. We sought to test whether a prolonged infusion of magnesium sulfate (MgSO₄; 40 mmol/24 hours) would normalize QT interval variability in patients with compensated heart failure. Fifteen patients (New York Heart Association class II to III; mean age 63 years) were enrolled in a placebo-controlled, double-blind study. Surface electrocardiograms were recorded and digitized at entry and at 48 and 168 hours (drug washout). Repolarization stability was assessed using an automated method measuring each QT interval in a 5-minute epoch. The QT variability index was derived as the ratio of normalized QT-to-normalized heart rate variability. Seven of 15 patients received MgSO₄. Mean heart rate and QT did not change in either group. The QT variability index was stable in the placebo group (-0.69 ± 0.15 at entry, -0.71 ± 0.22 at 48 hours, -0.70 ± 0.18 at 168 hours), but decreased significantly in the treated group at 48 hours (-0.95 ± 0.19 to -1.36 ± 0.13, p <0.05 repeated-measures analysis of variance), returning to baseline at 168 hours (-0.84 ± 0.18). Regression analyses showed that administration of MgSO₄ resulted in a stronger correlation between the QT and RR interval (p <0.01). Thus, MgSO₄ stabilizes cardiac repolarization in patients with compensated heart failure due to ischemic heart disease. Magnesium therapy may be useful in altering the proarrhythmic substrate in heart failure.