Magnesium sulfate tocolysis prolongs QT interval

Abstract

Objective: Magnesium sulfate (MgSO 4) tocolysis causes maternal ionized calcium (Ca ++) to fall precipitously. Changes in Ca ++ levels are known to alter electrical conduction in the heart, possibly resulting in arrhythmias. The goal of this study was to determine the effects on maternal QT interval when intravenous MgSO 4 is used for tocolysis in preterm labor. Methods: All patients admitted for MgSO 4 tocolysis were considered for the study. Patients with any underlying medical conditions were excluded. Levels of ionized calcium, calcium, potassium, phosphate, magnesium (Mg ++), and albumin, and an electrocardiogram (ECG) were obtained before initiation of MgSO 4. These labs and ECG were repeated after 12 hours of tocolysis and again after 24 hours. Magnesium sulfate was given as a 4-g bolus and continued as a 2–3 g/hour infusion. Results: In the initial six patients, the QTc was prolonged in all patients by 12 hours. At 24 hours, the QTc further lengthened in two out of six patients. In the other four women, the QTc remained longer than the initial measurement but did not continue to be prolonged. During the initial 12 hours, the average Mg ++ level was 6.2 mg/dL and the Ca ++ had declined an average of 19 % for five of the six patients. Conclusion: In this preliminary study, intravenous MgSO 4 tocolysis had an effect on the level of ionized calcium and the length of the QTc. This effect occurred within 12 hours of tocolysis. Whether this results in an adverse effect in certain individuals remains to be determined.