Maternal oxygen desaturation with intravenous magnesium therapy

To describe the occurrence, treatment, and outcome of maternal oxygen desaturation during magnesium sulfate therapy.

A post hoc analysis of a randomized double-blind trial, designed to determine if mothers at risk for premature delivery treated with phenobarbital and vitamin K had less frequent intracranial hemorrhage in their newborns, was done. A subset of these patients at imminent risk for delivery received both intravenous magnesium sulfate and intravenous study drug (phenobarbital or placebo) and was monitored with maternal oxygen saturation monitoring.

One hundred one women (29%) in the trial had pulse oximetry; 47 were assigned to placebo and 54 to the treatment group. The placebo and treatment groups had the following similarities: mean lowest oxygen saturation by pulse oximeter (93.4% +/- 3.0 compared with 93.1% +/- 3.3). mean highest magnesium levels (6.3 mEq/L +/- 1.5 compared with 6.2 mEq/L +/- 0.9), frequencies of desaturation events defined as oxygen saturation below 90% (11% compared with 11%), gestational age at delivery, birth weight, Apgar scores, and cord arterial pH. Using regression analysis, multiple gestation was the only one of 14 independent variables associated with low maternal oxygen saturation. Preeclampsia was not associated with a greater risk of desaturation. The statistical power of this study is limited by its small sample sizes.

Maternal oxygen desaturation occurs commonly with intravenous magnesium therapy, does not occur more frequently with simultaneous administration of intravenous phenobarbital, and does not cause decompensation in maternal or fetal status. Multiple gestation may be associated with lower maternal oxygen saturation.