Magnesium sulfate (MgSO₄) is the standard therapy for the treatment of preeclampsia and prevention of eclamptic seizures in labor. This study aimed to determine the effect of the magnesium administered intravenously as a bolus prior to spinal anesthesia on the speed of onset and duration of the spinal anesthesia in preeclamptic women.

Sixty women undergoing caesarean section were randomly divided into the MgSO₄ group and the control group. The MgSO₄ group received 50 mg/kg MgSO₄ (total 50 ml) infused intravenously within 15 min and the control group received the same volume normal saline, then spinal anesthesia was administered using a standardized technique. The onset and duration of sensory and motor block, and duration of spinal anesthesia were studied. Hemodynamic parameters were monitored and postoperative pain was measured using a visual analogue scale (VAS). Side effects if any were recorded.

The onset of sensory block was significantly shorter (5.15 ± 1.35 vs. 6.25 ± 1.75 min, P < 0.05), and the duration of sensory block, motor block and spinal anesthesia was significantly longer in the MgSO₄ group (261.3 ± 64.7 vs. 226.5 ± 56.4 min, 194.6 ± 35.4 vs.175.7 ± 27.6 min and 81.5 ± 18.6 vs. 71.4 ± 16.5 min, respectively, P < 0.05), while the onset of motor block was not significantly shorter in the MgSO₄ group (4.15 ± 1.05 min vs. 4.50 ± 0.95 min, P > 0.05). Side effects were similar between the groups.

Intravenous MgSO₄ can hasten the onset of sensory block, prolong the duration of sensory block, motor block and spinal anesthesia, alleviate postoperative pain, but not hasten the onset of motor block in preeclamptic women undergoing spinal anesthesia without additional side effects. (www.chictr.org.cn, registration number: ChiCTR-IOR-15006856).