Magnesium sulfate therapy, standard in preventing seizures in preeclampsia, is 
under active investigation as a neuroprotective agent. The authors studied the 
effect of magnesium as a cerebral vasodilator by measuring the cerebral blood 
flow velocity (CBFV) response to a 5g intravenous bolus of MgSO4 compared with a 
saline placebo after subarachnoid hemorrhage (SAH). Transcranial Doppler 
ultrasonography of the middle cerebral artery (MCA) was measured after each 
infusion. Patients were studied up to three times after SAH at prescribed time 
intervals. Fourteen patients (11 women, 3 men; mean age 58 years) underwent 29 
studies. All patients underwent hypertensive, hypervolemic therapy. Four 
patients developed cerebral vasospasm. Doubling serum magnesium levels did not 
affect MCA CBFV but slightly lowered mean arterial blood pressure and systemic 
vascular resistance. Intravenous magnesium bolus did not reduce elevated CBFV in 
the subset of SAH patients with clinical vasospasm. The role of magnesium 
sulfate as a cerebral vasodilator in patients with SAH requires further study.