Objectives: The objective was to investigate the efficacy of magnesium sulfate (MgSO 4 ) in decreasing the ventricular rate in emergency department (ED) patients presenting with new-onset, rapid atrial fibrillation (AF).Methods: A double-blinded, placebo-controlled randomized clinical trial was conducted in an adult university hospital. Patients aged ‡18 years with AF onset of less than 48 hours and a sustained ventricular rate of >100 beats ⁄ min were randomized to either intravenous (IV) MgSO 4 10 mmol or normal saline (NSal). Rhythm and instantaneous heart rate as measured by the monitor were recorded at baseline and every 15 minutes for 2 hours after starting the trial drug. Heart rate and rhythm were compared at 2 hours. A multilevel modeling analysis was performed to adjust for differences in baseline heart rate and any additional treatment and to examine changes in heart rate over time.Results: Twenty-four patients were randomized to MgSO 4 and 24 to NSal. Baseline heart rate was lower in the MgSO 4 group (mean ± standard deviation [±SD] = 125 ± 24 vs. 140 ± 21 beats ⁄ min]. One and 3 patients in the MgSO 4 and NSal groups, respectively, were given another antiarrhythmic or were electrically cardioverted within 2 hours after starting the trial drug. Heart rate (mean ± SD) at 2 hours in both MgSO 4 (116 ± 30 beats ⁄ min) and NSal groups (114 ± 31 beats ⁄ min) decreased below their respective baseline levels. However, the rate of heart rate decrease across time did not differ between groups (p = 0.124). The proportion of patients who converted to sinus rhythm 2 hours post-trial drug did not differ (MgSO 4 8.7% vs. NSal 25.0%, p = 0.25).Conclusions: This study was unable to demonstrate a difference between IV MgSO 4 10 mmol and saline placebo for reducing heart rate or conversion to sinus rhythm at 2 hours posttreatment in ED patients with AF of less than 48 hours duration.ACADEMIC EMERGENCY MEDICINE 2009; 16:295-300 ª