Comparing Magnesium 2g Versus 4g Versus Placebo in the Incidence of Treating AFF RVR

This study is comparing two doses of magnesium (2g vs 4g) versus a placebo (inactive treatment) to see which is most effective at slowing the heart rate in patients with atrial fibrillation with a rapid ventricular rate (a fast, irregular heartbeat) in the emergency setting. **You may be eligible if...** - You are 18 or older - You have been diagnosed with atrial fibrillation with a rapid heart rate (120 beats per minute or more) - You are currently being treated with diltiazem for rate control - You are English-speaking and able to give consent **You may NOT be eligible if...** - You have dangerously low blood pressure - You are in kidney failure requiring dialysis - You are having an acute heart failure episode - You are having a heart attack - You are pregnant - You are hemodynamically unstable or have impaired consciousness - Your rapid heart rate is thought to be compensating for another medical problem Talk to your doctor to see if this trial is right for you.