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

Inclusion Criteria: * Age \> 18 years or older * Able to provide informed consent * Primary diagnosis AFF RVR greater than or equal to 120 bpm * Diltiazem as rate control agent * English speaking Exclusion Criteria: * Hemodynamically unstable patients (SBP \<90, MAP \<65) * Impaired consciousness * End stage renal disease on hemodialysis or peritoneal dialysis * Acute heart failure exacerbation based on clinical diagnosis, physician exam, bedside echo, and/or additional imaging * Patient in AFF RVR that is highly suspected to be a compensatory mechanism for an alternative clinical diagnosis * Rhythms other than AF, such as sick sinus syndrome or wide-complex ventricular response * Acute myocardial infarction * Pregnancy defined as a positive urine HCG (human chorionic gonadotropin) * Contraindications to magnesium sulfate (including myasthenia gravis) * Allergy or sensitivity to any study drugs * Previously enrolled in this trial during a different patient encounter * Withdrew from study