All patients and their relatives will be informed about the hospital policy regarding the use of their medical records for scientific purposes upon admission. Informed consent will be obtained from the patients (those that were able to provide it) or from their relatives if the patients physical condition prevented them from signing it. Atrial fibrillation is   defined as irregular, chaotic atrial activity with no apparent P waves and irregular ventricular activity, with a ventricular rate consistently >110 beats/min lasting for at least 30 minutes. New-onset Atrial fibrillation is defined as newly developed Atrial fibrillation during the ICU stay in patients without a previous history of atrial tachyarrhythmias and antiarrhythmic drug use. The diagnosis will be confirmed by a 12-lead electrocardiogram  and 2D-Echo to rule out structural heart diseases and clots. Patients with a previous history of atrial tachyarrhythmias and those using specific antiarrhythmic drugs will be excluded. Identification, evaluation and treating underlying causes will be done simultaneously. The mean arterial pressure and heart rate just before and at the onset of Atrial fibrillation and subsequently at 1 hour, 4 hours, 8 hours, 12 hours and 24 hours will be recorded. DC Cardioversion will be done when the Atrial Fibrillation causes hemodynamic instability. Magnesium sulphate is given with a dose 4.5g as our standard dose over 20mins. Serum magnesium and serum potassium levels will be obtained in all patients priorly on admission and levels noted.  In case if there is no conversion to sinus rhythm or reduction of the ventricular rate to <110 beats/min within 30 minutes after the start of the magnesium sulphate bolus. According to our standard protocol we use rate and rhythm controlling drugs, the treating physician decides the drug, Amiodarone (loading dose 150 mg in 15 minutes, followed by another bolus of amiodarone of 150mg if not controlled, amiodarone infusion is initiated at 1mg/min for 6 hours and 0.5mg/minute for18hrs. The attending physician will determine when to discontinue the amiodarone infusion upon obtaining sinus rhythm. The rhythm and rate for both magnesium responders and non-responders each hour after the onset of atrial fibrillation will be noted. Other rate-controlling drugs like Betablocker-Metoprolol, Calcium channel-diltiazem and digoxin are also added depending on the treating physician. In the management of Atrial fibrillation in ICU, the objective is to rapidly decrease Ventricular rate with or without restoration of sinus rhythm. For most patients with new-onset Atrial fibrillation due to critical illness, the risks of anticoagulation seem to generally outweigh potential benefits.