A multi-centre, non-blinded, comparative effectiveness, randomised controlled trial. Patients will be prospectively enrolled from Critical Care Units and will be assessed for study enrollment based on inclusion/exclusion criteria at the time of the onset of fast atrial fibrillation (AF)(irregular and often rapid heart rate). The authors hypothesize that high dose Magnesium Sulphate with the addition of Digoxin as a second line treatment will improve the success rate in returning the heart to normal rhythm as well as speed of resolution of critical illness in new onset rapid atrial fibrillation in the critically ill cared for in general ICUs.