There are only few studies that show the depletion of electrolytes and serum electrolyte concentration changes after cardiac surgery . The role of potassium in pathogenesis of cardiac arrhythmias is well recognized. Low serum potassium level is often found in association with hypomagnesemia and predisposes to atrial fibrillation. Extracellular magnesium is broadly implicated in neuronal control, neuromuscular transmission, and cardiovascular tone. It has been shown that magnesium suppresses arrhythmias after acute myocardial infarction, and there are studies confirming correlation between hypomagnesemia and postoperative atrial fibrillation . The underlying mechanism of these effects is not well understood but most probably involves magnesium interaction with calcium channels within myocytes membrane. Still the role of magnesium in the pathogenesis of AF is not clear yet.