Hypokalemia is defined as serum potassium level less than 3.5 mEq/L. When the serum level of potassium is less than 3 mEq/L, intravenous potassium supplementation is warranted. A 23 yr old adult female with complaints of dyspnoea (NYHA II) since 6 yrs, dyspnoea (NYHA III) and paroxysmal nocturnal dyspnoea on and off since 2 months, diagnosed with severe mitral stenosis, was posted for mitral valve replacement. After the release of ACC, ECG revealed sine wave pattern, Transesophageal echocardiographic examination revealed global hypokinesia and ABG showed potassium of 2.3 mEq/L. Hypokalemia in cardiac patients can occur due to the effect of poor oral intake, increased renal loss by the secondary hyperaldosteronism in congestive heart failure, loss due to use of digoxin and diuretics like thiazide diuretics, loop diuretics etc. Hypokalemia should be avoided while weaning off cardiopulmonary support as it can lead to atrial and ventricular arrhythmias. Potassium ion is very important for the normal contractility of the heart. Hypokalemia if refractory to intravenous potassium supplementation, concomitant magnesium deficiency should be suspected and treated.