<prompt> A 62 years old man with carcinoma of lung presented to emergency depament with respiratory distress. His ECG showed electrical alternans. The most likely diagnosis isPneumothoraxPleural effusionCardiac tamponadeConstrictive pericarditis  <response>: Ref Harrison 19 th ed pg 1759-60 Electrical alternans refers to beat-to-beat alteration in one or more components of the ECG signal. Total electrical alternans with sinus tachycardia is a relatively specific sign for pericardial effusion, often with cardiac tamponade. 