<prompt> A 62-year old man with carcinoma lung presented to emergency depament with respiratory distress. ECG showed electrical alternans. Most likely diagnosis isPneumothoraxPleural effusionCardiac tamponadeConstrictive pericarditis  <response>: Electrical alterans-a beat-to-beat alternation in one or more components of the ECG signal-is a common type of nonlinear cardiovascular response to a a variety of hemodynamic and electro physiologic peurbations.Total electrical alterans with sinus tachycardia is a relatively specific sign of pericardial effusion,usually with cardiac tamponade. 