<prompt> A 62 years old man with carcinoma of lung presented to emergency depament with respiratory distress. His EKG showed electrical alternans. The most likely diagnosis isPneumothorax.Pleural effusionCardiac tamponadeConstrictive pericarditis.  <response>: Electrical alternans is an electrocardiographic phenomenon of alternation of QRS complex amplitude or axis between beats and a possible wandering base-line. It is seen in cardiac tamponade and severe pericardial effusionMassive pericardial effusion produces a triad of:Low QRS voltageTachycardiaElectrical alternansRef: Harrisons 18e p1972 