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 is

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.