Ramkumar, a 70 year old hypertensive male, was admitted to the intensive care unit with transmural anterolateral myocardial infarction. His condition was stable till the 5th day of admission, when he developed a pericardial friction rub and pleuritic chest pain which persisted despite narcotic and steroid therapy. On the 7th morning he suddenly developed marked hypotension. On examination, there was distension of jugular veins, accompanied by electromechanical dissociation. Most likely, the patient had developed: