A 65-year-old male with 10-year history of coronary artery disease (CAD) developed severe retrosternal pain 6 hours ago, described as crushing and radiating to the left arm. Took nitroglycerin 4 times with partial relief. Presents with restlessness, sweating. Examination: acute distress, temperature 36.5°C, BP 100/70 mmHg, pulse rate 110 bpm; normal cardiac size, regular rhythm, diminished heart sounds, no gallop or murmurs heard; few bilateral wet rales; liver/spleen not palpable. Most likely diagnosis is: