You are asked to review a 60-year-old man on the Cardiothoracic ward 24 hours after coronary bypass grafting. He has become breathless and feels dizzy and faint. BP 90/60 mmHg and PR 120 beats/minute, [His IVP is 8 cm above the sternal angle, and the apex beat is impalpable. On auscultation, heart sounds are softly audible and lung fields are clear. An ECG shows an alternas pattern. What is the most likely diagnosis?