A 70-year-old man on ICU develops a new pericardial effusion on echocardiography. The effusion is moderate in size but there is no evidence of tamponade physiology. He had a pericardiocentesis 3 days ago for a large effusion which drained 800 mL of straw-coloured fluid. The fluid has reaccumulated. He is on day 10 of ICU following coronary artery bypass grafting. What is the most appropriate investigation to determine the cause of the recurrent effusion?