A 20 year old man with a sinus venosus atrial septal defect underwent surgical correction with a pericardial baffle; the superior vena cava was not patched. Seven days postoperatively he developed pericardial effusion, which was initially treated with non-steroidal anti-inflammatory drugs. The pericardium was subsequently drained surgically, through a subxyphoid approach, after an unsuccessful percutaneous attempt. A pericardial drain was inserted the day after the procedure because of a hydropneumopericardium, revealed by standard chest x ray (left). We hypothesise that air trapping in the pericardium occurred because of a valve mechanism caused by the anti-Trendelenburg position.