Tumurs <5 cm in diameter and at the anal margin have a more favourable prognosis. PrognosisFor prognostic purposes most information of clinical value is obtained by distinguishing tumours arising at the anal margin from those in the anal canal and separating them into those greater or less than 5 cm in diameter. The tumour is relatively uncommon so there have been relatively few large follow up studies, but the five year survival rate is about 50%. The need to avoid air embolism when inserting and manipulating central venous catheters is widely appreciated but the risk after catheter removal is less widely known. This has implications regarding the delegation of practical procedures.' Case report A 73 year old man was admitted for elective repair of an 8 cm abdominal aortic aneurysm. The past history included a myocardial infarction complicated by ventricular fibrillation eight years previously and an episode of acute left ventricular failure one year before when mitral incompetence was noted. The preoperative echocardiogram showed minimal mitral regurgitation and an ejection fraction of 54%. The aneurysm repair was uneventful, lasting 90 minutes. An episode ofpulmonary oedema on the first day was easily treated with diuretics and oxygen. Thereafter, he had an uneventful course, returning from the intensive care unit to the ward on the second postoperative day.A pulmonary flotation catheter (7 5 French gauge) had been inserted into the right internal jugular vein at the time of operation and the catheter sheath was removed by the houseman on the fifth day. About two minutes later the patient collapsed and became unresponsive with no palpable cardiac output. Cardiac massage was begun. On arrival the arrest team noted a spontaneous respiration rate of 18/min with good ventilatory efforts. Pulse was 120/min, regular, and in sinus rhythm. Blood pressure was 90/60 mm Hg and there was good colour and no cyanosis. The patient was sweating profusely. He was unresponsive to painful stimuli. Pupils were of normal size and reacting symmetrically. There were no focal neurological signs. Chest sounds were normal but examination of the heart disclosed widespread loud, harsh, crunching sounds throughout systole and diastole over the whole precordium. The originally recorded murmur of mitral incompetence could not be heard.Examination of abdomen and legs showed nothing abnormal apart from the recent incision. Blood glucose concentration was 6-7 mmol/l, and haemoglobin and Prevention Central venous catheters must be removed with the patient supine or in slight Trendelenberg position and the entry site covered by an air occlusive dressing.