A 57-year-old man, with no past history of interest, attended the hospital emergency department for chest pain. Acute coronary syndrome with ST-segment elevation in the anterior aspect of the heart was diagnosed and percutaneous coronary angiography was performed. The patient had a lesion in the middle portion of the anterior descending coronary artery, so angioplasty was performed and a stent was placed, without complications, after administering 300 mg of acetylsalicylic acid, 300 mg of clopidogrel and 5,000 IU of intravenous heparin sodium. At the end of the procedure the patient was in Killip III and presented with massive haematemesis. Arterial O2 saturation was 92%, with a Ventimask at 50% 10 bpm. Digestive endoscopy was performed with respiratory support with CPAP-B, as per protocol described, and a Mallory Weiss lesion was observed. Haemoglobin dropped from 15.1 to 13g/dl and haematocrit from 43.6 to 39.1%. At the end of the procedure, the patient was 100% saturated and CPAP-B could be removed (arterial blood gas was not performed due to the risk of bleeding after administration of anticoagulants and antiplatelet agents).