The presence of blood in the PERICARDIUM, the membranous sac which surrounds the heart. It may result from myocardial infarction (see HEART, DISEASES OF), leaking ANEURYSM, injury, or tumour. Because the pericardial blood compresses the heart, the latter's pumping action is impeded, reducing the blood pressure and causing cardiac failure. Urgent surgical drainage of the blood may be required.