A pericardial effusion is an abnormal accumulation of fluid in the pericardial space between the visceral and parietal pericardium. On echocardiography, it appears as an anechoic or echo-poor space surrounding the heart. Small effusions are typically seen posteriorly; large effusions surround the entire heart. A large rapidly accumulating effusion can cause cardiac tamponade — compression of the heart chambers that compromises filling and cardiac output. Sonographic signs of tamponade include right atrial and right ventricular diastolic collapse.