Pulmonary embolism is a syndrome that develops when an embolus-like object blocks the pulmonary artery, interrupting blood flow in whole or in part, and can be accompanied by increased pulmonary artery pressure and changes in the right side of the heart. These emboli include blood clots, fat, amniotic fluid, air, tumor emboli, and infectious emboli, 99% of which are blood clots, hence the term pulmonary thromboembolism is often used. The emboli come from other parts of the body as well as may be formed locally. Pulmonary embolism can be divided into acute and chronic according to the course of the disease. Qing Gu, Department of Cardiovascular Medicine, Fu Wai Hospital, Beijing, China