Pulmonary embolism is a clinical and pathophysiological syndrome caused by obstruction of the main trunk or branches of the pulmonary artery by an endogenous or exogenous embolus, resulting in impaired pulmonary circulation. When further necrosis of lung tissue occurs on the basis of pulmonary embolism, it is called pulmonary infarction. What are the causes of pulmonary embolism? The following describes the causes of the onset of pulmonary embolism. 1, heart disease Heart disease is the most common cause of pulmonary embolism in China, accounting for 40%. Several times and all kinds of heart disease, combined with atrial fibrillation, heart failure and subacute bacterial endocarditis has a higher incidence. The right heart cavity thrombus is the most common, and a few also originate from the venous system. In addition to subacute bacterial endocarditis, bacterial emboli can also be caused by pacemaker infection. The former infectious emboli mainly come from the tricuspid valve, and occasionally mitral valve redundancy in patients with precordial heart can enter the right heart via defective shunt and reach the pulmonary artery from the left heart. 2. Pregnancy and childbirth Pulmonary embolism in pregnancy and childbirth occurs in pregnant women several times more than in age-matched non-pregnant women, and has the highest incidence after delivery and cesarean section. Increased intra-abdominal pressure and hormonal relaxation of vascular smooth muscle and pelvic vein compression during pregnancy can cause slow venous blood flow and change the rheological properties of blood, which aggravates venous thrombosis. This is accompanied by an increase in coagulation factors and platelets, and a decrease in the activity of the plasminogen-plasmin proteolytic system. However, these changes are not absolutely different from those in pregnant women without thromboembolism. Amniotic fluid embolism is also a serious complication during labor. 3. Thrombosis Thrombosis pulmonary embolism is often a comorbidity of venous thrombosis. The emboli usually originate from the deep veins of the lower extremities and pelvis and cause embolism through circulation to the pulmonary arteries. However, they rarely originate from the veins of the upper extremities, head and neck. Stagnant blood flow, increased blood coagulation and endothelial damage to the veins are factors that promote thrombosis. Therefore, trauma, prolonged bed rest, varicose veins, intravenous cannulation, pelvic and hip surgery, obesity, diabetes mellitus, contraceptive pills or other causes of hypercoagulability predispose to venous thrombosis. Early thrombosis is fragile, and the role of fibrinolytic system, so the risk of pulmonary embolism is highest in the first few days of thrombosis. 4.Tumor is the second cause in China, accounting for 35%, which is much higher than 6% abroad. Lung cancer, digestive system tumor, choriocarcinoma, leukemia, etc. are more common. Only about 1/3 of malignant tumor complicating pulmonary embolism is tumor embolism, and the rest are thrombus. It is speculated that there may be thrombokinase and other substances that can activate the coagulation system such as histone, tissue protease and protein hydrolase in the blood of tumor patients, so the incidence of pulmonary embolism in tumor patients is high, and it may even be the first symptom. Other rare causes include fat embolism caused by long bone fracture, air embolism caused by accident and decompression sickness, parasite and foreign body embolism. In the absence of obvious contributing factors, a decrease in hereditary anticoagulation factors or an increase in fibrinogen activation inhibitors should also be considered.