Deep vein thrombosis (DVT) formation is the abnormal clotting of blood in the deep veins, resulting in partial or complete blockage of the deep vein lumen. It occurs in the lower extremities and can cause a series of symptoms such as pain and swelling in the lower extremities. 1. The three main factors of deep vein thrombosis: stagnant blood flow; venous wall damage; hypercoagulable state. 2, the preferred site of deep vein thrombosis – lower extremity: human blood starts from the heart, is transported to the lower extremity through the arterial system, and then flows back to the heart through the venous system, the distance through which the blood flows is the longest, the power of the heart pump is relatively weak in the lower extremity, and due to the effect of gravity, the blood is easily stagnated in the veins of the lower extremity. If the calf muscles are in a relaxed state at this time, the veins of the lower limbs are relatively easy to form blood clots in the same state in all parts of the body. 3, why the left lower limb is more likely to form deep vein thrombosis than the right lower limb: this is related to the anatomical position of the left common iliac vein, the right common iliac artery crosses the front of the left common iliac vein, the left common iliac vein is sandwiched between the right common iliac artery and the sacral isthmus, which is easy to be pressurized, so that the return flow of the left common iliac vein is blocked and leads to thrombosis. 4.High-risk group of deep vein thrombosis: Patients with hip or knee joint replacement surgery, lower limb fracture, severe trauma and acute crural injury are the very high-risk group of lower limb deep vein thrombosis. Patients who are bedridden or inactive for a long time after major surgery; stroke patients; patients with nephrotic syndrome; patients with malignant tumors; patients with a history of long-term oral hormone or contraceptive pills; pregnant women during pregnancy; patients with varicose veins in the lower limbs or previous thrombosis. 5.Economy class syndrome refers to long time airplane ride, sitting in a narrow space, unable to move, the lower limb venous blood flow is slow, stagnation and the formation of lower limb deep vein thrombosis. After getting off the plane, the thrombus is dislodged when moving, and then it flows back to the right heart with the blood, and then enters the pulmonary artery to cause embolism, making the lungs ischemic and hypoxic, causing chest pain, shortness of breath, hemoptysis and other symptoms, which can lead to sudden death in serious cases. The broad sense of “economy class syndrome” also includes long time riding on trains and automobiles, which causes deep vein thrombosis of lower limbs or further leads to pulmonary embolism. 6.Clinical symptoms of lower limb deep vein thrombosis: lower limb swelling, pain and superficial varicose veins are the three main symptoms of lower limb deep vein thrombosis, pain is mostly swelling or dull pain, superficial varicose veins are mostly the manifestation of the establishment of collateral circulation in chronic period. 7.Types of lower limb deep vein thrombosis: peripheral type, central type and mixed type. There are also two special types: femoral cyanosis and femoral leukomalacia, both of which are emergencies of lower limb DVT and require emergency surgery to remove the embolus in order to save the affected limb. 8.Post-formation syndrome of lower extremity deep vein thrombosis: It refers to the lower extremity deep vein thrombosis after treatment, although the symptoms have improved, but the lower extremity swelling and edema appear after standing or activity, gradually causing lower extremity superficial varicose veins, calf skin pigmentation, sclerosis, and even ulcer formation, the cause of which is often related to the incomplete closure of the deep vein valve caused by thrombosis mechanization.