1.What is Deep Vein Thrombosis (DVT)? Deep vein thrombosis 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 limbs and can cause a series of symptoms such as pain and swelling in the lower limbs. 2.Factors of deep vein thrombosis? There are three major recognized causative factors of venous thrombosis: blood stasis, venous wall damage and hypercoagulation, among which “blood stasis” plays a key role in the process of venous thrombosis. Song Xiaojun, Vascular Surgery Center of Peking Union Medical College Hospital 3.Why deep vein thrombosis is more likely to occur in the lower limbs? The blood of human body starts from heart, is transported to lower limbs through arterial system, and then back to heart through venous system, the distance of blood flow is the longest, the pumping power of heart becomes relatively weak to this part. And due to the effect of gravity, the blood tends to slow down in the lower limbs and stagnate in the veins. If the calf muscles, which are known as the “second heart”, are at rest at this time, the veins of the lower extremities are relatively prone to thrombosis in the same state as all parts of the body. Among the bilateral lower limbs, the left lower limb is more likely to form deep vein thrombosis than the right, which is related to its anatomical position. The left common iliac vein is sandwiched between the right common iliac artery and the sacral isthmus, which makes it easy for the left common iliac vein to be in contact with the front and back walls for a long time, which not only obstructs the reflux of the left common iliac vein, but also forms intraventricular adhesions. 4.Who is prone to lower limb deep vein thrombosis? People with high risk of DVT are: bedridden or inactive for a long time after surgery; trauma, obesity, hyperlipidemia or age over 40; patients with myocardial infarction, heart failure, stroke, nephrotic syndrome; patients with malignant tumor; patients with oral contraceptives, pregnancy, varicose veins or previous history of thrombosis, etc. Patients with surgery and trauma are especially prone to lower extremity deep vein thrombosis. Patients with acute thoracic and abdominal surgery and hip or knee replacement surgery, hip fracture, severe trauma and acute spinal injury are at very high risk of thromboembolism. 5.Why does pregnancy increase the incidence of lower limb deep vein thrombosis? During pregnancy, the enlarged uterus compresses the blood vessels in the abdominal cavity, which hinders the blood flow back to the lower limbs and increases the blood stagnation in the lower limbs. At the same time, the blood in the body during pregnancy is secondary to hypercoagulation. The incidence of DVT in the lower extremities is greatly increased by the synergistic effect of the two factors. 6.What is economy class syndrome? Economy class syndrome refers to a prolonged airplane ride in which the left side cannot move in a narrow space, and the venous blood flow in the lower limbs slows down and stagnates, resulting in the formation of DVT in the lower limbs. After getting off the plane, the thrombus is dislodged when moving around, and then the blood flows back to the right heart, and then enters the pulmonary artery to cause embolism, making the lungs ischemic and hypoxic, causing symptoms such as chest pain, shortness of breath and blood clots, 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. 7.What is the risk of deep vein thrombosis? The incidence of asymptomatic DVT is 5-7% and is mostly confined to the distal veins of the lower extremities. In some patients, the first and only clinical manifestation is sudden death, and the cause is “pulmonary embolism”: deep vein thrombosis can be detected in 70-90% of patients with pulmonary embolism. In the United States, the mortality rate of pulmonary embolism is the third highest after cancer and coronary heart disease. Therefore, deep vein thrombosis is called the “silent killer”. In China, with the change of people’s dietary structure and living habits, the incidence of lower limb deep vein thrombosis is increasing every year. However, the medical community is still far from understanding this disease, about 70% of the patients with pulmonary embolism are missed or misdiagnosed as myocardial infarction, coronary heart disease and pulmonary disease, and about 30% of the patients die due to untimely diagnosis and treatment. 8.What are the symptoms of lower limb deep vein thrombosis? Lower limb swelling, pain and superficial varicose veins are the three main symptoms of DVT in lower limbs. The pain is mostly cramping or dull pain, and superficial varicose veins are mostly the manifestation of the establishment of collateral circulation in the chronic stage. 9.What types of DVT can be divided into? Generally, it can be divided into three types: peripheral type, central type and mixed type. There are also two special types: femoral cyanosis and femoral leukomalacia, both of which are emergency cases of DVT in the lower extremity and require emergency surgery to remove the embolus in order to save the affected limb. 10.How to determine the upper boundary of DVT in lower extremity simply? It can be judged according to the level of lower limb edema. Generally, edema below the middle of the calf is in the N vein; painful edema below the knee is in the superficial femoral vein; edema below the middle of the thigh is in the femoral vein; edema below the buttock is in the common iliac vein; edema in bilateral lower limbs is in the inferior vena cava. It should be noted that bilateral lower extremity edema in inferior vena cava thrombosis is often symmetrical and can easily be overlooked and misdiagnosed. 11.What tests can be performed to help diagnose lower extremity deep vein thrombosis? In addition to the physician’s careful physical examination, the following auxiliary examinations are useful in diagnosing and identifying lower extremity deep vein thrombosis: lower extremity deep vein ultrasonography, lower extremity venography, spiral CTA or MRI, and radionuclide examination. 12.What is the value of lower extremity venous ultrasonography in diagnosing DVT of lower extremity? Due to the high specificity and sensitivity of venous ultrasonography for thrombus detection, and its non-invasive, reproducible, easy operation and obvious price advantages, venous ultrasonography has become the main preferred method for diagnosing DVT of lower extremities. Combining the information provided by B-mode ultrasonography, Doppler spectral analysis and color Doppler flow imaging, experienced ultrasonographers have diagnostic sensitivity and specificity of up to 97% for central DVT and up to 75% for peripheral DVT. Moreover, for patients with DVT of lower extremities on standardized anticoagulation therapy, regular deep vein ultrasound follow-up and normal deep vein findings are safe for guiding clinical discontinuation of anticoagulants. 13.How to evaluate the lower extremity venography? Lower extremity venography has been regarded as the “gold standard” for the diagnosis of lower extremity DVT. It can not only effectively determine the presence or absence of thrombus, but also provide detailed information about the location, scope, shape and collateral circulation of thrombus, and further understand the thrombosis of pelvic and intra-abdominal venous system. Because of its accuracy and comprehensiveness in the diagnosis of DVT, phlebography can be used to identify the diagnostic value of other testing methods. However, venography is an invasive test, which can cause infection and even induce venous thrombosis in case of improper operation; moreover, the applied contrast agent can lead to allergic reaction, vascular damage, and in severe cases, renal failure. Therefore, the application of venography is somewhat restricted, and caution should be exercised when considering this method to diagnose DVT. 14.Treatment methods of lower limb DVT? The current treatment includes anticoagulation, thrombolytic therapy and surgery. 15.What is anticoagulation therapy? Under the human physiological condition, thrombosis and dissolution are in balance with each other. And once venous thrombosis occurs, the coagulation process of the body takes the dominant position. Targeted application of drugs to block the activation of clotting factors in the process of thrombosis is anticoagulation therapy. 16.What are the commonly used anticoagulant drugs? What should be noted in the anticoagulation process? The commonly used anticoagulants include two major categories: heparin and coumarin derivatives. The former are injectable drugs, short-acting anticoagulants, generally used for anticoagulation in surgery or treatment of acute thrombotic diseases. Low molecular heparin is a small molecular weight heparin fragment purified by cleavage of common heparin, which is simple to use, has a long half-life and few bleeding complications, and is gradually being widely used. The latter is represented by warfarin, an oral class of drugs, which is a long-acting anticoagulant, mostly used to prevent recurrence after thrombosis treatment or to prevent vascular occlusion after various revascularization procedures. Regardless of which type of anticoagulant is applied, there is a risk that a small dose will not achieve the anticoagulant effect and a large dose will greatly increase bleeding complications. Therefore, during the application process, the blood coagulation function changes must be monitored in order to adjust the drug dose. 17.How to decide the thrombolytic treatment for lower limb deep vein thrombosis? At present, it is still controversial in the vascular surgery medical community at home and abroad whether to use thrombolytic therapy for DVT of lower extremities. The immediate recanalization rate of lower extremity thrombosis with thrombolytic therapy is relatively high, but the occurrence of pulmonary embolism and the recurrence rate of DVT are still uncertain. It is generally believed that if there are no contraindications, the earlier thrombolysis is performed after thrombosis, the better the outcome, while more than 7 days is less effective. 18.What are the ways of thrombolytic therapy? The methods of thrombolytic therapy include: systemic application of thrombolytic drugs, application of thrombolytic drugs in the arteries of the affected limbs, and application of thrombolytic drugs in the deep veins of the affected limbs. 20.What are the advantages and disadvantages of various thrombolytic treatment methods? Systemic thrombolysis is performed by puncturing a superficial vein, which is easy to operate, reproducible and easy to care for, but the drug dose is larger, so bleeding complications are more likely to occur. Arterial thrombolysis of the affected limb requires femoral artery puncture, which is difficult to operate and more painful for the patient, and is prone to hematoma if operated improperly, but the drug concentration in the affected limb is large and the drug dose is smaller than that of systemic thrombolysis, so the bleeding complications are smaller. With deep vein thrombolysis of the affected limb, the drug enters the target vein directly, the drug is in full contact with the thrombus, and the drug dose is small, but the thrombus must be partially recanalized. 21.What are the indications for surgery of lower extremity deep vein thrombosis? The indications for surgery are mainly for acute DVT of the lower extremity: primary iliofemoral vein thrombosis and the duration of the disease does not exceed 72 hours, or within 7-10 days after the onset of the disease if there is a condition of DVT. In addition, femoral cyanosis and femoral leukomalacia require emergency surgery. 22.After surgery or thrombolytic therapy, will DVT of lower extremity recur? DVT of lower extremity is very likely to recur! After thrombolytic therapy or surgery, standardized anticoagulation therapy must be performed. 23.Why should I have vena cava filter placement? Because the biggest danger of DVT in lower extremity is that the embolus dislodged leads to pulmonary embolism, which can cause sudden death, and the inferior vena cava filter is like a protective umbrella to capture part of the dislodged embolus. Especially for patients who have already had a pulmonary embolism or who need thrombolytic therapy, the risk of embolus dislodgement is higher and inferior vena cava filter placement is feasible. Placement of an inferior vena cava filter can significantly reduce the incidence of pulmonary embolism. Venous filters can be divided into permanent and temporary types. They are carefully selected according to the different clinical characteristics of the patient. Anticoagulation therapy is still required after the placement of vena cava filters. In addition, the cost-benefit ratio of vena cava filter placement needs to be further observed. 24.In addition to the above treatment, what else should be noted during the treatment? Patients with DVT of lower extremities should take measures such as bed rest, elevation of affected limbs and local wet and hot compresses. The duration of bed rest is usually 10 days, and light activities can be carried out when the general symptoms and local pressure pain are relieved. When getting up and moving, they need to wear gradient decompression elastic stockings or use elastic bandage. 25.What is post-thrombotic syndrome of lower extremity deep vein? The post-thrombosis syndrome of lower limb DVT refers to the swelling and edema of the lower limbs after treatment of lower limb DVT, although the symptoms have improved, but after standing or moving around, which gradually causes varicose veins in the lower limbs, skin pigmentation and sclerosis of the lower legs, and even ulcer formation. 26.What is the typology of post-thrombotic syndrome of lower limb deep vein? According to the different types of original lesions, post DVT syndrome of lower limbs is also divided into peripheral type, central type and mixed type accordingly. 27.What is the difference between the treatment of post DVT syndrome and acute DVT of lower extremity? The treatment of post-DVT syndrome of lower extremity is mainly non-surgical, including elastic bandage or elastic stocking. For central and mixed types, venous bypass or venous diversion may be considered if there is limited stenosis or occlusion of the deep veins; venous stenting may also be considered.