What is Deep Vein Thrombosis (DVT), which veins are most commonly affected, and how does it lead to Pulmonary Embolism (PE)?

Deep Vein Thrombosis: Definition, Pathophysiology, and Complications

Definition of DVT

Deep vein thrombosis (DVT) is the formation of a blood clot (thrombus) in the deep veins of the body, most commonly affecting the deep leg veins including the calf veins, femoral vein, popliteal vein, or the deep veins of the pelvis. 1

• DVT is part of venous thromboembolism (VTE), which encompasses both DVT and pulmonary embolism (PE) 2
• VTE is a blood clot that most frequently starts in the deep veins of the legs or pelvis 2
• DVT affects approximately 0.1% of persons per year and is predominantly a disease of the elderly with a slight male preponderance 1

Veins Most Commonly Affected

The lower extremity deep veins are the primary sites of DVT, specifically the calf veins, popliteal vein, femoral vein, and iliac veins. 2

• The lower extremity system includes the inferior vena cava (IVC), pelvis, iliac, femoral, and popliteal veins 2
• Most DVTs start in the calf veins, and many resolve spontaneously without extending proximally 3
• Upper extremity DVT can occur, particularly involving the superior vena cava (SVC), though this is less common 2
• Other vascular territories can be affected, including the portal vein, mesenteric vein, and splanchnic vasculature 2

Virchow's Triad

Virchow's Triad identifies three fundamental pathophysiologic mechanisms that promote thrombus formation: venous stasis, vascular injury (endothelial damage), and hypercoagulability. 4

Venous Stasis

• Prolonged immobilization during hospitalization is a major contributor to stasis 2
• Direct compression of vessels from tumors or external pressure reduces blood flow 2
• Bed rest for more than 4 days significantly increases stasis risk 2

Vascular Injury

• Surgical procedures cause direct vessel wall damage 2
• Trauma to blood vessels disrupts the endothelial barrier 4
• Central venous access devices (CVADs) can cause endothelial injury 2

Hypercoagulability

• Cancer cells express procoagulants such as tissue factor, creating a hypercoagulable state 2
• Chemotherapeutic regimens increase coagulation risk 2
• Inherited thrombophilias (factor V Leiden, prothrombin 20210 mutation, antithrombin deficiency, protein C/S deficiency) enhance coagulation 5
• Acquired conditions like antiphospholipid syndrome promote hypercoagulability 5

Proximal vs. Distal DVT

Proximal DVT includes thrombosis in the popliteal vein and above (popliteal, femoral, and iliac veins), while distal DVT affects deep veins with the most proximal extent being distal to the popliteal vein. 6

Proximal DVT Characteristics

• The popliteal vein is classified as proximal DVT, not distal DVT 6
• Proximal DVT carries a significantly higher risk of pulmonary embolism 6
• Proximal DVT requires immediate anticoagulation therapy 6
• Recurrence rate after stopping anticoagulation is approximately 10.3 events per 100 person-years 6
• Treatment duration is generally at least 3 months 6

Distal DVT Characteristics

• Distal DVT has a lower risk of pulmonary embolism compared to proximal DVT 6
• Recurrence rate after stopping anticoagulation is approximately 1.9 events per 100 person-years 6
• Thrombi confined to the calf rarely cause leg symptoms or symptomatic PE 3
• Treatment decisions may be individualized based on risk factors including proximity to the popliteal vein, extensive thrombosis, positive D-dimer, active cancer, and history of VTE 6

How DVT Leads to Pulmonary Embolism

DVT leads to PE when a thrombus or part of it breaks free from the vein wall and travels through the venous circulation to the lungs, where it blocks some or all of the pulmonary blood supply. 2

Mechanism of Embolization

• Most patients with acute VTE have thrombosis at both leg and pulmonary sites at the time of diagnosis, even when symptoms are localized 3
• The probability that calf DVT will extend proximally and subsequently cause PE increases with the severity of the initiating prothrombotic stimulus 3
• Proximal DVTs pose substantially higher embolic risk than distal DVTs 6

Clinical Significance

• PE is one of the most common causes of nonsurgical death in hospitalized patients 2
• Approximately 10% of PEs are rapidly fatal, and an additional 5% cause death later despite diagnosis and treatment 3
• About 50% of diagnosed PEs are associated with right ventricular dysfunction, which increases in-hospital mortality approximately 5-fold 3
• An estimated 28,726 hospitalized patients diagnosed with VTE die each year in the USA 2
• In cancer patients, PE increases the risk of death by 2- to 6-fold 2