Virchow triad

**Components and examples**

1. **Stasis (abnormal blood flow)**: Prolonged immobilization (long flights, bed rest, post-op), atrial fibrillation (left atrial stasis), heart failure, varicose veins, pregnancy (compression of the IVC/pelvic veins), and hyperviscosity states (polycythemia vera, sickle cell disease). Stasis allows platelets to contact the endothelium and prevents dilution of clotting factors.

2. **Endothelial injury**: Trauma, surgery (especially orthopedic: hip and knee replacement), indwelling catheters, atherosclerotic plaque, vasculitis, and hypertension. Injury exposes subendothelial collagen and tissue factor, which activates platelets and the coagulation cascade. It also reduces endothelial anticoagulant products (prostacyclin, NO, tPA, thrombomodulin). Endothelial injury is the dominant factor in **arterial** and cardiac thrombi (e.g., MI, ulcerated plaques).

3. **Hypercoagulability**: - *Inherited*: Factor V Leiden (most common inherited cause, resistance to activated protein C), prothrombin G20210A mutation, protein C or S deficiency, antithrombin III deficiency (also causes heparin resistance). - *Acquired*: Malignancy (Trousseau syndrome, migratory thrombophlebitis, classically pancreatic adenocarcinoma), antiphospholipid syndrome, nephrotic syndrome (urinary loss of antithrombin III), estrogen use (OCPs, HRT), pregnancy, obesity, smoking, heparin-induced thrombocytopenia, and major trauma/burns.

**Clinical significance** - **DVT**: Unilateral leg swelling, pain, warmth, erythema, and a positive Homan sign (low sensitivity and specificity). Diagnose with Wells score, D-dimer (high negative predictive value, used when pretest probability is low or moderate), and compression duplex ultrasound. - **PE**: Dyspnea, pleuritic chest pain, tachycardia, hypoxia. Diagnose with CT pulmonary angiography (V/Q scan if contrast is contraindicated). - **Treatment**: Anticoagulation with heparin/LMWH or a DOAC (apixaban, rivaroxaban), then warfarin or a DOAC for 3+ months. Use an IVC filter if anticoagulation is contraindicated. - **Prophylaxis**: Early ambulation, compression stockings, intermittent pneumatic compression, and pharmacologic prophylaxis for high-risk patients.

**Pearls for boards** - Rudolf Virchow described the triad in the 1800s. - Know the type of thrombus: venous thrombi are red (fibrin and RBC rich, "red thrombi") and form in areas of stasis. Arterial thrombi are pale (platelet and fibrin rich) and form in areas of high flow and endothelial injury. Lines of Zahn (alternating pale platelet/fibrin layers and dark RBC layers) indicate a thrombus formed *before* death (antemortem), unlike postmortem clots. - Paradoxical embolism: a venous thrombus crosses a patent foramen ovale or ASD into systemic circulation, causing stroke. - A common stem: a long-haul flight or post-op patient develops unilateral calf swelling and then sudden dyspnea. The answer is DVT with PE. - Young patient with recurrent DVT or a family history: think Factor V Leiden. Unexplained DVT in an older patient: screen for occult malignancy.