Pulmonary Embolism for USMLE Step 2
Pulmonary Embolism for the USMLE Step 2 Exam
Etiology and Risk Factors
- Pulmonary Embolism (PE):
- PE occurs when a thrombus, usually originating from the deep veins of the lower extremities or pelvis (deep vein thrombosis, DVT), travels to the pulmonary arteries, obstructing blood flow.
- Virchow’s Triad:
- PE formation is associated with Virchow’s triad:
- Endothelial injury: Trauma, surgery, or inflammation of the vessel wall.
- Venous stasis: Prolonged immobility (e.g., bed rest, long flights).
- Hypercoagulability: Seen in malignancy, pregnancy, oral contraceptive use, and genetic conditions (e.g., factor V Leiden).
- Additional Risk Factors:
- Recent surgery (especially orthopedic surgery), immobilization, obesity, smoking, and malignancy (e.g., lung or pancreatic cancers).
- Pregnancy, postpartum state, and hormone replacement therapy.
Pathophysiology
- Thrombus Formation and Migration:
- A thrombus forms in the deep veins of the legs or pelvis, dislodges, and travels to the pulmonary circulation, causing an embolic obstruction in the pulmonary arteries.
- Hemodynamic Effects:
- The obstruction leads to increased pulmonary vascular resistance, causing right ventricular strain and potentially resulting in right heart failure or cardiogenic shock in massive PE.
- Ventilation-Perfusion (V/Q) Mismatch:
- Due to the embolus blocking blood flow, a V/Q mismatch occurs, impairing gas exchange and leading to hypoxemia.
Clinical Features
- Symptoms:
- Dyspnea: Sudden onset of shortness of breath, often the most common symptom.
- Pleuritic chest pain: Sharp, localized pain that worsens with deep breathing.
- Cough: Often non-productive; hemoptysis can occur in cases of pulmonary infarction.
- Tachypnea and tachycardia: Frequent findings in PE.
- Leg pain/swelling: Suggestive of DVT as the source of the thrombus.
- Massive PE:
- Presents with syncope, hypotension, or signs of shock due to severe right ventricular dysfunction.
Diagnosis
- Wells Score:
- A clinical scoring system used to estimate PE probability. Key components include:
- Signs of DVT, recent surgery, tachycardia, hemoptysis, and history of DVT/PE.
- D-dimer:
- Elevated in cases of PE, but nonspecific. A negative D-dimer can rule out PE in low-risk patients.
- Imaging:
- CT Pulmonary Angiography (CTPA): The gold standard for diagnosing PE, directly visualizing the thrombus.
- V/Q scan: Used when CTPA is contraindicated (e.g., contrast allergy or renal impairment). Shows a mismatch between ventilation and perfusion.
- Compression ultrasonography: Used to detect DVT, especially in patients with leg symptoms.
Management
Anticoagulation
- Immediate Anticoagulation:
- Low-molecular-weight heparin (LMWH) or unfractionated heparin is initiated as soon as PE is suspected unless contraindicated.
- Direct oral anticoagulants (DOACs): Rivaroxaban or apixaban are first-line agents for stable patients.
- Unfractionated heparin is preferred in patients who are hemodynamically unstable or those undergoing thrombolysis, as it is more easily reversed.
Thrombolysis and Thrombectomy
- Thrombolysis:
- Considered in massive PE with hemodynamic instability. Alteplase (tPA) is the most common thrombolytic agent used.
- Contraindicated in patients with a high risk of bleeding.
- Thrombectomy:
- Surgical or catheter-based thrombectomy may be considered in patients who fail thrombolysis or have contraindications to it.
Long-Term Anticoagulation
- Continuation of Therapy:
- Anticoagulation is continued for 3-6 months in provoked PE (e.g., after surgery), while unprovoked PE may require extended or indefinite anticoagulation.
Complications
- Chronic Thromboembolic Pulmonary Hypertension (CTEPH):
- Occurs in a minority of patients after unresolved PE, leading to progressive pulmonary hypertension and right heart failure.
- Right Heart Failure:
- Acute or chronic right ventricular failure due to increased afterload from the pulmonary embolus.
Prevention
- Prophylaxis:
- High-risk patients (e.g., post-surgical, immobilized) should receive prophylaxis with low-dose heparin, LMWH, or pneumatic compression devices to prevent DVT and PE.
Key Points
- PE results from thrombi originating from the deep veins of the legs or pelvis, with risk factors including immobility, surgery, cancer, and hypercoagulable states.
- Symptoms include sudden dyspnea, pleuritic chest pain, tachypnea, and tachycardia. Severe cases may present with hypotension or shock.
- Diagnosis is confirmed with CT pulmonary angiography (CTPA), while a D-dimer is useful for ruling out PE in low-risk patients.
- Treatment involves anticoagulation (e.g., LMWH, DOACs), with thrombolysis considered in massive PE.
- Long-term anticoagulation is based on whether the PE is provoked or unprovoked, with duration ranging from 3 months to indefinite treatment.