Pulmonary Embolism for USMLE Step 1
Pulmonary Embolism for the USMLE Step 1 Exam
Etiology and Risk Factors
- Pulmonary Embolism (PE):
- PE occurs when a thrombus (clot), typically originating from the deep veins of the lower extremities or pelvis (deep vein thrombosis, DVT), dislodges and obstructs a pulmonary artery.
- Virchow’s Triad:
- Three major factors contributing to thrombus formation:
- Endothelial injury: Trauma, surgery, or vascular damage.
- Venous stasis: Immobility, bed rest, or prolonged travel.
- Hypercoagulability: Conditions such as pregnancy, oral contraceptive use, malignancy, and genetic disorders (e.g., factor V Leiden, antiphospholipid syndrome).
- Additional Risk Factors:
- Recent surgery (especially orthopedic procedures)
- Immobilization (e.g., hospitalization, long flights)
- Malignancy (lung, pancreatic, or gastrointestinal cancers)
- Obesity and smoking
Pathophysiology
- Clot Migration:
- A thrombus formed in the deep veins travels through the venous circulation, enters the right side of the heart, and is pumped into the pulmonary arteries where it obstructs blood flow.
- Pulmonary and Hemodynamic Effects:
- The obstruction causes increased pulmonary vascular resistance, which leads to right ventricular strain. This can result in right heart failure in large PEs.
- Ventilation-perfusion (V/Q) mismatch occurs due to blocked blood flow to parts of the lung, resulting in hypoxemia (low blood oxygen).
- Hypoxia and Infarction:
- PE can reduce blood flow to lung tissue, leading to lung infarction in some cases. Hypoxemia also triggers respiratory symptoms like dyspnea.
Clinical Features
- Symptoms:
- Dyspnea: The most common symptom, often sudden in onset.
- Pleuritic chest pain: Sharp pain that worsens with breathing or coughing.
- Cough: Typically nonproductive, though hemoptysis may occur with pulmonary infarction.
- Tachypnea and tachycardia are common signs.
- Severe PE:
- In large or massive PE, patients may present with syncope, hypotension, or signs of shock (e.g., cold extremities, altered mental status). This results from decreased cardiac output due to right heart strain.
Diagnosis
Clinical Probability
- Wells Score:
- A clinical scoring system used to assess the likelihood of PE, based on factors like signs of DVT, tachycardia, recent surgery, and previous DVT/PE.
Diagnostic Testing
- D-dimer:
- A biomarker elevated in cases of PE. A negative D-dimer can rule out PE in low-risk patients, but elevated levels are nonspecific and require further imaging.
- CT Pulmonary Angiography (CTPA):
- The gold standard for diagnosing PE. It visualizes the thrombus in the pulmonary arteries.
- Ventilation-perfusion (V/Q) scan:
- Used when CTPA is contraindicated (e.g., in patients with contrast allergies or renal insufficiency). A V/Q mismatch suggests PE.
- Ultrasound:
- Compression ultrasonography is used to detect DVT, especially in patients with leg symptoms or when CTPA is unavailable.
Management
Anticoagulation
- Initial Treatment:
- Immediate anticoagulation is critical to prevent further clot formation. Common options include:
- Low-molecular-weight heparin (LMWH): Preferred for most cases.
- Unfractionated heparin: Used in unstable patients or those at high risk for bleeding, as it can be rapidly reversed.
- Direct oral anticoagulants (DOACs): Rivaroxaban and apixaban are increasingly used as first-line treatment in stable patients.
- Thrombolysis:
- Considered in massive PE with hemodynamic instability. Thrombolytic agents like alteplase (tPA) dissolve the clot.
Long-Term Management
- Continuation of Anticoagulation:
- Anticoagulation is continued for 3-6 months in provoked cases (e.g., after surgery). Longer durations are recommended for unprovoked PE or patients with ongoing risk factors.
- IVC Filter:
- Used in patients with contraindications to anticoagulation to prevent further clots from traveling to the lungs.
Complications
- Chronic Thromboembolic Pulmonary Hypertension (CTEPH):
- A long-term complication of unresolved PE, leading to persistent pulmonary hypertension and right ventricular failure.
- Right Heart Failure:
- Due to chronic increased pulmonary vascular resistance.
Key Points
- PE is caused by a thrombus obstructing the pulmonary arteries, commonly originating from the deep veins of the legs (DVT).
- Risk factors include immobility, recent surgery, malignancy, and hypercoagulability.
- Common symptoms include sudden dyspnea, pleuritic chest pain, tachypnea, and tachycardia. Severe cases may present with hypotension and shock.
- CT pulmonary angiography (CTPA) is the diagnostic gold standard. D-dimer is used for initial screening, and a V/Q scan is an alternative in selected patients.
- Treatment includes anticoagulation (LMWH or DOACs) and, in severe cases, thrombolysis with agents like alteplase. Long-term anticoagulation may be needed depending on the cause of PE.