Pulmonary Embolism: A High-Yield USMLE Review

Pulmonary embolism is vascular obstruction of the pulmonary circulation, most often from a lower-extremity deep venous thrombosis that embolizes to the lungs. It is one of the most frequently missed diagnoses in medicine because it has no pathognomonic presentation and mimics many other conditions. Untreated, roughly 30% of patients die; with anticoagulation mortality drops to about 2–8%, which is why PE belongs on the differential for any unexplained dyspnea, chest pain, syncope, or hypoxemia.

Presentation

• Sudden (instantaneous, seconds-to-minutes) onset dyspnea — PE sits at the top of the differential for abrupt-onset breathlessness
• Hypoxemia with a relatively normal (clear) lung exam, because the pathology is vascular obstruction rather than airway or parenchymal disease
• Pleuritic chest pain and tachycardia
• Unilateral leg swelling, pointing to a DVT as the embolic source
• Syncope or, in up to 25% of patients, sudden death as the first manifestation
• May present atypically — always consider PE in COPD exacerbations that don't fit and in older patients with vague complaints such as "weakness"

Diagnosis

• Pre-test probability scoring (Wells, Geneva, PERC) to stratify risk and guide whether to test — the test must follow a hypothesis, not precede it
• D-dimer: useful to rule OUT PE in low-probability patients, but a mildly elevated value in a low pre-test probability patient does not meaningfully raise the risk of PE and can trigger unnecessary imaging
• CT pulmonary angiography (CTPA): the gold standard for diagnosis, reserved for higher pre-test probability (or a positive D-dimer)
• ABG may show hypoxemia with an elevated A-a gradient and an early respiratory alkalosis
• Echocardiography and biomarkers (troponin, BNP) to identify RV dysfunction/strain for risk stratification of confirmed PE

Management

• Anticoagulation is the foundation of treatment for confirmed PE
• Risk-stratify to guide treatment intensity: massive (high-risk) PE — hypotension/obstructive shock/cardiac arrest — requires immediate anticoagulation plus systemic thrombolysis or embolectomy
• Submassive (intermediate-risk) PE — hemodynamically stable but with RV dysfunction and/or elevated troponin or BNP — is treated with anticoagulation, with thrombolysis considered if the patient deteriorates
• Low-risk PE (hemodynamically stable, no RV dysfunction, normal biomarkers) is treated with anticoagulation and may be candidates for outpatient management, using the PESI score to identify them

High-yield

• Hypoxemia + clear lungs = think vascular (PE) — the lungs are often "quiet" on exam despite severe pathology
• Unilateral leg swelling with hypoxemia and clear lungs is the classic PE stem
• Risk factors: immobility, malignancy, recent surgery
• CTPA is the gold standard; D-dimer only rules out PE in low-probability patients
• Massive PE mortality ~25–50%; low-risk PE 30-day mortality <2%; submassive ~5–15%
• Up to 25% of PE patients present with sudden death; ~50% recurrence without treatment
• Elevated troponin has many causes — including PE — so a positive value never closes the case on its own
• PE is a cause of exudative pleural effusion

Pitfalls

• Ordering a D-dimer without a pre-test hypothesis: a mildly elevated D-dimer in a genuinely low-risk patient (Wells 0) drags you toward an unnecessary CT and incidental findings rather than confirming PE
• Being falsely reassured by a normal lung exam — clear breath sounds do not exclude PE; listen to the vital signs and oxygenation
• Anchoring on ACS or another single diagnosis in chest pain and forgetting PE is one of the lethal must-not-miss diagnoses
• Missing atypical presentations — attributing a COPD exacerbation or an older patient's "weakness" to something benign when PE is hiding underneath
• Distinguish from look-alikes: pulmonary edema/pneumonia (bilateral crackles), heart failure (elevated JVP, S3, edema), and pneumothorax (hyperresonance, absent breath sounds)