A 58-year-old woman presents to the emergency department with acute onset dyspnea and pleuritic chest pain. She returned from a 12-hour flight 3 days ago. On examination, heart rate is 118/min, BP 102/68 mmHg, respiratory rate 24/min, and O₂ saturation 91% on room air. Unilateral leg swelling is noted. ECG shows sinus tachycardia and T-wave inversions in leads V1–V4. Chest X-ray is normal. D-dimer is elevated at 2.8 µg/mL. What is the most appropriate next diagnostic step?

Explanation

## Clinical Presentation & Risk Stratification **Key Point:** This patient has a high pretest probability of PE: recent immobilization (long flight), acute dyspnea, pleuritic chest pain, tachycardia, hypoxemia, and unilateral leg swelling (DVT sign). **High-Yield:** The combination of clinical suspicion + elevated D-dimer + ECG changes (right heart strain pattern) mandates immediate imaging confirmation. ## Diagnostic Algorithm for PE ```mermaid flowchart TD A["Clinical suspicion of PE"]:::outcome --> B{"Pretest probability?"}:::decision B -->|"High (Wells > 6 or PERC fail)"| C["Elevated D-dimer?"]:::decision C -->|"Yes"| D["CTPA (gold standard)"]:::action C -->|"No"| E["PE unlikely (but recheck)"]:::outcome B -->|"Intermediate"| F["D-dimer + risk assessment"]:::action F --> G{"D-dimer elevated?"}:::decision G -->|"Yes"| D G -->|"No"| E B -->|"Low"| H["D-dimer alone"]:::action H --> I{"Elevated?"}:::decision I -->|"Yes"| D I -->|"No"| E ``` ## Why CTPA is the Correct Answer 1. **Gold standard imaging** for PE diagnosis in hemodynamically stable patients [cite:Harrison 21e Ch 298] 2. **High sensitivity (94–98%)** and specificity (95–98%)** for central and segmental PE 3. **Fast, non-invasive**, widely available, and can visualize alternative diagnoses 4. **Indicated here** because: - High clinical probability (immobility, leg swelling, acute dyspnea, hypoxemia, ECG changes) - Elevated D-dimer confirms thrombotic risk - Chest X-ray normal (rules out pneumonia, pneumothorax) ## Why Other Options Are Suboptimal | Option | Limitation | |--------|------------| | **V/Q scan** | Requires normal baseline CXR (✓ here), but lower sensitivity (~80%) in intermediate probability; CTPA preferred in modern practice | | **Echocardiography** | Detects RV dilatation/dysfunction (supportive) but cannot confirm PE diagnosis; used for prognosis/risk stratification, not diagnosis | | **Compression ultrasound** | Confirms DVT but does NOT diagnose PE; ~50% of PE patients have no detectable DVT on ultrasound | **Clinical Pearl:** A normal D-dimer in low-risk patients safely excludes PE; however, elevated D-dimer + high clinical suspicion = proceed to CTPA without delay. **High-Yield:** PERC criteria (Pulmonary Embolism Rule-out Criteria) can safely exclude PE in low-risk patients without imaging; this patient fails PERC (hypoxemia, tachycardia, leg swelling) → imaging mandatory.