A 58-year-old man with a history of recent orthopedic surgery (tibial fracture fixation 5 days ago) presents to the emergency department with acute onset dyspnea, pleuritic chest pain, and tachycardia. Vital signs: BP 128/82 mmHg, HR 112/min, RR 28/min, SpO₂ 88% on room air. Physical examination reveals unilateral calf swelling and tenderness. Chest X-ray is normal. ECG shows sinus tachycardia with T-wave inversion in leads V1–V3. 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 for pulmonary embolism (PE) based on: - Recent immobilization and orthopedic surgery (major thromboembolism risk factor) - Classic triad: dyspnea, pleuritic chest pain, tachycardia - Unilateral calf swelling (signs of deep vein thrombosis) - Hypoxemia (SpO₂ 88%) - ECG changes consistent with right heart strain (T-wave inversion V1–V3) ## Diagnostic Algorithm for Suspected PE ```mermaid flowchart TD A[Suspected PE with high pretest probability]:::outcome --> B{Hemodynamically stable?}:::decision B -->|Yes| C{Contraindication to CTPA?}:::decision B -->|No| D[Echocardiography + CTPA urgently]:::action C -->|No| E[CTPA is gold standard]:::action C -->|Yes| F[V/Q scan or alternative imaging]:::action E --> G[Diagnosis confirmed/excluded]:::outcome F --> G ``` ## Why CTPA is the Correct Choice **High-Yield:** CTPA (computed tomography pulmonary angiography) is the first-line diagnostic imaging modality for suspected PE in hemodynamically stable patients with: - Normal or near-normal chest X-ray - High clinical suspicion (Wells score ≥4 or PERC rule positive) - No contraindications to contrast (normal renal function assumed) **Clinical Pearl:** The combination of elevated D-dimer, clinical risk factors, and ECG changes indicating right heart strain mandates imaging confirmation. D-dimer alone cannot diagnose PE; it only guides the need for imaging in intermediate-to-high probability cases. **Key Point:** Although this patient is hemodynamically stable (BP normal), the high pretest probability and objective findings (hypoxemia, ECG changes, clinical signs of DVT) warrant immediate CTPA rather than observation or less sensitive modalities. ## Sensitivity & Specificity | Modality | Sensitivity | Specificity | Best Use | |----------|-------------|-------------|----------| | CTPA | 94–98% | 95–98% | First-line for high pretest probability | | V/Q scan | 80–90% | 90–95% | Contraindication to CTPA (renal failure, contrast allergy) | | Echocardiography | 50–60% | 90% | Assess RV dysfunction; not diagnostic for PE | | D-dimer | 95% | 40% | Rule out PE only if negative + low pretest probability | [cite:Harrison 21e Ch 298]