A 58-year-old man with a 10-year history of type 2 diabetes mellitus presents to the emergency department with acute-onset dyspnea and pleuritic chest pain of 4 hours' duration. He had undergone total hip replacement 5 days ago. On examination, blood pressure is 115/72 mmHg, heart rate 108/min, respiratory rate 24/min, SpO₂ 92% on room air. Unilateral leg swelling is noted on the right. 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 (normal <0.5). What is the most appropriate next investigation?

Explanation

## Clinical Presentation Analysis This patient has a **high pretest probability** for pulmonary embolism (PE): - Recent major surgery (hip replacement 5 days ago) — major risk factor - Acute dyspnea and pleuritic chest pain - Unilateral leg swelling (DVT sign) - Tachycardia and hypoxemia - ECG changes consistent with right heart strain (T-wave inversion V1–V3) - Elevated D-dimer ## Diagnostic Algorithm for Suspected PE ```mermaid flowchart TD A["Suspected PE<br/>High pretest probability"]:::outcome A --> B{"Contraindication to<br/>contrast or renal<br/>insufficiency?"}:::decision B -->|No| C["CTPA<br/>Gold standard<br/>Sensitivity >95%"]:::action B -->|Yes| D["V/Q scan or<br/>alternative imaging"]:::action C --> E["Diagnosis confirmed<br/>or excluded"]:::outcome ``` ## Why CTPA is the Answer **Key Point:** CTPA is the **first-line imaging modality** for suspected PE in patients with: - High clinical probability - Normal or non-diagnostic chest X-ray - Adequate renal function (no contraindication) **High-Yield:** In this case: - D-dimer is positive (supports need for imaging) - Clinical probability is HIGH (recent surgery + leg swelling + hypoxemia + ECG changes) - Chest X-ray is normal (does not exclude PE) - No renal contraindication mentioned - CTPA has >95% sensitivity and specificity for PE [cite:Harrison 21e Ch 298] ## Alternative Imaging | Modality | Use Case | Limitation | |----------|----------|------------| | **CTPA** | High probability, normal CXR, normal renal function | Requires IV contrast | | **V/Q scan** | Renal insufficiency, contrast allergy, pregnancy | Lower sensitivity in abnormal CXR; requires normal baseline CXR | | **Echocardiography** | Hemodynamic assessment, RV dysfunction | Not diagnostic for PE; used for risk stratification | | **Troponin/BNP** | Risk stratification, prognosis | Not diagnostic; supportive only | **Clinical Pearl:** Troponin elevation and BNP elevation in PE indicate **right ventricular strain** and higher mortality risk, but they are **not diagnostic** — imaging is mandatory. ## Rationale for CTPA Over V/Q **Key Point:** CTPA is preferred over V/Q scan when: - Chest X-ray is normal (V/Q requires normal baseline for interpretation) - High clinical probability (V/Q may be non-diagnostic in ~30% of cases) - Rapid diagnosis needed (CTPA faster, more specific)