A 58-year-old man from Delhi presents to the emergency department with acute onset dyspnea and pleuritic chest pain. He had undergone total knee replacement 10 days ago. On examination, he is tachycardic (HR 112/min), tachypneic (RR 28/min), and hypoxic (SpO₂ 88% on room air). His right calf shows mild swelling and tenderness. Chest X-ray is unremarkable. 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 to confirm the diagnosis?

Explanation

## Clinical Diagnosis: Pulmonary Embolism (PE) **Key Point:** This patient presents with a classic triad of risk factors (recent surgery), clinical presentation (acute dyspnea, pleuritic chest pain, hypoxia), and supportive investigations (elevated D-dimer, ECG changes). ### Why CTPA is the Gold Standard CTPE (Computed Tomography Pulmonary Angiography) is the imaging modality of choice for suspected PE in modern practice because: 1. **High sensitivity and specificity** — >95% for detecting PE, especially central and segmental emboli [cite:Harrison 21e Ch 298] 2. **Rapid acquisition** — critical in acute, unstable patients 3. **Widely available** — present in most tertiary centres in India 4. **Provides alternative diagnoses** — can identify pneumonia, pneumothorax, aortic dissection if PE is ruled out 5. **Risk stratification** — can assess RV strain (RV/LV ratio >0.9 indicates hemodynamic compromise) **High-Yield:** In this case, the elevated D-dimer (>0.5) combined with high clinical suspicion (Wells score or PERC criteria positive) mandates imaging confirmation. CTPA is the standard of care. ### Why Other Options Are Suboptimal | Investigation | Limitation | Current Role | |---|---|---| | **V/Q scan** | Low specificity in abnormal CXR; requires normal baseline; operator-dependent | Reserved for contrast allergy or renal failure (eGFR <30) | | **Pulmonary artery catheterization** | Invasive; reserved for hemodynamic monitoring in shock; diagnostic yield low | Therapeutic/monitoring, not diagnostic | | **Transthoracic echo** | Cannot exclude PE; only shows RV strain (indirect evidence) | Useful for risk stratification (massive PE) but not diagnostic | **Clinical Pearl:** The ECG findings (sinus tachycardia + T-wave inversion in V1–V3) suggest right ventricular strain, consistent with PE, but are non-specific. CTPA is needed for definitive diagnosis. ### Management Algorithm ```mermaid flowchart TD A[Suspected PE: dyspnea + pleuritic pain + risk factor]:::outcome A --> B[Assess clinical probability + D-dimer]:::action B --> C{D-dimer elevated?}:::decision C -->|Yes| D[CTPA]:::action C -->|No| E[PE excluded]:::outcome D --> F{PE confirmed?}:::decision F -->|Yes| G[Anticoagulation ± thrombolysis]:::action F -->|No| H[Investigate alternative diagnosis]:::action ``` **Mnemonic: CTPA Advantages — FAST** - **F**ast acquisition - **A**ccurate (>95% sensitivity) - **S**tratification (RV strain assessment) - **T**ertiary imaging (widely available)