A 65-year-old woman with COPD presents with acute dyspnea. CTPA is being considered, but she has severe renal impairment (eGFR 18 mL/min/1.73m²) and a history of anaphylaxis to iodinated contrast. Her D-dimer is markedly elevated and clinical suspicion for PE is high. What is the most appropriate next investigation?

## Alternative Imaging When CTPA is Contraindicated **Key Point:** Ventilation-perfusion (V/Q) scan is the appropriate second-line investigation when CTPA cannot be performed due to contrast allergy or severe renal impairment. ### Clinical Scenario Analysis **High-Yield:** This patient has TWO contraindications to contrast-enhanced CTPA: 1. **Severe renal impairment** (eGFR 18) — contrast-induced nephropathy risk 2. **Prior anaphylaxis to iodinated contrast** — absolute contraindication These mandate an alternative imaging strategy despite high clinical suspicion. ### V/Q Scan: Rationale & Performance | Aspect | Detail | |--------|--------| | **Contrast requirement** | Technetium-99m radionuclide (no iodinated contrast) | | **Renal safety** | Safe in renal impairment; no nephrotoxicity | | **Sensitivity** | 80–90% for PE (lower than CTPA) | | **Specificity** | 90–95% when baseline CXR is normal | | **Interpretation** | High probability = PE likely; intermediate = further testing needed | | **Baseline CXR** | ESSENTIAL — abnormalities reduce specificity | **Clinical Pearl:** In this patient, COPD causes baseline CXR abnormalities, which may result in "intermediate probability" V/Q scans. However, with high clinical suspicion + elevated D-dimer + intermediate V/Q, PE is still likely and anticoagulation is justified. ### Why V/Q is Better Than Alternatives Here ```mermaid flowchart TD A[PE suspected + Contrast contraindication]:::outcome --> B{Renal function?}:::decision B -->|Normal| C[Consider MR angiography]:::action B -->|Impaired| D[V/Q scan preferred]:::action D --> E[Normal CXR?]:::decision E -->|Yes| F[High/Low probability interpretation]:::outcome E -->|No| G[Intermediate probability likely]:::outcome G --> H{Clinical suspicion high?}:::decision H -->|Yes| I[Anticoagulate + consider alternative]:::action H -->|No| J[Serial imaging or D-dimer]:::action ``` **Warning:** Do NOT use premedication to "overcome" anaphylaxis history — the risk of repeat reaction outweighs benefit. Premedication (steroids + antihistamines) reduces mild-moderate reactions but is unreliable for prior anaphylaxis. ### Why Other Options Fail **Gadolinium MR angiography:** While gadolinium avoids iodine, it is: - Not widely available for acute PE diagnosis - Contraindicated in eGFR < 30 (nephrogenic systemic fibrosis risk) - Slower acquisition than CTPA - Lower sensitivity than CTPA for subsegmental PE **Serial D-dimer + observation:** Dangerous — high clinical suspicion + elevated D-dimer demand imaging confirmation, not watchful waiting. [cite:Harrison 21e Ch 297]