In a 38-year-old woman with persistent dyspnea four months after an acute pulmonary embolism treated with three months of anticoagulation, which test should be ordered next: a 6‑minute walk test, CT pulmonary angiography, ventilation‑perfusion (V/Q) lung scan, or pulmonary angiography?

Order a Ventilation-Perfusion (V/Q) Lung Scan

You should order a ventilation-perfusion (V/Q) lung scan now to evaluate for chronic thromboembolic pulmonary disease (CTEPD) or chronic thromboembolic pulmonary hypertension (CTEPH) as the cause of her persistent dyspnea. 1

Clinical Reasoning

This patient presents with persistent dyspnea 4 months after acute PE despite completing 3 months of anticoagulation—a presentation consistent with post-PE syndrome. The 2019 ESC Guidelines explicitly recommend that symptomatic patients with mismatched perfusion defects on V/Q lung scan beyond 3 months after acute PE should be referred to a pulmonary hypertension/CTEPH expert centre 1. The guidelines also state to routinely re-evaluate patients 3-6 months after acute PE 1.

Why V/Q Scan Over Other Options?

V/Q scanning is superior to CT pulmonary angiography (CTPA) for detecting CTEPD/CTEPH. The evidence demonstrates that:

• CTPA has lower sensitivity than V/Q imaging for CTEPD and can produce false-negative results even when clinical suspicion is high 2
• A case report specifically documented CTEPD missed by CTPA but detected by V/Q imaging, emphasizing that "a negative CTPA does not necessarily exclude CTEPD" 2
• V/Q SPECT has been validated for PE follow-up and can quantitatively assess treatment effectiveness and identify chronic PE 3

Why Not the Other Tests?

• CT pulmonary angiography: While useful for acute PE, it has inferior sensitivity for chronic thromboembolic disease compared to V/Q scanning 2
• 6-minute walk test: This is a functional assessment tool that may help quantify exercise limitation but does not diagnose the underlying cause of dyspnea 4
• Pulmonary angiography: This is an invasive procedure reserved for confirming CTEPH diagnosis after non-invasive screening and for pre-procedural planning, not as an initial screening test 5

Diagnostic Algorithm

1. Perform V/Q lung scan first as the screening test of choice for CTEPD/CTEPH 1, 2
2. If mismatched perfusion defects are present, proceed with:
   • Echocardiography
   • Natriuretic peptide levels
   • Cardiopulmonary exercise testing 1
3. If these suggest pulmonary hypertension, right heart catheterization is required for definitive CTEPH diagnosis 5
4. CT pulmonary angiography and/or conventional pulmonary angiography are then used to characterize chronic thromboembolic lesions and guide treatment planning 5

Important Clinical Considerations

Up to 20% of patients remain symptomatic with exertional dyspnea after PE even without pulmonary hypertension 6. However, identifying CTEPH is critical because it requires specific management beyond anticoagulation, including pulmonary endarterectomy, medical therapy, or balloon pulmonary angioplasty 5.

Given this patient has a prothrombotic predisposition, she is at higher risk for CTEPH 5. The timing (4 months post-PE, 1 month after stopping anticoagulation) and severity of symptoms (unable to walk up one flight of stairs despite normal vital signs and physical exam) warrant immediate investigation for chronic thromboembolic disease rather than functional testing alone.