What are the criteria for a Ventilation-Perfusion (VQ) scan to suggest Chronic Thromboembolic Pulmonary Hypertension (CTEPH)?

VQ Scan Criteria for CTEPH Diagnosis

A VQ scan showing at least one segmental or larger mismatched perfusion defect is the key criterion suggesting CTEPH, while a normal or low-probability VQ scan effectively excludes the diagnosis with 90-100% sensitivity and 94-100% specificity. 1

Primary Diagnostic Criteria

The presence of segmental mismatched perfusion defects is the hallmark finding:

• At least 2.5 segmental mismatched perfusion defects provides optimal diagnostic accuracy with 100% sensitivity and 94.7% specificity 2
• The majority (95%) of CTEPH patients demonstrate more than 4 segmental mismatched perfusion defects 2
• These defects appear as wedge-shaped, segmental patterns that are characteristic of chronic thromboembolic disease 1
• The defects must be mismatched (perfusion defect without corresponding ventilation abnormality) to distinguish from other pulmonary conditions 1

Negative Predictive Value (Ruling Out CTEPH)

A normal perfusion scan is the most powerful tool for excluding CTEPH:

• A normal or low-probability VQ scan essentially excludes CTEPH as a diagnostic consideration 1
• This provides 90-100% sensitivity and 94-100% specificity for ruling out the disease 1
• The negative predictive value is so high that patients with normal VQ scans are typically not offered surgery 1

Important Distinctions from Other Conditions

VQ scan patterns differ between CTEPH and other forms of pulmonary hypertension:

• In pulmonary arterial hypertension (PAH), the VQ scan may be normal or show only small peripheral, unmatched, non-segmental defects 1
• No patient with idiopathic PAH should have a high-probability VQ scan 1
• Unmatched perfusion defects can also occur in pulmonary veno-occlusive disease (PVOD), creating a diagnostic caveat 1

Technical Considerations

VQ scintigraphy is superior to CT pulmonary angiography for CTEPH screening:

• VQ scanning demonstrates 96-97% sensitivity compared to only 51% sensitivity for CT angiography 1, 3
• This superiority is especially pronounced in centers without significant CTEPH experience 1
• VQ SPECT (single-photon emission CT) further improves accuracy to 94% compared to planar imaging 1

Critical Clinical Pitfall

Never rely on CT alone to exclude CTEPH:

• CT can appear relatively normal despite significant abnormalities on VQ scintigraphy 3
• Multiple studies document cases where CT missed surgically accessible CTEPH that was detected by VQ scanning 1, 3
• This represents a critical missed opportunity for potentially curative surgical therapy 1, 3

Guideline-Recommended Diagnostic Algorithm

The ESC/ERS guidelines establish a clear pathway:

• VQ scanning is the mandatory first-line imaging modality for all patients with unexplained pulmonary hypertension 1, 3
• If multiple segmental perfusion defects are present, proceed to CT pulmonary angiography for anatomic confirmation and surgical planning 3
• Right heart catheterization provides definitive hemodynamic diagnosis and is required before treatment decisions 3
• Pulmonary angiography is generally required to confirm surgical accessibility of chronic thromboemboli 1

Interpretation Based on Perfusion Alone

While interpretation can be based solely on perfusion defects (without ventilation imaging), this approach provides similar sensitivity but reduced specificity (81.8% versus 94.7% when ventilation is included) 2. Therefore, matched ventilation-perfusion imaging is preferred for optimal diagnostic accuracy.