What is the role of a ventilation perfusion (V/Q) scan in diagnosing pulmonary embolism?

Ventilation-Perfusion (V/Q) Scan in Diagnosing Pulmonary Embolism

V/Q scanning is an established, validated diagnostic test for suspected PE that should be preferentially used in specific patient populations where CTPA is contraindicated or poses excessive risk, including young women, pregnant patients, those with severe renal failure, and patients with contrast allergy. 1, 2

Primary Role and Clinical Positioning

V/Q scanning serves as a robust alternative to CT pulmonary angiography (CTPA) with strong validation in prospective management outcome studies. 1 The test identifies pulmonary embolism by detecting perfusion defects in lung segments with preserved ventilation (ventilation-perfusion mismatch). 1

Key Advantages Over CTPA

• Lower radiation exposure: Effective dose ~2 mSv compared to 3-10 mSv for CTPA, with significantly reduced breast tissue radiation in young women 1
• Almost no contraindications: No iodinated contrast required, making it safe for patients with contrast allergies, severe renal failure, and hyperthyroidism 1, 2
• Relatively inexpensive with few allergic reactions reported 1

Optimal Patient Selection

V/Q scanning should be the first-line imaging test in:

• Young patients, particularly women (to minimize breast radiation exposure) 1
• Pregnant women 1
• Patients with severe renal failure (contraindication to iodinated contrast) 1
• History of contrast-induced anaphylaxis 1, 2
• Outpatients with low clinical probability and normal chest X-ray 1

Interpretation Framework

Three-Tier Classification System

The European Society of Cardiology recommends a simplified three-tier classification rather than complex probabilistic criteria: 1

1. Normal scan: Excludes PE—safe to withhold anticoagulation 1
2. High-probability scan: Confirms PE in most patients (≥2 mismatched segmental perfusion defects) 1
3. Non-diagnostic/indeterminate scan: Requires further testing 1

Diagnostic Performance

• Normal perfusion scan: Validated in prospective outcome studies as safe for excluding PE, with low event rates during follow-up 1
• High-probability scan: Positive predictive value of 88-92% when ≥1 segmental mismatch is present 1
• Major limitation: Approximately 50% of planar V/Q scans are non-diagnostic, necessitating additional testing 1

Technical Considerations

Planar vs. SPECT Imaging

V/Q SPECT is strongly preferred over planar imaging when available, as it dramatically reduces non-diagnostic results to <3-5% compared to 50% with planar scanning. 1, 3 SPECT provides:

• Binary interpretation ("PE" versus "no PE") 1
• Higher sensitivity and specificity than planar imaging 3
• Accurate diagnosis even with comorbid conditions like COPD 3

Ventilation vs. Perfusion-Only Scanning

While combined ventilation-perfusion imaging is standard, perfusion scanning alone may be acceptable in patients with normal chest X-ray, as any perfusion defect would represent a mismatch. 1 The PISA-PED study demonstrated comparable diagnostic yield using perfusion-only imaging. 1

Critical Pitfalls and Limitations

Clinical Scenarios Where V/Q Scanning May Be Misleading

V/Q scan interpretation is difficult or unreliable in: 1

• Previous pulmonary embolism (unless follow-up scan available showing resolution) 1
• Left heart failure (causes regional perfusion variations) 1
• Chronic obstructive airways disease (local ventilation/perfusion abnormalities from hypoxic vasoconstriction) 1
• Lung fibrosis (patchy unmatched defects) 1
• Proximal lung cancer (vascular occlusion mimicking PE) 1

In these situations, CTPA is preferred as it may provide alternative diagnoses. 1

Integration with Clinical Probability

Pre-test clinical probability assessment is essential for proper V/Q scan interpretation. 2 The European Society of Cardiology emphasizes that:

• High-probability V/Q scans may have insufficient positive predictive value in patients with low clinical probability 1
• Clinical information including Wells score should be provided before scan interpretation 2
• Non-diagnostic scans in patients with low clinical probability have low PE prevalence 2

Management Algorithm for Non-Diagnostic Scans

When V/Q scanning yields non-diagnostic results:

1. Consider compression ultrasonography of lower extremities to detect proximal DVT 2
2. Proceed to CTPA if ultrasound negative and clinical suspicion remains 2
3. Incorporate clinical probability: Low probability patients with non-diagnostic scans may not require further testing 2

Comparison with CTPA

While CTPA has become the dominant imaging modality due to 24/7 availability and low inconclusive rate (3-5%), 1 V/Q scanning remains equally safe in diagnostic algorithms and offers critical advantages in radiation-sensitive populations. 4 The choice between modalities should be driven by patient-specific factors rather than institutional convenience, particularly given CTPA's tendency toward overuse. 1