When is a ventilation‑perfusion (V/Q) scan indicated in a first‑trimester pregnant patient with suspected pulmonary embolism, and what protocol should be used?

V/Q Scan in First Trimester Pregnancy

In a first-trimester pregnant patient with suspected pulmonary embolism and a normal chest X-ray, V/Q scan (lung scintigraphy) is the preferred imaging modality over CT pulmonary angiography due to significantly lower fetal radiation exposure during this early gestational period.

Diagnostic Algorithm for First Trimester

The approach should follow this sequence 1:

1. Chest radiography first - Perform CXR as the initial radiation-associated procedure (strong recommendation)

2. If CXR is normal - Proceed directly to V/Q scan (lung scintigraphy) as the preferred test (strong recommendation)

3. If CXR is abnormal - Consider CTPA as an alternative

4. If V/Q scan is nondiagnostic - Perform CTPA rather than invasive angiography

Critical Radiation Considerations Specific to First Trimester

The first trimester is uniquely important for imaging choice. CTPA delivers 50-97% higher fetal radiation dose compared to V/Q scan during the first two trimesters 2. This radiation differential reverses only after 6 months gestation, when V/Q becomes only 15% lower than CTPA 2.

Practical Protocol Details:

• V/Q scan efficiency is highest in first trimester - The pregnancy-adapted YEARS algorithm avoided CTPA in 65% of first-trimester patients versus only 32% in third trimester 3

• Perfusion-only scanning may be considered - Using 99mTc-MAA with reduced activity (54-70 MBq rather than standard 200 MBq) keeps fetal dose within acceptable limits 4

• Ventilation tracers are negligible - 133Xe and 81mKr contribute essentially no fetal dose 4

Additional Diagnostic Considerations

Before any imaging with radiation:

• If signs/symptoms of DVT are present, perform bilateral lower extremity compression ultrasound first 1. A positive result confirms venous thromboembolism and allows treatment initiation without pulmonary imaging

• D-dimer testing is not recommended to exclude PE in pregnancy due to poor specificity 1

Common Pitfalls to Avoid

⚠️ Do not default to CTPA in first trimester - Despite CTPA being the most commonly used modality overall (55.7% in first trimester per registry data 5), this represents clinical practice patterns rather than optimal evidence-based care. The guideline explicitly prioritizes V/Q scan when CXR is normal 1.

⚠️ Do not use standard activity doses - Reduce radiopharmaceutical activities below standard protocols to minimize fetal exposure, particularly for 99mTc-MAA 4

⚠️ Do not delay imaging due to radiation concerns - PE is a leading cause of maternal mortality. The radiation risk from appropriate diagnostic testing is far lower than the mortality risk of untreated PE 1, 6

Patient Counseling

Pregnant patients experience significant distress about fetal health during diagnostic testing 6. Counsel that:

• Both V/Q scan and CTPA are considered safe in pregnancy
• Fetal radiation exposure from V/Q scan in first trimester is minimal
• Missing a PE diagnosis poses far greater risk than the imaging itself
• The choice of test is based on minimizing cumulative radiation while ensuring diagnostic accuracy