Is a Ventilation-Perfusion (VQ) scan safe for a pregnant patient?

VQ Scan Safety in Pregnancy

VQ scanning is safe and appropriate for pregnant patients with suspected pulmonary embolism, with a very low false-negative rate (0.5%) and significantly lower maternal breast radiation exposure compared to CTPA. 1

When to Use VQ Scan in Pregnancy

The American Thoracic Society recommends VQ scanning for pregnant women with a normal chest X-ray and suspected PE. 1 This is particularly relevant when:

• The chest radiograph is normal (VQ scans are less accurate with abnormal chest X-rays) 1
• VQ scanning is readily available at your institution 1
• You want to minimize maternal breast radiation exposure 1

Diagnostic Performance

VQ scanning performs similarly to CTPA in pregnancy with key advantages:

• False-negative rate of 0.5% versus 0.4% for CTPA - essentially equivalent diagnostic accuracy 1
• For every 1000 pregnant women scanned, approximately 5 pulmonary emboli would be missed with VQ versus 4 with CTPA 1
• Unlike non-pregnant populations, the rate of non-diagnostic scans is NOT appreciably higher with VQ scanning compared to CTPA in pregnant women 1

Radiation Exposure Considerations

The radiation profile favors VQ scanning for maternal tissue but requires nuanced consideration for the fetus:

• VQ scanning exposes the mother to significantly lower breast-absorbed radiation dose compared to CTPA 1
• Fetal radiation dose may be slightly greater with VQ scanning than CTPA, depending on protocol and gestational age 1, 2
• Both modalities deliver fetal radiation doses far below the accepted maximal threshold for fetal radiation exposure 1
• The potential absolute increase in risk of childhood cancer from either modality is very small 1
• CTPA causes 50-97% lower fetal doses during the first two trimesters, while VQ SPECT leads to 15% lower fetal dose after 6 months gestation 2

Clinical Algorithm

When PE is suspected in pregnancy, follow this approach:

1. Obtain chest radiograph first - this informs the choice between CTPA and VQ scanning, and may reveal alternative diagnoses 3

2. Assess for DVT symptoms - if present, perform bilateral lower extremity compression ultrasound 1

3. If chest X-ray is normal and VQ scanning is available, proceed with VQ scan 1

4. If chest X-ray is abnormal, CTPA is preferred 1

5. If indeterminate results occur, initiate empiric anticoagulation while pursuing additional testing 1

Common Pitfalls to Avoid

• Do not rely solely on D-dimer or clinical decision rules - these have not been validated in pregnancy and D-dimer increases physiologically throughout gestation 4
• Do not withhold imaging due to radiation concerns - both VQ and CTPA are safe, and untreated PE is a leading cause of maternal mortality (20% of pregnancy-related deaths) 1
• Do not assume VQ scans will be non-diagnostic more often in pregnancy - this is a misconception from non-pregnant populations 1
• Do not delay anticoagulation if clinical suspicion is high - start therapeutic LMWH while awaiting definitive imaging 1

When CTPA May Be Preferred

CTPA should be considered instead of VQ scanning when:

• Chest radiograph is abnormal 1
• VQ scanning is not readily available 1
• Patient is in first or second trimester and fetal radiation is the primary concern 2
• Alternative diagnoses need to be excluded 3