What is the preferred imaging test, Computed Tomography Pulmonary Angiography (CTPA) or Ventilation-Perfusion scan (VQ scan), for diagnosing pulmonary embolism in an 8-week pregnant patient?

VQ Scan is Preferred Over CTPA for Suspected Pulmonary Embolism in an 8-Week Pregnant Patient

For an 8-week pregnant patient with suspected pulmonary embolism (PE), a Ventilation-Perfusion (VQ) scan is the preferred imaging test over Computed Tomography Pulmonary Angiography (CTPA), particularly if the chest X-ray is normal. 1

Diagnostic Algorithm for PE in Pregnancy

1. Initial Assessment:

   • Chest X-ray (CXR) should be the first radiation-associated procedure 1
   • This helps triage between VQ scan and CTPA

2. Based on CXR Results:

   • Normal CXR: Proceed with VQ scan (strong recommendation) 1
   • Abnormal CXR: Consider CTPA (weak recommendation) 1

3. If VQ scan is non-diagnostic:

   • Proceed to CTPA rather than clinical management alone 1

Rationale for VQ Scan Preference

Radiation Exposure Considerations

• Maternal Breast Radiation:

  • VQ scan: 0.28-0.37 mGy 2
  • CTPA: 44 mGy 2
  • This significant difference in breast radiation exposure is particularly important as it relates to future maternal breast cancer risk 1

• Effective Dose:

  • VQ scan: 1.04-1.29 mSv 2
  • CTPA: 21 mSv 2

• Fetal Radiation:

  • Both modalities deliver doses well below the threshold for fetal risk
  • VQ scan: 0.25-0.40 mGy 2
  • CTPA: 0.46 mGy 2

Diagnostic Performance

• Both modalities have similar false-negative rates:

  • VQ scan: 0.5% (95% CI, 0.2%-1.3%) 1
  • CTPA: 0.4% (95% CI, 0.2%-1.3%) 1

• Non-diagnostic rates in pregnancy:

  • VQ scan: 9.3% 3
  • CTPA: 9.3% 3
  • However, more recent data suggests higher non-diagnostic rates with CTPA in pregnancy 4

Important Clinical Considerations

• PE is uncommon in pregnancy but is a leading cause of maternal mortality, accounting for 20% of maternal deaths in the US 1

• The American Society of Hematology (2018) and American Thoracic Society/Society of Thoracic Radiology (2011) both recommend VQ scanning over CTPA when chest X-ray is normal 1

• Low-dose perfusion scanning (LDQ) has shown excellent negative predictive value (100%) in pregnancy 3

• The positive scan rate for PE in pregnancy is low (3.5%), highlighting the importance of appropriate patient selection for imaging 4

Pitfalls to Avoid

1. Do not skip the chest X-ray: It's essential for determining the optimal subsequent imaging modality 1

2. Do not use D-dimer to exclude PE in pregnancy: D-dimer levels naturally increase during pregnancy and have poor specificity 1

3. Availability considerations: If VQ scanning is not readily available, CTPA is an acceptable alternative 1

4. Do not assume CTPA is always superior: While CTPA is generally more sensitive for PE detection in non-pregnant patients, the radiation exposure considerations in pregnancy shift the risk-benefit balance toward VQ scanning when the chest X-ray is normal 1, 2

5. Do not overlook alternative diagnoses: If the chest X-ray is abnormal, CTPA may be preferred as it can identify alternative causes of symptoms 5