What are the risks of computed tomography pulmonary angiography (CTPA) in pregnant patients with suspected pulmonary embolism (PE)?

Risks of CTPA in Pregnant Patients with Suspected Pulmonary Embolism

V/Q scanning is safer than CTPA for pregnant patients with suspected PE when the chest X-ray is normal due to significantly lower maternal breast radiation exposure. 1, 2

Radiation Exposure Risks

Maternal Radiation Exposure

• Breast tissue radiation:

  • CTPA: 10-60 mGy
  • V/Q scan: 0.98-1.07 mGy (approximately 95% lower) 1, 2, 3

• Lung tissue radiation:

  • CTPA: 39.5 mGy
  • V/Q scan: 5.7-13.5 mGy (approximately 85% lower) 1, 3

• Effective dose:

  • CTPA: 21 mSv
  • Perfusion scan: 1.04 mSv
  • V/Q scan: 1.29 mSv 3

Fetal Radiation Exposure

• Fetal radiation exposure is minimal with both V/Q scan and CTPA when proper shielding is used 2
• For early pregnancy (first two trimesters), CTPA actually results in 50-97% lower fetal doses 4
• For gestational age >6 months, V/Q SPECT leads to approximately 15% lower fetal dose 4

Cancer Risk

• Using BEIR VII risk models, a 25-year-old woman undergoing CTPA has a calculated lifetime relative risk of:
  • Breast cancer: 1.011
  • Lung cancer: 1.022 1
• However, precise quantification of cancer risk from low radiation levels is currently not possible 1

Diagnostic Accuracy and Utility

Non-diagnostic Results

• V/Q scans in pregnant women have a lower rate of non-diagnostic results (4-7%) compared to the general population 2, 5
• CTPA has a non-diagnostic rate of 0.9-36% (median 5.9%) in pregnant patients 6

Negative Predictive Value

• Both modalities have excellent negative predictive values:
  • CTPA: 96-100% (median 100%)
  • V/Q scan: 99-100% (median 100%) 6, 7

Recommended Diagnostic Algorithm

1. Initial evaluation: Chest radiograph (CXR) as the first radiation-associated procedure 1, 2
2. If leg symptoms present: Perform bilateral compression ultrasound (CUS) of lower extremities; if positive, begin anticoagulation without further imaging 2
3. If CXR is normal: Proceed with V/Q scan 1, 2
4. If CXR is abnormal: Proceed with CTPA 1, 2

Important Considerations

• D-dimer testing is not recommended in pregnant women due to poor specificity (15%) and inadequate sensitivity (73%) 2
• Immediate anticoagulation is necessary if PE is confirmed 2
• For patients in whom there is a high clinical suspicion of PE and low risk of bleeding, anticoagulant therapy is recommended while awaiting diagnostic test results 1
• MRA is not recommended to rule out PE in pregnancy 1

Pitfalls to Avoid

• Do not rely solely on D-dimer to exclude PE in pregnant women 2
• Do not perform CT venography as an adjunct to CTPA due to increased radiation exposure 1
• Do not assume that V/Q scan is always the safer option for the fetus - CTPA may be safer in early pregnancy 4
• Do not delay imaging in unstable patients; empiric therapy and alternate diagnostic strategies should be considered 1

The benefits and risks of diagnostic imaging should be discussed with the pregnant woman, acknowledging the limitations and uncertainties of estimated radiation-induced cancer risk while emphasizing the well-understood clinical benefits of accurate diagnosis 1.