What is the safest diagnostic approach for a pregnant woman at 10 weeks gestation with pleuritic chest pain and suspected pulmonary embolism, and is CT (Computed Tomography) angiography safe in the first trimester?

Diagnostic Approach for Suspected Pulmonary Embolism in Early Pregnancy

Both CT pulmonary angiography (CTPA) and ventilation-perfusion (V/Q) scanning are safe diagnostic options for suspected pulmonary embolism in the first trimester, with CTPA being preferred due to lower fetal radiation exposure during early pregnancy. 1, 2

Initial Evaluation

• Begin with D-dimer testing, as a normal result has the same exclusion value for PE in pregnant women as in non-pregnant patients, potentially avoiding unnecessary radiation exposure 1
• Perform lower extremity compression ultrasound if there are signs/symptoms of DVT, as a positive result warrants anticoagulation without further imaging 1
• Obtain a chest radiograph as the first radiation-associated test due to its minimal radiation exposure (0.01 mSv) 1, 3

Diagnostic Algorithm Based on Trimester

First Trimester (10 weeks):

1. If D-dimer negative: PE excluded (though less likely to be negative in pregnancy)
2. If D-dimer positive and CUS negative:
   • CTPA is preferred over V/Q scan in first trimester as it delivers 50-97% lower fetal radiation dose 2
   • Fetal radiation exposure from CTPA in first trimester: 3-20 μGy (0.003-0.02 mSv) 1
   • Maternal breast radiation is higher with CTPA (10-60 mGy vs 0.98-1.07 mGy with V/Q scan) 3

Second Trimester:

• CTPA remains preferred with fetal dose of 8-77 μGy (0.008-0.08 mSv) 1

Third Trimester:

• V/Q scan may be preferred as it delivers approximately 15% lower fetal dose than CTPA 2
• CTPA fetal dose increases to 51-130 μGy (0.051-0.13 mSv) 1

Safety Considerations

• All diagnostic imaging tests for PE deliver radiation doses well below the 50 mSv threshold considered dangerous for fetal injury 1
• CTPA has equivalent clinical negative predictive value to perfusion scanning (99% vs 100%) 4
• The risk of missing a PE diagnosis (potentially fatal for both mother and fetus) outweighs the minimal radiation risk 1

Radiation Dose Reduction Techniques

• For CTPA: adjust technical factors for maternal size, limit scan length, use dynamic collimation techniques 1
• For V/Q scan: use half the usual administered activity of Tc-99m, increase scan time, perform perfusion scan first and omit ventilation scan if perfusion is normal 1
• Ensure proper hydration to encourage frequent urination and reduce fetal exposure 1

Important Caveats

• Optimize CTPA protocols for pregnancy by using automated bolus triggering, high iodine flux (4.5-6 ml/s), high iodine concentration (350-400 mg I/ml), and clear breathing instructions 1
• If clinical suspicion for PE is high and bleeding risk is low, initiate anticoagulation while awaiting diagnostic results 1, 3
• Avoid pulmonary angiography during pregnancy as it carries significantly higher radiation exposure to the fetus (2.2-3.7 mSv) 1

In summary, while both CTPA and V/Q scanning are safe options with radiation doses well below dangerous thresholds, CTPA is preferred in early pregnancy due to lower fetal radiation exposure, despite higher maternal breast radiation. The diagnostic approach should prioritize minimizing radiation while ensuring accurate diagnosis of this potentially life-threatening condition.