What is the approach to diagnosing and managing pulmonary embolism (PE) in a pregnant patient presenting to the Emergency Department (ED) with suspected PE, considering the use of Computed Tomography (CT) scans?

Diagnostic Approach to Pulmonary Embolism in Pregnant Patients in the Emergency Department

Initial Evaluation

In pregnant patients presenting to the ED with suspected PE, begin with a chest X-ray as the first radiation-based imaging study, followed by lower extremity compression ultrasound if DVT symptoms are present, reserving CT pulmonary angiography (CTPA) for patients with abnormal chest X-rays or V/Q scanning for those with normal chest X-rays. 1

Clinical Assessment and D-dimer

• Do not rely on D-dimer testing to exclude PE in pregnancy, particularly in the third trimester where only 4.2% of women with non-high pre-test probability have negative results 1, 2
• D-dimer has limited utility due to physiologic increases during pregnancy, though recent data suggest it may exclude PE in 11.7% of women with non-high pre-test probability 1
• The American Thoracic Society recommends against using D-dimer to exclude PE in pregnant women, prioritizing avoidance of false negatives over reducing unnecessary testing 1

Lower Extremity Ultrasound Strategy

• If signs or symptoms of DVT are present, perform bilateral compression ultrasound (CUS) of lower extremities first 1
• If CUS is positive for proximal DVT, initiate therapeutic anticoagulation immediately without further imaging 1
• If CUS is negative but DVT symptoms persist, proceed to pulmonary vascular imaging 1
• If no DVT symptoms are present, proceed directly to pulmonary vascular imaging rather than CUS, as this is more cost-effective 1

Imaging Algorithm Based on Chest X-ray Results

Normal Chest X-ray

Perform V/Q (perfusion) scintigraphy as the next imaging test rather than CTPA 1

Rationale for V/Q scanning over CTPA:

• Significantly lower maternal breast radiation exposure (0.98-1.07 mGy vs 10-60 mGy with CTPA) 1
• Similar diagnostic accuracy with false-negative rates of 0.5% for V/Q vs 0.4% for CTPA 3
• Excellent diagnostic yield in pregnancy with only 7-8% non-diagnostic rate in recent studies 4, 5
• Negligible increase in maternal lifetime cancer risk 1

Abnormal Chest X-ray

Perform CTPA as the next imaging test rather than V/Q scanning 1, 3

Rationale:

• Higher likelihood of achieving a diagnostic study when chest X-ray is abnormal 1
• Minimizes time delay to diagnosis 1
• Provides alternative diagnoses that may explain symptoms 1
• V/Q scans are less accurate when chest X-ray is abnormal 3

CTPA Protocol Optimization for Pregnancy

Critical Technical Modifications

Standard CTPA protocols must be specifically adapted for pregnancy to avoid the 35.7% non-diagnostic rate seen with routine protocols 6, 7, 5

Essential protocol modifications include: 6, 7

• Automated bolus triggering with short start delays to account for hyperdynamic circulation
• High iodine flux and high concentration contrast (high volume, high rate injection)
• Shallow inspiration or suspended breathing rather than deep inspiration to avoid Valsalva maneuver
• Reduced anatomical coverage along the z-axis to minimize radiation exposure 1, 8
• Low kVp techniques and iterative reconstruction to reduce dose 1

Common Pitfall: Transient Contrast Interruption

• Unopacified blood from the inferior vena cava (IVC) interrupting the contrast bolus is the most common cause of non-diagnostic CTPA in pregnancy, occurring in 39% of studies with standard protocols 7, 5
• This phenomenon is significantly reduced (to 10%) with optimized pregnancy protocols 7
• Physiologic changes during pregnancy create hyperdynamic circulation that reduces pulmonary vascular enhancement 6

Management of Non-diagnostic or Indeterminate Results

If V/Q Scan is Non-diagnostic

Proceed to CTPA rather than clinical management alone or digital subtraction angiography 1, 3

• The American Thoracic Society strongly recommends further diagnostic testing given the potentially fatal consequences of undiagnosed PE 1
• Diagnostic certainty is prioritized over additional radiation exposure in this scenario 1

If CTPA is Indeterminate

Initiate empiric therapeutic anticoagulation immediately while pursuing additional diagnostic testing if clinical suspicion is high and bleeding risk is low 3

• Consider V/Q scanning if chest X-ray was normal 3
• Perform bilateral CUS if not already done to assess for DVT 3
• Do not withhold anticoagulation while awaiting additional testing when clinical suspicion is high 3
• PE accounts for 20% of pregnancy-related maternal deaths in the United States, making the risk-benefit ratio strongly favor treatment in diagnostic uncertainty 3

Radiation Safety Considerations

Fetal Safety

• Both CTPA and V/Q scanning expose the fetus to radiation doses well below the 50-100 mSv threshold for fetal complications 1, 2
• Modern CTPA techniques with proper protocols result in negligible fetal risk 1, 2
• Fetal radiation dose is slightly higher with V/Q scanning but still far below safety thresholds 3

Maternal Breast Radiation

• Modern optimized CTPA exposes maternal breast tissue to 3-4 mGy, resulting in lifetime cancer risk increase of only 1.0003-1.0007 1, 2
• This represents negligible risk and should not preclude CTPA when clinically indicated 1
• V/Q scanning provides significantly lower breast radiation exposure 1, 4

Third Trimester Specific Considerations

• Inconclusive imaging results occur more frequently in late pregnancy (43% of investigations) 1, 2
• D-dimer utility is particularly limited in the third trimester 1, 2
• Ensure pregnancy-optimized CTPA protocols are used to minimize non-diagnostic studies 2, 6

Key Pitfalls to Avoid

1. Using standard CTPA protocols without pregnancy-specific modifications leads to 35.7% non-diagnostic rate 5
2. Relying on D-dimer testing, especially in third trimester 1, 2
3. Performing deep inspiration breath-holds during CTPA, which increases Valsalva risk and contrast interruption 6, 7
4. Withholding anticoagulation with indeterminate results and high clinical suspicion 3
5. Avoiding CTPA entirely due to radiation concerns when it is clinically indicated, as modern techniques pose negligible risk 1, 2