What is the most important initial diagnostic procedure for a pregnant patient in the second trimester (2nd trimester) presenting with progressive dyspnea (shortness of breath) and a swollen right lower limb, with clear lungs and a normal chest radiograph (chest X-ray)?

Initial Diagnostic Procedure for Suspected PE in Pregnancy

In a pregnant patient presenting with progressive dyspnea and unilateral leg swelling, perform bilateral Doppler ultrasonography of the lower extremities first (Answer D). 1

Rationale for Lower Extremity Ultrasound

The American Thoracic Society/Society of Thoracic Radiology explicitly recommends that in pregnant women with suspected PE who have signs and symptoms of DVT (such as unilateral leg swelling), bilateral compression ultrasound (CUS) of the lower extremities should be performed first, followed by anticoagulation if positive, thereby avoiding radiation exposure entirely. 1

Key Clinical Features Supporting This Approach

• The patient has a swollen right lower limb, which constitutes a clear sign of possible DVT 1
• Progressive dyspnea over 24 hours in pregnancy raises concern for venous thromboembolism 1
• Clear lungs and normal chest X-ray do not exclude PE but indicate no alternative pulmonary diagnosis 1

Why This Strategy is Optimal

• A positive lower extremity ultrasound warrants immediate anticoagulation treatment and makes further radiation-based testing unnecessary 1
• This approach prioritizes avoidance of fetal radiation exposure when a non-radiation diagnostic test can establish the need for treatment 1
• The prevalence of DVT in pregnant women presenting with leg symptoms ranges from 7-9%, making ultrasound a high-yield initial test 1
• PE remains the leading cause of pregnancy-related maternal death in developed countries, so diagnostic certainty is critical 1, 2

Why Other Options Are Incorrect

D-dimer (Option A)

• The American Thoracic Society explicitly recommends against using D-dimer to exclude PE in pregnancy (weak recommendation, very-low-quality evidence) 1
• D-dimer levels increase physiologically throughout pregnancy, reducing diagnostic utility 1
• While approximately 50% of women have normal D-dimer at 20 weeks gestation, the false-positive rate is unacceptably high 1
• A negative D-dimer cannot reliably exclude PE in this population 3

CT Pulmonary Angiography (Option B)

• CTPA should not be the initial test when signs of DVT are present 1
• The guideline algorithm specifically directs patients with DVT symptoms to ultrasound first 1
• CTPA delivers significantly higher radiation to maternal breast tissue (10-60 mGy) compared to V/Q scanning (0.98-1.07 mGy) 1
• If ultrasound is positive, CTPA becomes unnecessary; if negative, then pulmonary imaging should follow 1

Lung Perfusion Scintigraphy (Option C)

• Like CTPA, perfusion scanning should only be performed after a negative lower extremity ultrasound in patients with DVT symptoms 1
• The American Thoracic Society recommends chest X-ray first (already done and normal in this case), then lung scintigraphy for patients with normal chest X-ray 1
• However, this sequence applies to patients without signs of DVT 1

Clinical Algorithm Summary

For pregnant patients with suspected PE:

1. If signs/symptoms of DVT present (unilateral leg swelling): Perform bilateral lower extremity ultrasound first 1

   • If positive → Start anticoagulation, no further imaging needed 1
   • If negative → Proceed to pulmonary imaging 1

2. If no signs/symptoms of DVT: Perform chest X-ray, then proceed directly to pulmonary vascular imaging 1

   • Normal chest X-ray → Lung scintigraphy preferred over CTPA 1
   • Abnormal chest X-ray → CTPA preferred 1

Critical Safety Context

• All radiation-based diagnostic tests in pregnancy deliver doses well below the 50 mSv (50,000 mGy) safety threshold for fetal harm 1, 3
• However, the risk of missing PE (leading cause of maternal death) far outweighs radiation concerns 1, 2
• The goal is diagnostic certainty while minimizing unnecessary radiation when non-radiation tests can establish treatment need 1