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Difficulty: HardInterpreting Imaging Results

A primigravid patient at 26 weeks gestation is evaluated in the obstetric triage unit with new-onset maternal tachycardia, pleuritic chest discomfort, and shortness of breath. Examination reveals asymmetric edema and deep tenderness in the left calf. The clinical team strongly suspects a venous thromboembolic event. Assuming each successive imaging modality yields a negative or nondiagnostic interpretation, and baseline lung imaging is structurally normal, arrange the following radiological studies in the correct step-wise chronological sequence recommended for evaluating this condition.

  1. 1Compression ultrasonography of the lower extremities
  2. 2Chest radiography
  3. 3Ventilation-perfusion (V/Q) scanning
  4. 4Computed tomography pulmonary angiography (CTPA)

Answer

The correct sequence is: Lower extremity compression ultrasonography, followed by Chest radiography, then Ventilation-perfusion (V/Q) scanning, and finally Computed tomography pulmonary angiography (CTPA).
The diagnostic algorithm for suspected pulmonary embolism in a pregnant patient presenting with symptoms of deep vein thrombosis begins with lower extremity compression ultrasonography to potentially avoid radiation. If this is negative, a chest radiograph is interpreted to rule out alternative diagnoses (e.g., pneumonia, pneumothorax) and to guide the choice of subsequent imaging. A normal radiograph interpretation dictates that a ventilation-perfusion (V/Q) scan should follow, as it carries a significantly lower risk of maternal breast radiation compared to CTPA. CTPA is reserved as the final diagnostic step for cases where the V/Q scan is nondiagnostic.

Step-by-Step Solution

1
Evaluate for deep vein thrombosis using a non-radiating modality.
Compression ultrasonography of the lower extremities is selected as the initial step.
The patient has unilateral leg swelling and tenderness. Confirming deep vein thrombosis allows for the initiation of anticoagulation without exposing the fetus or maternal breast to ionizing radiation.
2
Obtain baseline thoracic imaging.
Chest radiography is performed next.
If the ultrasound is negative, a chest radiograph is required to rule out alternative causes of dyspnea (e.g., pneumonia) and to properly interpret or guide the choice of definitive pulmonary imaging.
3
Select the appropriate definitive pulmonary imaging based on radiograph results.
Ventilation-perfusion (V/Q) scanning is chosen over CTPA.
Because the stem specifies that the baseline lung imaging (chest radiograph) is structurally normal, a V/Q scan is preferred to minimize the lifetime risk of radiation-induced breast cancer in the pregnant patient.
4
Resolve the diagnosis if the preceding test is inconclusive.
Computed tomography pulmonary angiography (CTPA) is performed as the final step.
Since the sequence assumes each preceding test is nondiagnostic, an indeterminate V/Q scan mandates a CTPA to definitively rule in or rule out a pulmonary embolism.

Key Concept

Step-wise imaging algorithm for suspected pulmonary embolism in pregnancy
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Interpreting Imaging Results

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