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Difficulty: MediumOrdering Imaging Studies

A 32-year-old woman is evaluated in the emergency department for sudden-onset shortness of breath and right-sided pleuritic chest pain that began 2 hours ago. She has a history of an unprovoked deep vein thrombosis 3 years ago, for which she completed a 6-month course of anticoagulation. Her only current medication is an oral contraceptive pill. Two years ago, she underwent a computed tomography (CT) scan of the abdomen with intravenous contrast for suspected appendicitis and developed severe anaphylaxis requiring intubation. Her temperature is 37.1°C (98.8°F), blood pressure is 114/72 mmHg, heart rate is 118/min, and respiratory rate is 24/min. Pulse oximetry shows 92% on room air. Physical examination reveals clear breath sounds bilaterally and normal heart sounds. The lower extremities are symmetric with no edema, erythema, or tenderness. An electrocardiogram demonstrates sinus tachycardia. A chest radiograph is unremarkable.

Which of the following is the most appropriate next imaging study to order for this patient?

  1. Ventilation-perfusion scanAnswer
  2. B
    Computed tomography angiography of the chest
  3. C
    Non-contrast computed tomography of the chest
  4. D
    Transthoracic echocardiography
  5. E
    Venous ultrasonography of the lower extremities

Answer

The most appropriate imaging study is a ventilation-perfusion (V/Q) scan.
This patient has a high clinical probability of pulmonary embolism (PE), necessitating definitive diagnostic imaging. The standard first-line test is a computed tomography angiography (CTA) of the chest. However, this patient's history of intubation due to contrast-induced anaphylaxis is an absolute contraindication to CTA. In such scenarios, particularly when the baseline chest radiograph is normal, a ventilation-perfusion (V/Q) scan is the most appropriate alternative imaging study.

Step-by-Step Solution

1
Determine the clinical probability of a pulmonary embolism.
The patient has a high pretest probability for PE based on her symptoms, tachycardia, oral contraceptive use, and history of deep vein thrombosis.
Establishing the pretest probability guides whether to order screening labs (like D-dimer) or proceed directly to definitive imaging.
2
Identify any contraindications to standard diagnostic testing.
The patient has an absolute contraindication to intravenous iodinated contrast due to a history of severe anaphylaxis.
Computed tomography angiography (CTA) of the chest is the typical first-line imaging study, but it cannot be used if the patient is allergic to contrast.
3
Select the appropriate alternative imaging modality.
A ventilation-perfusion (V/Q) scan is selected.
In a patient with a high probability of PE and a normal chest radiograph, a V/Q scan is the most accurate alternative imaging study when CTA is contraindicated.

Key Concept

In patients with suspected pulmonary embolism who have a high pretest probability and an absolute contraindication to iodinated contrast, a ventilation-perfusion (V/Q) scan is the diagnostic imaging modality of choice, provided the chest radiograph is normal.
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Ordering Imaging Studies

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