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Difficulty: HardDiagnostic Workup and Test Sequencing

A 46-year-old man is evaluated in the emergency department for the abrupt onset of severe shortness of breath and right-sided chest pain that worsens with deep inspiration. The symptoms began 2 hours ago. He recently returned from a 14-hour cross-country road trip. His medical history is notable for severe persistent asthma, which has been poorly controlled over the last month, and a well-documented anaphylactic reaction to iodinated contrast media that previously required intubation. Vital signs show a temperature of 37.3°C (99.1°F), blood pressure of 135/85 mmHg, pulse of 118/min, and respirations of 26/min. Pulse oximetry is 91% on room air. Physical examination reveals mild expiratory wheezing bilaterally. The right lower extremity is visibly swollen, erythematous, and tender to palpation, with a 4-cm increase in calf circumference compared to the left. An electrocardiogram reveals sinus tachycardia. A portable chest radiograph shows mild hyperinflation but no focal consolidations or pleural effusions. Which of the following is the most appropriate next step in the diagnostic evaluation of this patient?

  1. Compression ultrasonography of the lower extremitiesAnswer
  2. B
    Ventilation-perfusion (V/Q) lung scan
  3. C
    Computed tomography pulmonary angiography (CTPA)
  4. D
    Measurement of plasma D-dimer concentration
  5. E
    Right heart catheterization with pulmonary angiography

Answer

Compression ultrasonography of the lower extremities is the most appropriate next step.
The correct answer is compression ultrasonography of the lower extremities. This patient presents with a high clinical probability of a pulmonary embolism (PE) based on tachycardia, recent prolonged immobilization, and clear clinical signs of a deep vein thrombosis (DVT). While computed tomography pulmonary angiography (CTPA) is the standard imaging modality for suspected PE, it is contraindicated here due to his severe contrast allergy. Although a ventilation-perfusion (V/Q) scan is an alternative, this patient's poorly controlled asthma increases the risk of an indeterminate result due to abnormal airway ventilation. Because venous thromboembolism (VTE) is treated identically whether it is a DVT or a PE (i.e., with systemic anticoagulation), confirming the diagnosis via a safe, non-invasive lower extremity ultrasound in a patient with active leg symptoms effectively completes the diagnostic workup without requiring pulmonary imaging.

Step-by-Step Solution

1
Calculate the pretest probability for pulmonary embolism (PE).
The patient has a high pretest probability (modified Wells score > 4) based on tachycardia, recent prolonged immobilization, and clinical signs of deep vein thrombosis (DVT).
Establishing the pretest probability dictates whether to proceed with diagnostic imaging or a rule-out laboratory test.
2
Evaluate for contraindications to standard initial imaging.
Computed tomography pulmonary angiography (CTPA) is absolutely contraindicated due to the patient's history of contrast-induced anaphylaxis.
Avoiding severe adverse pharmacologic and allergic events is a critical component of patient safety.
3
Select the optimal alternative diagnostic sequence.
Because the patient has overt signs of a DVT (swollen, tender right calf), a lower extremity ultrasound is the most direct diagnostic step.
Confirming a DVT establishes the diagnosis of venous thromboembolism. Because the treatment for DVT and PE is identical (therapeutic anticoagulation), a positive ultrasound precludes the need for complex or risky pulmonary imaging.

Key Concept

In patients with suspected pulmonary embolism who have concurrent clinical signs of deep vein thrombosis, compression ultrasonography of the lower extremities is the most appropriate initial diagnostic step, especially when primary pulmonary imaging is contraindicated or likely to be indeterminate.
Estimated Time:1m 30s
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Diagnostic Workup and Test Sequencing

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