A 65-year-old woman is evaluated in the emergency department for the sudden onset of short… — USMLE Step 3

A 65-year-old woman is evaluated in the emergency department for the sudden onset of shortness of breath and right-sided pleuritic chest pain that began two hours ago. She underwent a total right knee arthroplasty two weeks ago and has been recovering at home. Her medical history is significant for hypertension, type 2 diabetes mellitus, and chronic kidney disease stage 4, with a baseline serum creatinine of $3.2 \text{ mg/dL}$ . Vital signs show a temperature of $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure of $135/85 \text{ mmHg}$ , heart rate of $112 \text{ beats/min}$ , and respiratory rate of $24 \text{ breaths/min}$ . Oxygen saturation is $90\%$ on room air. Physical examination reveals clear breath sounds bilaterally. Cardiac examination reveals a regular rhythm with tachycardia but no murmurs or gallops. The bilateral lower extremities are symmetric without significant edema or calf tenderness. A chest radiograph is unremarkable. Electrocardiography shows sinus tachycardia. Which of the following is the most appropriate imaging study to establish the diagnosis?

Ventilation-perfusion lung scanAnswer
B
Computed tomography pulmonary angiography
C
Magnetic resonance angiography of the chest
D
Lower extremity venous compression ultrasonography
E
Transthoracic echocardiography

Answer

Ventilation-perfusion lung scan

This patient's presentation of sudden-onset dyspnea, pleuritic chest pain, hypoxia, and tachycardia following recent orthopedic surgery creates a high clinical suspicion for a pulmonary embolism (PE). The definitive initial imaging modality of choice for suspected PE is generally computed tomography pulmonary angiography (CTPA). However, this patient has stage 4 chronic kidney disease with a significantly elevated serum creatinine (

3.2 \text{ mg/dL}

). The administration of intravenous iodinated contrast required for CTPA is contraindicated in this setting due to a high risk of contrast-induced nephropathy. When CTPA is contraindicated, a ventilation-perfusion (V/Q) lung scan is the best alternative to establish the diagnosis, particularly when the patient has a normal baseline chest radiograph (which is necessary to prevent indeterminate V/Q results).

Step-by-Step Solution

Assess the pre-test clinical probability of pulmonary embolism (PE).

The probability is high.

The patient has sudden-onset dyspnea and pleuritic chest pain, tachycardia, a history of major surgery within the last 4 weeks, and no more likely alternative diagnosis, yielding a high Wells score.

Identify the standard diagnostic imaging study for high-probability PE and evaluate for any patient-specific contraindications.

Computed tomography pulmonary angiography (CTPA) is the standard test, but it is contraindicated.

CTPA requires iodinated contrast, which is strictly contraindicated in this patient due to her stage 4 chronic kidney disease (baseline creatinine of 3.2 mg/dL).

Select the most appropriate alternative definitive imaging study.

Ventilation-perfusion (V/Q) lung scan.

A V/Q scan is the diagnostic test of choice for suspected PE when CTPA is contraindicated, provided the patient has a normal chest radiograph (an abnormal radiograph would render the V/Q scan indeterminate).

Key Concept

In patients with a high pre-test probability of a pulmonary embolism who have a contraindication to iodinated contrast (such as severe chronic kidney disease) and a normal chest radiograph, a ventilation-perfusion (V/Q) scan is the diagnostic imaging study of choice.

Estimated Time:1m 15s

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