A 72-year-old woman is evaluated in the hospital for sudden-onset severe dyspnea and right… — USMLE Step 3

A 72-year-old woman is evaluated in the hospital for sudden-onset severe dyspnea and right-sided pleuritic chest pain. She was admitted 3 days ago for a severe exacerbation of chronic obstructive pulmonary disease (COPD) and has been receiving intravenous corticosteroids, bronchodilators, and antibiotics. Her medical history is significant for stage 4 chronic kidney disease with a baseline serum creatinine of $3.4\text{ mg/dL}$ and an estimated glomerular filtration rate (eGFR) of $12\text{ mL/min/1.73 m}^2$ . Temperature is $37.4^\circ\text{C}$ , blood pressure is $118/76\text{ mm Hg}$ , pulse is $115/\text{min}$ , and oxygen saturation is 88% on 4 L/min supplemental oxygen. Physical examination reveals a prolonged expiratory phase with diffuse wheezing. The right calf is visibly swollen and tender to palpation compared to the left. A chest radiograph demonstrates hyperinflation and flattened diaphragms but no new focal consolidations. An electrocardiogram shows sinus tachycardia. Which of the following is the most appropriate next step in the diagnostic workup of this patient's acute deterioration?

A
Computed tomography pulmonary angiography
B
Plasma D-dimer assay
C
Transthoracic echocardiography
Lower extremity venous duplex ultrasonographyAnswer
E
Ventilation-perfusion (V/Q) scan

Answer

The most appropriate next step is lower extremity venous duplex ultrasonography.

Lower extremity venous duplex ultrasonography is the most appropriate next step. The patient has a high pre-test probability for a pulmonary embolism (PE), necessitating definitive imaging. The gold standard, CTPA, is contraindicated due to her severe chronic kidney disease. The standard alternative, a V/Q scan, is strongly relatively contraindicated because her severe COPD exacerbation will likely result in a non-diagnostic scan. When both primary thoracic imaging modalities are inappropriate, bilateral lower extremity compression ultrasonography is the next best step, especially given her unilateral calf swelling. Demonstrating a proximal DVT confirms venous thromboembolism, providing the necessary indication to initiate systemic anticoagulation without further pulmonary imaging.

Step-by-Step Solution

Calculate the patient's pre-test probability for a pulmonary embolism.

The patient has a high clinical probability based on the modified Wells score (symptoms of DVT, immobilization, alternative diagnosis less likely, tachycardia).

Determining pre-test probability dictates the initial branch of the diagnostic algorithm (definitive imaging versus D-dimer screening).

Evaluate the safety of the gold standard diagnostic test, Computed Tomography Pulmonary Angiography (CTPA).

CTPA is strongly contraindicated.

The patient has stage 4 chronic kidney disease, placing her at an unacceptably high risk for contrast-induced nephropathy and subsequent renal failure.

Assess the viability of the preferred alternative imaging study, the Ventilation-Perfusion (V/Q) scan.

A V/Q scan is highly likely to yield an indeterminate result.

The patient is experiencing a severe COPD exacerbation, which commonly causes matched ventilation-perfusion defects that confound V/Q scan interpretation.

Determine the safest and most effective alternative diagnostic test.

Lower extremity venous duplex ultrasonography is selected.

Since the patient has an actively swollen calf, a positive ultrasound confirming deep vein thrombosis (DVT) establishes the diagnosis of venous thromboembolism, mandating therapeutic anticoagulation and obviating the need for high-risk or low-yield pulmonary imaging.

Key Concept

Diagnostic sequencing for pulmonary embolism in patients with concurrent comorbidities that contraindicate standard imaging modalities.

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