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Difficulty: HardDiagnostic Workup and Test Sequencing

An internal medicine physician is evaluating a 72-year-old man for worsening renal function discovered on routine laboratory monitoring. The patient has a history of resistant hypertension that has been difficult to control despite maximum tolerated doses of amlodipine, metoprolol, and chlorthalidone. Three weeks ago, lisinopril was added to his medication regimen to achieve better blood pressure control. The patient currently feels well and denies any headache, chest pain, shortness of breath, or changes in urination.

Vital signs today show a blood pressure of 182/98 mmHg and a heart rate of 74/min. Physical examination is notable for a high-pitched holosystolic murmur heard best over the right upper quadrant of the abdomen.

Current laboratory studies are compared to baseline from three weeks ago:

ParameterCurrent3 Weeks Ago
Serum sodium139 mEq/L140 mEq/L
Serum potassium4.8 mEq/L4.1 mEq/L
Blood urea nitrogen34 mg/dL16 mg/dL
Serum creatinine2.6 mg/dL1.1 mg/dL

Urinalysis reveals no red blood cells, white blood cells, or casts.

Which of the following is the most appropriate next step in the diagnostic evaluation of this patient's hypertension?

  1. A
    Renal computed tomographic angiography
  2. B
    Radionuclide renal scanning with captopril
  3. Duplex Doppler ultrasonography of the kidneysAnswer
  4. D
    Measurement of plasma aldosterone to renin ratio
  5. E
    Magnetic resonance angiography of the abdomen

Answer

Duplex Doppler ultrasonography of the kidneys is the most appropriate next diagnostic step because it safely evaluates for renal artery stenosis without the use of nephrotoxic contrast in a patient with acute kidney injury.
The correct answer is the non-invasive ultrasound study. This patient's presentation of resistant hypertension, a right-sided abdominal bruit, and a sudden, significant rise in serum creatinine (from 1.1 mg/dL to 2.6 mg/dL) following the initiation of an ACE inhibitor is the classic clinical picture of renovascular hypertension due to bilateral renal artery stenosis. In the setting of bilateral disease, the glomerular filtration rate is highly dependent on angiotensin II-mediated efferent arteriole vasoconstriction. Blocking this mechanism with an ACE inhibitor leads to a precipitous drop in renal function. To establish the diagnosis in a patient with acute kidney injury, the chosen diagnostic modality must not further deteriorate renal function. Duplex Doppler ultrasonography of the kidneys is the most appropriate next step because it is non-invasive, avoids nephrotoxic contrast, and provides reliable evaluation of renal blood flow and arterial waveforms.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and medication history.
The patient has resistant hypertension, a new abdominal bruit, and developed acute kidney injury immediately after the initiation of an ACE inhibitor (lisinopril).
Identifying this classic triad of findings is essential to formulate the correct differential diagnosis of renovascular hypertension.
2
Identify the underlying physiological mechanism causing the acute presentation.
The presentation is highly characteristic of bilateral renal artery stenosis.
In bilateral disease, glomerular filtration is maintained by angiotensin II-mediated efferent arteriole vasoconstriction. ACE inhibitors block this compensation, leading to an acute rise in serum creatinine.
3
Determine the safest and most appropriate initial diagnostic imaging test.
Duplex Doppler ultrasonography is selected.
The patient has acute kidney injury, which contraindicates the use of iodinated contrast (used in CT angiography) and gadolinium (used in MR angiography). Ultrasound safely assesses renal perfusion.

Key Concept

In patients with suspected renal artery stenosis who present with concomitant acute kidney injury, duplex Doppler ultrasonography is the preferred initial diagnostic imaging modality because it avoids the administration of nephrotoxic contrast agents.
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Diagnostic Workup and Test Sequencing

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