A 68-year-old male is admitted to the hospital with acute decompensated heart failure. He … — USMLE Step 3

A 68-year-old male is admitted to the hospital with acute decompensated heart failure. He has a history of ischemic cardiomyopathy with a left ventricular ejection fraction of $25\%$ , stage 2 chronic kidney disease with a baseline serum creatinine of $1.1 \text{ mg/dL}$ , and type 2 diabetes mellitus. Upon admission, he is started on a continuous intravenous infusion of furosemide. On day 3 of hospitalization, his urine output decreases to $25 \text{ mL/hour}$ . His physical examination reveals elevated jugular venous pressure, bibasilar crackles, and $2+$ bilateral pitting lower extremity edema.

Laboratory results on day 3 are as follows:

Parameter	Result	Reference Range
Serum
Sodium	$135 \text{ mEq/L}$	$136 - 145 \text{ mEq/L}$
Potassium	$3.6 \text{ mEq/L}$	$3.5 - 5.0 \text{ mEq/L}$
Chloride	$96 \text{ mEq/L}$	$95 - 105 \text{ mEq/L}$
Bicarbonate	$29 \text{ mEq/L}$	$22 - 28 \text{ mEq/L}$
Blood urea nitrogen (BUN)	$56 \text{ mg/dL}$	$7 - 18 \text{ mg/dL}$
Creatinine	$2.4 \text{ mg/dL}$	$0.6 - 1.2 \text{ mg/dL}$
Urine
Sodium	$54 \text{ mEq/L}$	Variable
Urea nitrogen	$280 \text{ mg/dL}$	Variable
Creatinine	$48 \text{ mg/dL}$	Variable

Given the patient's current pharmacotherapy, calculate the value (in percentage) of the most appropriate fractional excretion index to determine whether his acute kidney injury is intrinsic or hemodynamically mediated.

Answer: 25 %

Answer

The correct calculation is

25\%

. In evaluating acute kidney injury in a patient actively receiving loop diuretics, the Fractional Excretion of Sodium (FENa) is unreliable due to pharmacologically induced natriuresis. The Fractional Excretion of Urea (FEUrea) is the correct index to use, as urea reabsorption occurs predominantly in the proximal tubule and is unaffected by loop diuretics. Using the provided values,

FE_{Urea} = \frac{U_{Urea} \times S_{Cr}}{S_{BUN} \times U_{Cr}} \times 100 = \frac{280 \times 2.4}{56 \times 48} \times 100 = 25\%

. Because

25\%

is less than

35\%

, it correctly points to hemodynamically mediated (prerenal) AKI driven by cardiorenal syndrome.

Step-by-Step Solution

Identify the clinical context and the physiological impact of the patient's medications on laboratory indices.

The patient has acute kidney injury (AKI) secondary to decompensated heart failure (cardiorenal syndrome) and is on a continuous furosemide infusion, which artificially increases urinary sodium excretion.

Loop diuretics inhibit the Na-K-2Cl cotransporter in the thick ascending limb, rendering sodium-based fractional excretion indices inaccurate.

Determine the most appropriate fractional excretion index to use.

Fractional Excretion of Urea (FEUrea) must be used instead of Fractional Excretion of Sodium (FENa).

Unlike sodium, urea is primarily reabsorbed in the proximal tubule. Its reabsorption is coupled with water and is unaffected by loop diuretics, making it a reliable marker of hemodynamically mediated (prerenal) AKI.

Recall the formula for calculating FEUrea.

FE_{Urea} = \frac{U_{Urea} \times S_{Cr}}{S_{Urea} \times U_{Cr}} \times 100

This formula normalizes the urea clearance to the creatinine clearance to determine the fractional excretion.

Substitute the patient's laboratory values into the formula and calculate.

FE_{Urea} = \frac{280 \times 2.4}{56 \times 48} \times 100 = \frac{672}{2688} \times 100 = 0.25 \times 100 = 25\%

Mathematical execution of the clinical index. A value

< 35\%

confirms prerenal azotemia.

Key Concept

In patients receiving diuretic therapy, the Fractional Excretion of Urea (FEUrea) should be utilized instead of the Fractional Excretion of Sodium (FENa) to differentiate between prerenal azotemia and intrinsic acute tubular necrosis. An FEUrea

< 35\%

indicates a prerenal etiology.

Alternative Method

While calculating the BUN-to-Creatinine ratio (

56 / 2.4 \approx 23.3

) provides a rapid bedside clue for prerenal azotemia, it is not a formally derived fractional excretion index. Determining the FEUrea provides diagnostic certainty when differentiating the exact mechanism of AKI in patients with complex hemodynamics on heavy diuresis.

Estimated Time:3m 0s

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