A 70-year-old woman post-cardiac surgery develops oliguric AKI. Urine output is 180 mL in the past 8 hours. Urine sodium is 8 mEq/L, serum creatinine rose from 0.9 to 2.8 mg/dL in 24 hours. Fractional excretion of sodium (FENa) is 0.4%. The MOST likely category of AKI and appropriate first management step is:
- A Intrinsic AKI (acute tubular necrosis); start continuous renal replacement therapy
- B Pre-renal AKI; cautious fluid challenge with 250–500 mL isotonic saline with response monitoring ✓
- C Post-renal AKI; urgent bladder catheterisation and renal ultrasound
- D Contrast-induced nephropathy; N-acetylcysteine infusion
Explanation
FENa < 1% (here 0.4%) with low urine sodium (< 20 mEq/L) and oliguria in the post-cardiac surgery context is consistent with pre-renal AKI due to reduced renal perfusion (low cardiac output or hypovolaemia). The kidney is avidly retaining sodium (intact tubular function). Appropriate initial management is a cautious fluid challenge (250–500 mL isotonic saline or balanced crystalloid) to restore perfusion, with close monitoring of urine output and haemodynamic response. Post-renal (obstructive) AKI would show bilateral hydronephrosis on ultrasound. ATN typically shows FENa > 2%.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.