Oliguria on Postoperative Day 2: Volume or Tubule?
A 74-year-old woman with CKD stage 3b (baseline creatinine 1.6 mg/dL, eGFR 34) and heart failure with preserved ejection fraction is admitted after an open reduction of a hip fracture. Intraoperatively she received 3 L crystalloid for a blood pressure that drifted to 88/50; she also received ketorolac once and contrast for a CT. On POD 2 she is alert, BP 112/64 (no pressors), HR 84, SpO2 96% on room air, weight up 3.2 kg from admission, with trace pedal edema and clear lungs. Urine output has fallen to 0.3 mL/kg/h for 6 hours. Labs: Na 134, K 5.6, Cl 108, HCO3 17, BUN 58, creatinine 2.9 mg/dL, glucose 140. Urine studies: urine Na 48, urine creatinine 62 mg/dL, serum creatinine 2.9, urine osmolality 310, FeNa 3.1%. Bladder scan 40 mL.
1. Calculate the FeUrea and interpret the urine indices in this specific patient. What is the most likely diagnosis, and why are the classic 'prerenal' indices unreliable here?
Read the worked answer
FeUrea = (urine urea × serum creatinine) / (serum urea × urine creatinine) × 100.
Using BUN 58 mg/dL (≈20.7 mg/dL urea nitrogen as BUN; if using urea, multiply by 2.14 - be explicit about units). With BUN 58 and urine urea not given directly, the key teaching point: FeUrea is the preferred index when the patient is on diuretics. A FeUrea <35% suggests prerenal; >50% suggests intrinsic. Here the FeNa of 3.1% is falsely elevated because the patient received diuretics/volume and has a salt-wasting tendency, and FeNa is unreliable with diuretics, contrast, and CKD.
Most likely diagnosis: a mixed picture - volume overload with prerenal physiology plus likely contrast- and NSAID-associated acute tubular injury, superimposed on CKD. The high urine Na, isosthenuric urine osmolality (~310, close to serum), and rising creatinine after contrast plus a single ketorolac dose fit ATI; the 3.2 kg weight gain and edema fit volume excess.
Why indices mislead: diuretics raise FeNa independent of tubular function; CKD baseline blunts concentrating ability; contrast causes a transient natriuresis; and a single NSAID dose in a CKD/HFpEF patient can precipitate AKI via afferent vasoconstriction and prostaglandin inhibition.
Practical approach: interpret urine indices only alongside the clinical volume exam and a fluid challenge/response, and use FeUrea rather than FeNa when diuretics are on board.
2. The patient's K is 5.6 with HCO3 17. Decide whether to treat the hyperkalemia now, how, and whether renal replacement therapy is indicated today. Justify thresholds.
Read the worked answer
Yes - treat now, but it is not an emergency dialysis indication yet. K 5.6 with a non-anion-gap acidosis and ECG changes unknown → get an ECG immediately.
Stepwise management:
- ECG first. If peaked T waves, widened QRS, or sine wave → IV calcium gluconate (membrane stabilization) regardless of number.
- Shift intracellularly: insulin 10 units with 25 g glucose (monitor glucose for 4–6 h, especially with CKD), and/or nebulized albuterol 10–20 mg. These lower K transiently; they do not remove potassium.
- Remove potassium: loop diuretic (she is volume-overloaded with edema - furosemide is attractive) plus a potassium binder (patiromer or sodium zirconium cyclosilicate) for ongoing control. Avoid NSAIDs and potassium-containing fluids; review for hidden sources (salt substitutes, trimethoprim, heparin, succinylcholine).
- Treat the acidosis cautiously: sodium bicarbonate only if severe metabolic acidosis (pH <7.1–7.2) or as an adjunct; it is not a primary hyperkalemia therapy and can worsen volume/alkalosis.
Is RRT indicated today? No - she is hemodynamically stable, making urine (0.3 mL/kg/h), and has no refractory hyperkalemia, severe acidosis, pulmonary edema, or uremic complications. **Refractory hyperkalemia, diuretic-resistant volume overload with hypoxia, pH <7.1, or uremic pericarditis/encephalopathy** would trigger urgent RRT. Plan: reassess K and volume after diuresis, trend creatinine, and involve nephrology if oliguria persists or K remains >6 despite medical therapy.