A 42-year-old woman with severe sepsis from urosepsis is admitted to the ICU. She has been oliguric (urine output 0.3 mL/kg/hr) for 18 hours despite aggressive fluid resuscitation. Serum creatinine has risen from 1.2 to 4.1 mg/dL. Urinalysis shows muddy brown casts, and fractional excretion of sodium (FENa) is 3.2%. Serum potassium is 6.8 mEq/L. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis: ATN in Sepsis-Induced AKI **Key Point:** This patient has established acute tubular necrosis (ATN) with sepsis-induced acute kidney injury (AKI). The muddy brown casts, elevated FENa (>2%), and oliguria despite fluid resuscitation confirm intrinsic renal failure, not prerenal azotaemia. ## Diagnostic Criteria for ATN | Feature | ATN | Prerenal | Postrenal | |---------|-----|---------|----------| | **FENa** | >2% | <1% | Variable | | **Urine casts** | Muddy brown, granular | Hyaline | None | | **Response to fluids** | Poor | Rapid improvement | Depends on obstruction | | **Urine osmolality** | <400 mOsm/kg | >500 mOsm/kg | Variable | ## Management Algorithm for ATN with Hyperkalaemia and Oliguria ```mermaid flowchart TD A[ATN with sepsis + oliguria + K+ 6.8]:::outcome --> B{Life-threatening hyperkalaemia?}:::decision B -->|Yes| C[Immediate K+ lowering agents]:::urgent C --> D[Calcium gluconate IV<br/>Insulin-dextrose IV<br/>Sodium bicarbonate IV]:::action D --> E[Initiate RRT]:::action E --> F[Supportive care + vasopressors<br/>Antibiotic therapy]:::action F --> G[Monitor K+, creatinine, urine output]:::outcome B -->|No| H[RRT without emergency K+ lowering] ``` ## Why RRT is Indicated Now **High-Yield:** RRT is indicated in ATN when: 1. **Oliguria persists despite fluid resuscitation** (>18 hours) 2. **Serum creatinine rises rapidly** (>2-fold increase) 3. **Life-threatening hyperkalaemia** (K+ >6.5 mEq/L with ECG changes) 4. **Severe metabolic acidosis** (pH <7.15) 5. **Pulmonary oedema** (fluid overload unresponsive to diuretics) This patient meets criteria 1, 2, and 3. ## Immediate Management of Hyperkalaemia **Clinical Pearl:** Hyperkalaemia in ATN is an emergency because the kidneys cannot excrete potassium. Immediate interventions shift K+ intracellularly and stabilize the myocardium: 1. **Calcium gluconate 10% (10 mL IV over 2–5 min)** — stabilizes cardiac membrane; does NOT lower K+ but prevents dysrhythmia 2. **Insulin 10 units IV + dextrose 25 g IV** — shifts K+ into cells; onset 10–20 min; duration 4–6 hours 3. **Sodium bicarbonate 50 mEq IV** — alkalinizes serum, shifts K+ intracellularly; less effective in acidosis 4. **RRT** — definitive removal of potassium and correction of acidosis **Warning:** Loop diuretics are contraindicated in oliguric ATN because: - The tubules are damaged and cannot respond to diuretics - Diuretics may worsen dehydration and further reduce glomerular filtration rate - Furosemide will not increase urine output in established ATN ## Why Fluid Resuscitation Alone Is Insufficient **Key Point:** The patient has already received aggressive fluid resuscitation for 18 hours with no improvement in urine output or creatinine. Continuing fluids without RRT will cause: - Pulmonary oedema - Hypertension - Worsening hyperkalaemia (K+ cannot be excreted) - Metabolic acidosis Fluid responsiveness has been exhausted; RRT is now the definitive therapy.