A 35-year-old woman is admitted 4 hours after sustaining a 40% TBSA burn from a house fire. She is alert, HR 128/min, BP 102/68 mmHg, RR 26/min, urine output 0.3 mL/kg/hr. Serum potassium is 6.2 mEq/L (normal 3.5–5.0), creatinine 1.8 mg/dL (baseline 0.9), and urine myoglobin is positive. What is the primary immediate concern, and what is the most appropriate next step in management?

Explanation

## Rhabdomyolysis and Acute Kidney Injury in Severe Burns ### Pathophysiology of Burn-Induced Rhabdomyolysis **Key Point:** Severe burns (>20% TBSA) cause massive muscle cell necrosis, releasing myoglobin and potassium into the circulation. Myoglobin precipitates in renal tubules, causing acute tubular necrosis (ATN) and acute kidney injury (AKI). ### Clinical Indicators of Rhabdomyolysis | Finding | Significance | |---------|-------------| | **Positive urine myoglobin** | Myoglobinuria; risk of AKI | | **Elevated creatinine (1.8 mg/dL)** | Renal dysfunction already developing | | **Elevated potassium (6.2 mEq/L)** | Secondary to cell lysis; hyperkalemia | | **Low urine output (0.3 mL/kg/hr)** | Inadequate renal perfusion; risk of tubular obstruction | | **Oliguria + myoglobinuria** | High risk for acute kidney injury | ### Management of Burn-Induced Rhabdomyolysis **High-Yield:** The cornerstone of preventing myoglobin-induced AKI is **aggressive IV hydration** to: 1. Increase glomerular filtration rate (GFR) and flush myoglobin from tubules 2. Maintain urine output **>1 mL/kg/hr** (higher than standard burn resuscitation target of 0.5 mL/kg/hr) 3. Alkalinize urine to **pH >6.5** by adding sodium bicarbonate to IV fluids - Myoglobin is less soluble in acidic urine and precipitates more readily - Alkaline urine prevents tubular obstruction **Clinical Pearl:** In rhabdomyolysis with myoglobinuria, the Parkland formula alone is often insufficient; fluid requirements may be 1.5–2 times the calculated amount. ### Why Other Options Are Incorrect in This Context **Hyperkalemia (Option B):** While K^+^ is elevated at 6.2 mEq/L, the immediate life threat is **renal failure from myoglobin precipitation**, not the potassium level itself. Calcium gluconate, insulin, and bicarbonate are temporizing measures; they do not prevent AKI. Aggressive hydration addresses both the hyperkalemia (by dilution and enhanced renal clearance) and the primary pathology (myoglobinuria). **Burn Shock (Option C):** The patient's BP (102/68) is acceptable at this early stage (4 hours post-burn). The low urine output is not primarily from shock but from **myoglobin-induced tubular injury**. Increasing Parkland infusion without addressing myoglobinuria will not prevent AKI and may cause pulmonary edema. **Inhalation Injury (Option D):** No history of enclosed-space fire, stridor, or carbonaceous sputum. RR 26/min is elevated but consistent with pain and burn shock, not hypoxia from CO poisoning. ### Parkland Formula Modification for Rhabdomyolysis ``` Standard Parkland: 4 mL × TBSA (%) × BW (kg) over 24 hrs ↓ With myoglobinuria: Increase to 6–10 mL × TBSA (%) × BW (kg) ↓ Target urine output: 1–2 mL/kg/hr (vs. 0.5 mL/kg/hr in uncomplicated burns) ↓ Add sodium bicarbonate to IV fluids to maintain urine pH >6.5 ``` **Mnemonic:** **MYOGLOBIN CRISIS** — **Massive hydration, Urine output >1 mL/kg/hr, Alkalinize urine (pH >6.5), Flush tubules, Prevent AKI**.