A 35-year-old woman is admitted 4 hours after a severe crush injury to her right forearm sustained in a motor vehicle collision. She has severe swelling and pain in the forearm. Compartment pressure measurement shows anterior compartment pressure of 32 mmHg (diastolic BP 70 mmHg). She undergoes emergent fasciotomy. Postoperatively, her urine becomes dark brown (cola-colored), and serum creatinine rises from 0.9 to 2.8 mg/dL within 24 hours. What is the most likely complication, and what is the most appropriate immediate intervention?

Explanation

## Rhabdomyolysis and Acute Kidney Injury Following Compartment Syndrome ### Pathophysiology of Crush Injury Complications **Key Point:** Crush injuries cause massive muscle necrosis with release of intracellular contents (myoglobin, potassium, phosphate, uric acid) into the circulation, leading to **rhabdomyolysis** and **acute tubular necrosis (ATN)**. ### Clinical Features of Rhabdomyolysis | Feature | Finding in This Case | |---------|----------------------| | **Muscle damage** | Crush injury + compartment syndrome | | **Myoglobinuria** | Dark brown (cola-colored) urine | | **Serum myoglobin** | Elevated (not routinely measured) | | **Creatine kinase (CK)** | Markedly elevated (typically > 5,000 U/L) | | **Acute kidney injury** | Creatinine rise from 0.9 to 2.8 mg/dL in 24 hours | | **Mechanism** | Myoglobin precipitation in renal tubules + direct tubular toxicity | **High-Yield:** The dark urine (myoglobinuria) is pathognomonic for rhabdomyolysis. Myoglobin is a small molecule that is freely filtered by the glomerulus and precipitates in acidic urine within the tubules, causing ATN. ### Prevention and Management of Rhabdomyolysis-Induced AKI **Mnemonic:** FLUSH — **F**luids, **L**oop diuretics, **U**rine **S**tatus, **H**yperkalemia management #### 1. Aggressive IV Hydration - **Goal:** Maintain urine output **200–300 mL/hour** (or 1 L/day minimum) - **Fluid choice:** **Normal saline (0.9% NaCl)** — isotonic, maintains intravascular volume without diluting serum osmolality - **Mechanism:** High urine flow dilutes myoglobin concentration in tubules, reducing precipitation and tubular obstruction - **Rate:** Often requires 1–1.5 L/hour initially; titrate to urine output **Warning:** Hypotonic saline (0.45% NaCl) is NOT recommended — it can worsen myoglobinuria by increasing urine flow but diluting serum osmolality, potentially worsening rhabdomyolysis. #### 2. Urine Alkalinization - **Goal:** Maintain urine pH > 6.5 (ideally 7–8) - **Method:** Add **sodium bicarbonate** to IV fluids (target serum HCO~3~^−^ 20–25 mEq/L) - **Mechanism:** Myoglobin is less soluble in acidic urine; alkaline urine prevents precipitation in tubules - **Monitoring:** Check urine dipstick (myoglobin) and urine pH hourly initially #### 3. Loop Diuretics (Furosemide) - **Indication:** If fluid overload develops or urine output plateaus despite hydration - **Mechanism:** Increases urine flow, further diluting myoglobin - **Caution:** Use only *after* adequate hydration; diuretics alone worsen dehydration and AKI #### 4. Hyperkalemia Management - **Risk:** Massive K^+^ release from muscle necrosis - **Interventions:** Calcium gluconate, insulin + glucose, beta-2 agonists, diuretics - **Dialysis:** Reserved for severe hyperkalemia (K^+^ > 6.5 mEq/L) or refractory AKI ### Why Hemodialysis Is NOT First-Line **Clinical Pearl:** Hemodialysis is indicated for: - **Refractory hyperkalemia** (K^+^ > 6.5 mEq/L unresponsive to medical management) - **Severe metabolic acidosis** (pH < 7.1) - **Fluid overload** with pulmonary edema - **Uremia** (BUN > 100 mg/dL, creatinine > 10 mg/dL) In this case, the patient has early AKI (creatinine 2.8 mg/dL) and no mention of hyperkalemia or pulmonary edema. Aggressive medical management with fluids and alkalinization is the appropriate first step; dialysis is reserved for progression or complications. ### Management Algorithm ```mermaid flowchart TD A[Crush injury with rhabdomyolysis]:::outcome --> B[Dark urine + elevated CK + rising creatinine]:::outcome B --> C[Aggressive IV hydration with normal saline]:::action C --> D[Target urine output 200-300 mL/hour]:::action D --> E[Add sodium bicarbonate for urine alkalinization]:::action E --> F{Urine output adequate + pH > 6.5?}:::decision F -->|Yes| G[Continue hydration + monitoring]:::action F -->|No| H[Add furosemide if fluid overload]:::action H --> I{Hyperkalemia or refractory AKI?}:::decision I -->|Yes| J[Hemodialysis]:::urgent I -->|No| K[Continue medical management]:::action ```