13.2 Severe Rhabdomyolysis & Crush Injury

13.2 Severe Rhabdomyolysis & Crush Injury

Clinical Scenario: A 64-year-old female with hypertension and hyperlipidemia treated with atorvastatin 80 mg daily is admitted to the ICU after being found down on a floor for 18 hours following a fall. She presents with severe gluteal pain, confusion, and dark reddish-brown urine. Vital signs: BP 94/58 mmHg, HR 112 bpm, temperature 38.1°C. Initial labs reveal serum creatine kinase (CK) 68,000 U/L, potassium 6.4 mEq/L, phosphorus 6.2 mg/dL, BUN 54 mg/dL, creatinine 3.4 mg/dL (baseline 0.8 mg/dL), and total calcium 6.8 mg/dL. Urinalysis dipstick is strongly positive for blood, but microscopy reveals only 0–2 red blood cells per high-power field. The acute care nurse practitioner must prioritize immediate cardioprotection, crystalloid expansion, and metabolic surveillance.

Pathophysiology of Muscle Breakdown & Myoglobin Nephrotoxicity

Rhabdomyolysis is a life-threatening syndrome characterized by skeletal muscle necrosis and discharge of toxic intracellular contents into the systemic circulation. Normal myocytes maintain a low cytosolic calcium level via energy-dependent $\text{Na}^+/\text{K}^+\text{-ATPase}$ and $\text{Ca}^{2+}\text{-ATPase}$ pumps. Direct trauma, prolonged compression, or energy exhaustion depletes cellular ATP, inactivating these pumps. Intracellular sodium accumulates, reversing the $\text{Na}^+/\text{Ca}^{2+}$ exchanger and flooding myocytes with calcium. Free cytosolic calcium activates neutral proteases (calpains) and phospholipases, destroying myofibrils and releasing myoglobin, creatine kinase (CK), potassium, phosphate, purines, and thromboplastin.

Pathophysiologic Triad of Myoglobinuric AKI

Filtered myoglobin drives acute tubular necrosis via three distinct pathways:

1. Intrarenal Vasoconstriction: Third-spacing of fluid into injured muscle beds causes intravascular hypovolemia. This activates the renin-angiotensin-aldosterone axis, endothelin-1, and sympathetic tone. Free heme myoglobin also scavenges endothelial nitric oxide (NO), eliminating microvascular vasodilatory reserve.
2. Tubular Obstruction: In the distal nephron, myoglobin concentrates as water is reabsorbed. In acidic urine ($\text{pH} < 5.6$), myoglobin complexes with Tamm-Horsfall protein (uromodulin), forming obstructing pigmented casts.
3. Direct Cytotoxicity: Endocytosed ferrihemate releases free iron ($\text{Fe}^{2+}$), driving Fenton reactions that generate toxic hydroxyl free radicals ($\text{OH}^\bullet$). This induces lipid peroxidation and proximal tubular apoptosis.

Critical Care Etiologies & Risk Factors

• Crush Trauma & Immobilization: Industrial entrapment, building collapse, motor vehicle extrication, prolonged surgical lithotomy, and prolonged immobilization ("found down" after stroke, fall, or intoxication).
• Exertional & Hyperkinetic States: Status epilepticus, delirium tremens, extreme exertion (especially in sickle cell trait), heat stroke, and malignant hyperthermia / neuroleptic malignant syndrome.
• Toxins & Pharmacotherapy: HMG-CoA reductase inhibitors (statins), especially with fibrates or CYP3A4 inhibitors (macrolides, azoles); cocaine, methamphetamine, synthetic cannabinoids, and alcohol.
• Metabolic & Sepsis: Severe hypokalemia ($<2.0\text{ mEq/L}$ impairs exercise hyperemia), severe hypophosphatemia, influenza, Legionella, and septic shock.

Diagnostic Evaluation & Laboratory Kinetics

Diagnostic Distinction: Myoglobin vs. Hemoglobin

In intravascular hemolysis, free hemoglobin binds haptoglobin, depleting it and producing reddish serum. In rhabdomyolysis, rapid myoglobin clearance often leaves plasma clear; this appearance supports but does not prove the distinction from hemoglobinuria. Track serial CK levels every 6 to 12 hours; a plateau or secondary rise prompts reassessment for ongoing muscle injury, including compartment syndrome when clinical findings suggest it.

Resuscitation Protocols & Metabolic Management

1. Aggressive Crystalloid Resuscitation: Early IV hydration is essential to restore renal perfusion and flush tubular casts. Initiate balanced crystalloids (Plasma-Lyte or Lactated Ringer's) or 0.9% normal saline at an individualized starting rate, often 400 mL/hr, titrated to perfusion and urine output.
   • Balanced crystalloids are preferred over large volumes of normal saline, which induces hyperchloremic acidosis that acidifies urine and accelerates cast precipitation. Although Lactated Ringer's contains 4 mEq/L potassium, it does not worsen hyperkalemia compared to normal saline-induced acidemic shifts. Treat potassium>6.0 mEq/L promptly; select fluid according to perfusion, electrolytes and acid–base status.
2. Urine Output Targets: Target 1–3 mL/kg/hr, up to 300 mL/hr, while monitoring perfusion, renal function and overload. Persistent oliguria/anuria warrants reassessment and possible RRT; do not force fluids until CK<5,000 U/L.
3. Cardioprotection & Hyperkalemia: For peaked T waves or QRS widening, immediately administer:
   • Calcium gluconate 1 to 2 g IV over 5 to 10 minutes (or calcium chloride 1 g IV central line).
   • Intracellular shifting: regular insulin 10 units IV with 50 mL D50W, plus nebulized albuterol 10 to 20 mg.
   • Potassium binders (patiromer or sodium zirconium cyclosilicate).
4. Bicarbonate: Routine urine alkalinization is not recommended to prevent AKI. Reserve bicarbonate for a separate systemic indication, with pH, sodium and ionized-calcium monitoring; it can worsen hypocalcemia.
5. Diuretics & RRT: Diuretics and mannitol are not routinely recommended. Indications for emergent renal replacement therapy (RRT) include refractory hyperkalemia ($>6.5\text{ mEq/L}$), severe acidemia ($\text{pH} < 7.15$), refractory volume overload, and uremia. Use intermittent or continuous RRT according to the patient’s clinical status. Myoglobin clearance depends on membrane and technique; no RRT modality has established superiority for rhabdomyolysis.