6.2 Acute Kidney Injury, Rhabdomyolysis / Myoglobinuria, and Fluid-Electrolyte Shifts
Key Takeaways
- Burn-associated acute kidney injury (AKI) displays a distinct biphasic etiology: Early AKI (<48 hours) stems from severe hypovolemic prerenal hypoperfusion, microthrombi, or obstructive pigment nephropathy, whereas Late AKI (>48 hours) is driven by hypermetabolic SIRS, burn sepsis, and cumulative nephrotoxic medications (aminoglycosides, vancomycin, colistin, IV contrast).
- Fluid and electrolyte dynamics undergo dramatic, phase-specific transformations: initial hyponatremia and hyperkalemia (first 24–48 hours) from thermal cellular lysis transition during the post-resuscitation flow phase into profound hypokalemia, hypomagnesemia, hypophosphatemia (refeeding syndrome risk), and late hypernatremia driven by massive open-wound evaporative water loss.
- Succinylcholine is absolutely contraindicated in burn patients beginning 24 to 48 hours post-injury and extending up to 1 to 2 years post-burn due to massive extrajunctional nicotinic acetylcholine receptor upregulation across skeletal muscle membranes, which triggers catastrophic, refractory hyperkalemic cardiac arrest.
- Rhabdomyolysis and myoglobinuria are frequent, severe complications of high-voltage electrical trauma, crush injuries, and deep circumferential burns; free myoglobin precipitates in renal tubules under acidic conditions (forming obstructive ferrihemate casts) and exerts direct cytotoxic and vasoconstrictive damage on tubular epithelium.
- The standard clinical protocol for pigmenturia mandates: (1) immediately increasing crystalloid resuscitation to maintain an adult urine output of 75–100 mL/hr (1.5–2.0 mL/kg/hr in pediatric patients) until macroscopic clearing, (2) urinary alkalinization with sodium bicarbonate to achieve a urine pH > 6.5, and (3) judicious administration of mannitol only after euvolemia has been confirmed.
6.2 Acute Kidney Injury, Rhabdomyolysis / Myoglobinuria, and Fluid-Electrolyte Shifts
Core Knowledge: Renal failure in major burn trauma carries a mortality rate exceeding 50% to 80%. Protecting renal function requires continuous surveillance of glomerular filtration, rapid identification of pigmenturia in high-voltage electrical and deep thermal injuries, aggressive titration of hourly urine output endpoints, and meticulous correction of dynamic electrolyte shifts. A specialized understanding of early versus late Acute Kidney Injury (AKI) and drug contraindications (particularly succinylcholine) is essential for CBRN practice.
1. Biphasic Pathophysiology of Burn Acute Kidney Injury (AKI)
Acute kidney injury in burn patients occurs in two distinct clinical phases, each governed by unique pathophysiological mechanisms:
BIPHASIC SPECTRUM OF BURN ACUTE KIDNEY INJURY
┌────────────────────────────────────────┬────────────────────────────────────────┐
│ EARLY AKI (<48 Hours) │ LATE AKI (>48 Hours) │
├────────────────────────────────────────┼────────────────────────────────────────┤
│ • Primary Mechanism: Prerenal Ischemia │ • Primary Mechanism: Sepsis & Toxins │
│ • Intravascular hypovolemia & low CO │ • Septic acute tubular necrosis (ATN) │
│ • Angiotensin II & Vasopressin spasm │ • Hypermetabolic cytokine storm (TNF-α)│
│ • Renal cortical vasoconstriction │ • Nephrotoxic antimicrobials: │
│ • Intravascular hemolysis / Myoglobin │ Aminoglycosides, Vancomycin, Colistin│
│ • Microvascular fibrin thrombi deposit │ • Systemic endotoxemia & MODS │
│ • Progression: Ischemic ATN if delayed │ • Fluid overload / Creep complications │
└────────────────────────────────────────┴────────────────────────────────────────┘
Early AKI Pathogenesis (<48 Hours)
Early AKI develops as a direct consequence of burn shock. Intense sympathetic stimulation releases high levels of catecholamines, angiotensin II, aldosterone, and endothelin-1, triggering profound afferent arteriolar vasoconstriction in the renal cortex. Glomerular filtration rate (GFR) plummets. If intravascular volume is not restored rapidly with crystalloid resuscitation, prolonged prerenal ischemia leads directly to ischemic Acute Tubular Necrosis (ATN) characterized by tubular cell sloughing, granular cast formation, and loss of tubular concentrating ability.
Late AKI Pathogenesis (>48 Hours)
Late AKI occurs after initial resuscitation has been completed and is primarily driven by systemic sepsis, hyperinflammatory SIRS, and cumulative nephrotoxic insults:
- Septic Vasodilation & Microvascular Shunting: Bacterial endotoxins and inflammatory cytokines induce renal microvascular shunting and bioenergetic tubular shutdown.
- Nephrotoxic Antimicrobials: Burn intensive care frequently requires broad-spectrum empiric and targeted antibiotics (e.g., vancomycin, tobramycin, amikacin, polymyxin B, colistin) to combat multidrug-resistant pathogens (Pseudomonas, Acinetobacter, MRSA). Drug accumulation rapidly induces direct tubular toxicity.
- IV Contrast & Surgical Insults: Repeated CT imaging with iodinated contrast and periods of intraoperative hypotension during extensive burn excision exacerbate tubular injury.
2. Rhabdomyolysis, Myoglobinuria, and Pigment Nephropathy
Rhabdomyolysis occurs when damaged striated skeletal muscle releases intracellular contents—specifically myoglobin, potassium, creatine kinase (CK), and phosphate—into the systemic circulation. It is most frequently encountered in:
- High-voltage electrical injuries ($\ge 1,000\text{ Volts}$) where deep muscular current passage causes massive electrothermal coagulation necrosis.
- Crush injuries associated with structural collapse or blast trauma.
- Deep, circumferential full-thickness extremity burns causing unreleased compartment syndrome.
PATHOGENESIS OF PIGMENT NEPHROPATHY
┌────────────────────────────────────────────────────────────────────────────────────────┐
│ Massive Electrothermal / Deep Muscle Necrosis ──► Systemic Release of Free Myoglobin │
│ │ │
│ ▼ │
│ Unbound Myoglobin Exceeds Plasma Haptoglobin Binding Capacity & Passes Glomerulus │
│ │ │
│ ▼ │
│ Under Acidic Tubular Conditions (pH < 5.6): Myoglobin Precipitates with Tamm-Horsfall │
│ Protein ──► Forms Obstructive Pigmented Ferrihemate / Uromodulin Casts in Distal Tubule│
│ │ │
│ ▼ │
│ Direct Cytotoxicity: Ferrihemate Generates Toxic Free Radicals & Lipid Peroxidation │
│ │ │
│ ▼ │
│ Myoglobin Scavenges Endogenous Nitric Oxide ──► Severe Intrarenal Vasoconstriction │
│ │ │
│ ▼ │
│ ACUTE OLIGURIC / ANURIC RENAL PIGMENTARY FAILURE (ATN) │
└────────────────────────────────────────────────────────────────────────────────────────┘
Clinical Presentation and Urine Screening
- Visual Inspection: Urine displays a classic dark, port-wine, reddish-brown, or "tea-colored" appearance.
- Urinalysis Discrepancy: Dipstick testing is strongly positive for "blood" (due to myoglobin's heme moiety), but microscopic examination reveals few or no intact red blood cells (0–2 RBCs/HPF).
- Serum Biomarkers: Serum Creatine Kinase (CK) levels are markedly elevated (often $>10,000$ to $>100,000\text{ U/L}$). Serum myoglobin is elevated.
Specialized Resuscitation Protocol for Pigmenturia
When macroscopic pigmenturia is observed, standard fluid formulas (Parkland/Brooke) are immediately superseded by the ABA Pigmenturia Resuscitation Protocol:
| Intervention | Target / Dosing | Mechanism & Clinical Rationale |
|---|---|---|
| 1. Increase Crystalloid Resuscitation | Adults: 75–100 mL/hr<br>Pediatrics: 1.5–2.0 mL/kg/hr | Flushes tubular lumen, prevents cast precipitation, and dilutes toxic pigment concentrations. Maintained until urine clears macroscopically. |
| 2. Urinary Alkalinization | Add 50 mEq Sodium Bicarbonate ($NaHCO_3$) per liter of IV fluid (e.g., $D_5W$ or 0.45% NS) | Alkalinizes urine to target urine pH > 6.5. Prevents dissociation of myoglobin into toxic ferrihemate and inhibits cast formation. |
| 3. Osmotic Diuresis (Mannitol) | 12.5 to 25 g IV bolus, repeated cautiously (max 100 g/day) | Induces osmotic diuresis, flushes obstructing tubular casts, and scavenges hydroxyl radicals. Administer only AFTER intravascular volume is fully restored. |
[!WARNING] Never administer loop diuretics (furosemide) or mannitol to an unresuscitated, hypovolemic patient with pigmenturia. Furosemide acidifies tubular fluid, which accelerates myoglobin precipitation, and causes acute volume depletion that worsens renal ischemia.
3. Dynamic Fluid and Electrolyte Shifts Across Burn Phases
Burn trauma causes some of the most extreme electrolyte fluctuations in clinical medicine. Management requires distinguishing between the Resuscitation Phase (0–48 hours) and the Post-Resuscitation / Flow Phase (Days 3 to 14+).
ELECTROLYTE EVOLUTION ACROSS THE BURN CARE SPECTRUM
┌──────────────┬────────────────────────────────────┬────────────────────────────────────┐
│ Electrolyte │ Resuscitation Phase (0–48 Hours) │ Flow / Recovery Phase (Days 3–14+) │
├──────────────┼────────────────────────────────────┼────────────────────────────────────┤
│ Sodium │ HYPONATREMIA │ HYPERNATREMIA │
│ (Na+) │ Third-space shift; massive loss │ Massive evaporative free-water │
│ │ through open blisters and LR fluid │ loss through open wounds (L/day) │
├──────────────┼────────────────────────────────────┼────────────────────────────────────┤
│ Potassium │ HYPERKALEMIA │ HYPOKALEMIA │
│ (K+) │ Thermal cellular lysis; tissue │ Hypermetabolic urinary wasting; │
│ │ necrosis; acidosis; prerenal GFR │ aldosterone; refeeding shift │
├──────────────┼────────────────────────────────────┼────────────────────────────────────┤
│ Phosphate │ Variable / Mild elevation │ PROFOUND HYPOPHOSPHATEMIA │
│ (PO4 3-) │ Cellular release │ Refeeding; ATP synthesis; glycolysis│
├──────────────┼────────────────────────────────────┼────────────────────────────────────┤
│ Calcium │ HYPOCALCEMIA │ HYPOCALCEMIA │
│ (Ca2+) │ Albumin loss (decreased total Ca); │ Must monitor Ionized Calcium (iCa);│
│ │ citrate toxicity from blood │ hypoparathyroidism of trauma │
├──────────────┼────────────────────────────────────┼────────────────────────────────────┤
│ Magnesium │ Normal to slightly elevated │ HYPOMAGNESEMIA │
│ (Mg2+) │ Initial tissue release │ Renal wasting & wound exudate loss │
└──────────────┴────────────────────────────────────┴────────────────────────────────────┘
Sodium Dynamics
- Early Hyponatremia ($Na^+ < 135\text{ mEq/L}$): Driven by massive shift of sodium into the intracellular compartment (sodium-potassium pump failure), exudative blister fluid loss, and dilution from massive crystalloid resuscitation.
- Late Hypernatremia ($Na^+ > 145–150\text{ mEq/L}$): The loss of the epidermal stratum corneum allows pure free water to evaporate at rates of 3,000 to 5,000 mL/day. If insensible free-water losses are not matched with enteral free water or IV $D_5W$, severe hyperosmolar hypernatremic dehydration results, causing cerebral shrinkage and intracranial hemorrhage.
Potassium Dynamics & The Succinylcholine Contraindication
- Early Hyperkalemia ($K^+ > 5.5\text{ mEq/L}$): Direct thermal destruction of trillions of cell membranes dumps intracellular potassium into the extracellular space, compounded by metabolic acidosis.
- Late Hypokalemia ($K^+ < 3.5\text{ mEq/L}$): Hypermetabolic cortisol and aldosterone release stimulates excessive renal potassium excretion in exchange for sodium. Intracellular uptake during enteral refeeding further depletes serum potassium, requiring aggressive potassium supplementation.
THE SUCCINYLCHOLINE HYPERKALEMIC LETHAL CASCADE
┌────────────────────────────────────────────────────────────────────────────────────────┐
│ Thermal Burn Trauma / Skeletal Muscle Denervation / Severe Systemic Inflammation │
│ │ │
│ ▼ │
│ Massive Proliferation & Spread of Extrajunctional Acetylcholine Receptors │
│ (Alpha-7 and Gamma Subunits) Across the ENTIRE Skeletal Muscle Sarcolemma │
│ │ │
│ ▼ │
│ Succinylcholine Administered (Depolarizing Neuromuscular Blocker) │
│ │ │
│ ▼ │
│ Prolonged Depolarization of Billions of Upregulated Extrajunctional ACh Receptors │
│ │ │
│ ▼ │
│ Massive, Uncontrollable Efflux of Intracellular Potassium into the Extracellular Space│
│ │ │
│ ▼ │
│ Serum Potassium Spikes by +5 to +10 mEq/L Within Minutes (Serum K+ > 8–10 mEq/L) │
│ │ │
│ ▼ │
│ PEAKED T-WAVES ──► WIDE QRS ──► SINE WAVE ──► VENTRICULAR FIBRILLATION / ASYSTOLE │
└────────────────────────────────────────────────────────────────────────────────────────┘
[!CAUTION] Succinylcholine is strictly contraindicated starting 24 to 48 hours after burn injury and remains contraindicated for at least 1 to 2 years post-burn (or until full rehabilitation and muscle restoration). For rapid sequence intubation (RSI), always utilize non-depolarizing neuromuscular blockers, such as rocuronium or vecuronium (reversal with sugammadex if necessary).
Calcium, Magnesium, and Phosphate Dynamics
- Hypocalcemia: Total serum calcium drops due to profound hypoalbuminemia (calcium binds to albumin). Clinicians must monitor Ionized Calcium ($iCa$) (target $1.15–1.30\text{ mmol/L}$). Hypocalcemia impairs cardiac contractility and blunts coagulation factor cascades.
- Hypophosphatemia ($PO_4^{3-} < 2.5\text{ mg/dL}$): During the hypermetabolic flow phase and initiation of enteral nutrition, rapid cellular carbohydrate metabolism consumes inorganic phosphate for ATP and 2,3-DPG synthesis. Severe hypophosphatemia ($<1.0\text{ mg/dL}$) induces respiratory muscle failure (failure to wean from mechanical ventilation), diaphragmatic weakness, myocardial depression, and hemolytic anemia.
- Hypomagnesemia ($Mg^{2+} < 1.7\text{ mg/dL}$): Excreted through wound exudate and aminoglycoside-induced tubular wasting. Must be corrected to prevent refractory hypokalemia and ventricular dysrhythmias (Torsades de Pointes).
4. Continuous Renal Replacement Therapy (CRRT) in Burns
When medical management fails to control AKI, Continuous Veno-Venous Hemodiafiltration (CVVHDF) or Hemofiltration (CVVH) is the modality of choice for hemodynamically unstable burn patients.
Burn-Specific CRRT Indications:
- Refractory Uremic Encephalopathy or Pericarditis (BUN $>100\text{ mg/dL}$).
- Severe, Uncorrectable Hyperkalemia ($K^+ > 6.5\text{ mEq/L}$ with ECG changes).
- Intractable Metabolic Acidosis ($ ext{pH} < 7.15$, base deficit $>10$).
- Volume Overload & Fluid Creep unresponsive to diuretics, threatening abdominal compartment syndrome or refractory gas exchange failure.
- Cytokine Clearance / Adsorption: High-volume hemofiltration helps attenuate circulating inflammatory mediator peaks in severe septic shock.
A 32-year-old utility lineman sustains a high-voltage electrical burn (7,200 Volts) to both upper extremities. Upon placement of a Foley catheter, the nurse observes dark reddish-brown urine. Urinalysis reveals strong positive results for occult blood, but microscopic examination demonstrates 0 RBCs per high-power field. The patient weighs 80 kg. What is the immediate priority nursing action?
A 28-year-old female with a 35% TBSA deep partial-thickness burn is admitted to the ICU on post-burn day 5. She suddenly develops acute respiratory distress requiring emergency endotracheal intubation. The provider orders succinylcholine for rapid sequence intubation. Which action by the burn nurse is mandatory?
A patient with a 55% TBSA flame burn is receiving aggressive enteral nutrition on post-burn day 6. Morning laboratory values reveal: Sodium 149 mEq/L, Potassium 3.2 mEq/L, Magnesium 1.4 mg/dL, and Phosphate 1.1 mg/dL. The nurse recognizes that the profound hypophosphatemia is primarily caused by which physiological process?