7.5 Burn Physiology: Calculations (Parkland Formula) and Escharotomy

Burn Shock Pathophysiology and Jackson's Zones of Injury

The pathophysiology of major thermal injury extends far beyond the skin. A burn covering more than 20% of the total body surface area (TBSA) triggers a systemic inflammatory response, leading to a unique state known as burn shock. This is a hybrid of hypovolemic and distributive shock.

At the cellular level, the local injury is described by Jackson's Burn Zones:

1. Zone of Coagulation: The central area of the burn that sustains maximum heat exposure. It is characterized by coagulation necrosis of proteins and complete tissue destruction. This tissue is dead and cannot be salvaged.
2. Zone of Stasis: Surrounding the zone of coagulation, this region suffers from moderate injury, characterized by microvascular congestion, endothelial damage, and decreased tissue perfusion. Cells in this zone are ischemic but viable; however, without prompt and adequate fluid resuscitation, they will progress to necrosis (zone conversion).
3. Zone of Hyperemia: The outermost zone, which exhibits minimal cellular injury and increased blood flow due to the release of inflammatory mediators (vasodilation). This tissue typically recovers fully within days.

Systemically, the release of inflammatory cytokines (such as interleukins and tumor necrosis factor) causes a widespread increase in capillary permeability. This capillary leak syndrome allows water, electrolytes, and large plasma proteins (such as albumin) to escape from the intravascular space into the interstitium, causing massive third-spacing and edema. This process is exacerbated by a decrease in interstitial hydrostatic pressure and a concurrent release of myocardial depressive factors, which impair cardiac output. Burn shock peaks within the first 8 to 24 hours post-injury, necessitating precise fluid resuscitation.

Assessment of Burn Size: Rule of Nines and Lund-Browder

Accurately calculating the percentage of TBSA burned is critical, as it directly guides fluid resuscitation. Only second-degree (partial thickness) and third-degree (full thickness) burns are included in this calculation; first-degree burns (superficial redness without blistering) are excluded.

The Rule of Nines is a rapid tool used to estimate TBSA:

• Adult: Head = 9%, Anterior Torso = 18%, Posterior Torso = 18%, Each Arm = 9%, Each Leg = 18%, Perineum = 1%.
• Infant/Pediatric: Children have larger heads and smaller lower extremities relative to adults. Infant Head = 18%, Anterior Torso = 18%, Posterior Torso = 18%, Each Arm = 9%, Each Leg = 14%, Perineum = 1%. For pediatric patients, a useful rule of thumb is to subtract 1% from the head and add 0.5% to each leg for every year of age over 1 year, until adult proportions are reached.

The Lund-Browder Chart is the gold standard for pediatric burn assessment. It provides detailed, age-adjusted percentages for different body segments, reducing the risk of over-estimating burn size in children. For small, scattered, or irregular burns, the Patient's Palm Method can be used: the patient's hand (including fingers) represents approximately 1% of their TBSA.

Resuscitation Fluid Calculations: The Parkland and Consensus Formulas

Resuscitation is initiated with Lactated Ringer's (LR), as its osmolarity and electrolyte concentrations closely match human plasma, and its lactate content acts as a buffer against metabolic acidosis. Normal saline (0.9% NaCl) is avoided because large volumes can cause hyperchloremic metabolic acidosis.

The Parkland (Consensus) Formula calculates the volume of crystalloid required in the first 24 hours:

Total 24-Hour Fluid (mL) = 4 mL × Weight (kg) × % TBSA

(Note: Current American Burn Association guidelines recommend $2 \text{ mL/kg/%TBSA}$ for adult thermal burns to avoid the complications of "fluid creep" (such as pulmonary edema and abdominal compartment syndrome), but $4 \text{ mL/kg/%TBSA}$ remains the standard for electrical burns to clear myoglobin).

Regardless of the multiplier used, the administration schedule is strict:

• First Half (50%): Administered over the first 8 hours from the time of the burn injury (not from the time the transport team arrives or when the IV is started). If transport begins 3 hours after the injury, the first half of the fluid must be infused over the remaining 5 hours.
• Second Half (50%): Administered over the subsequent 16 hours.

Pediatric Resuscitation and Maintenance Fluids

Pediatric patients (<30 kg) have unique physiological needs. They have limited glycogen stores and are at high risk for hypoglycemia during stress. Therefore, in addition to the calculated Parkland fluid volume, pediatric patients must receive maintenance fluids containing dextrose (e.g., D5LR or D5 1/4 NS) at their calculated maintenance rate.

Resuscitation adequacy is monitored by Urine Output (UOT):

• Adults: Target $0.5 \text{ mL/kg/hr}$ (or approximately 30 to 50 mL/hr).
• Pediatrics: Target $1.0 \text{ mL/kg/hr}$ for children weighing less than 30 kg.
• Electrical Burns: Target 1.0 to 1.5 mL/kg/hr to flush myoglobin through the kidneys, preventing acute tubular necrosis.

Circumferential Burns and Escharotomy

Full-thickness (third-degree) burns result in a tough, leathery, and inelastic tissue layer called eschar. When a full-thickness burn is circumferential around a limb or the chest, it acts as a rigid band. As fluid resuscitation progresses, underlying tissue edema increases tissue pressure. Since the eschar cannot stretch, this creates a compartment syndrome effect.

• Extremity Compromise: Characterized by loss of distal pulses (assessed by Doppler), delayed capillary refill, paresthesias, pain out of proportion to the injury, and cool skin.
• Thoracic Compromise: Characterized by restricted chest wall expansion, resulting in decreased ventilatory compliance, elevated peak inspiratory pressures (PIP) on the ventilator, hypoventilation, hypercapnia, and severe hypoxia.

An emergency escharotomy is indicated if compartment pressures exceed 30 mmHg, distal pulses are lost, or chest wall compliance prevents ventilation. This is a surgical incision through the dead eschar down to the subcutaneous fat, relieving the pressure.

• Limb incisions: Performed along the mid-medial and mid-lateral lines of the extremity. Incisions must avoid key neurovascular structures: the ulnar nerve at the medial elbow (medial epicondyle) and the common peroneal nerve at the lateral neck of the fibula.
• Chest incisions: Performed along the bilateral anterior axillary lines from the 2nd to the 12th rib, connected by transverse incisions at the top of the chest and the costal margin (creating an "H" or "shield" pattern). This releases the chest wall, allowing it to expand during respiration.