Thermal injuries result in the release of cytokines and other inflammatory mediators at the site of injury. These mediators have a systemic effect once the burn reaches about 30% of the total body surface area (TBSA). Among the cardiovascular effects, there is a loss of microvascular integrity, causing intravascular fluid to leak into the interstitial space. Peripheral and splanchnic vasoconstriction also occurs, and myocardial contractility has been shown to decrease, possibly due to a release of tumor necrosis factor-α. The end result is burn shock: systemic hypotension and end-organ hypoperfusion. The metabolic rate may also increase up to three times its basal rate, furthering the demand for tissue perfusion. In addition, a concomitant inhalation injury will typically increase resuscitation fluid requirements.

It has been shown that maintaining adequate intravascular volume is important to perfuse vital organs and may also limit the depth/extent of a burn. Use of crystalloid alone, specifically lactated Ringer’s solution (LR), is a safe and effective way to resuscitate burn patients during the first 24 h after injury. Using the Parkland-Baxter formula, 4 mL LR × weight (kg) × %TBSA burn, one can estimate the total volume of lactated Ringer’s solution needed in the first 24 h after thermal injury. Because the increase in capillary permeability is greatest over the first 8 h after injury, half of the volume of lactated Ringer’s solution as calculated by the Parkland-Baxter formula is given over the first 8 hours from the time of injury and the remainder is given over the subsequent 16 hours. An accurate assessment of burn depth and %TBSA is important for the Parkland-Baxter formula to be effective. The “rule of nines” is commonly used as a quick bedside assessment to estimate TBSA: each arm is about 9% of the TBSA, each leg is about 18%, the anterior trunk is about 18%, the posterior trunk is about 18%, and the head is about 9%, with the perineum encompassing the remaining approximately 1%. Children have greater evaporative losses because they have vastly different body proportions and a greater percentage of total body water. Thus, age-appropriate diagrams (e.g., Lund-Browder or Berkow) are more accurate in determining the TBSA burned in children.

Example. A 70-kg adult with a 40% burn requires 11,200 mL (4 × 70 × 40) over 24 h from the time of injury, with 5,600 mL being given in the first 8 h from the time of injury and 5,600 mL being given in the subsequent 16 h.

It should be stressed that any resuscitation formula only provides an estimate for required fluids. Objective parameters such as urine output (0.5 cc/kg for adults and 1.0 cc/kg for children) or blood pressure should be used to adjust fluid needs. Mean arterial pressure (MAP) and urine output are considered to be the most reliable measures of adequate tissue perfusion for burn resuscitation. A MAP maintained above 60 mm Hg typically ensures adequate cerebral perfusion. Hypotension or decreased urine output should be treated by judiciously increasing the resuscitation fluid rate and not by bolusing fluids because boluses will only increase the volume of the capillary leak, resulting in worse edema. There is no consistent evidence to support the use of pulmonary arterial catheter measurements for routine resuscitation. In fact, their use may lead to over-resuscitation and subsequent complications. Nonetheless, invasive hemodynamic monitoring may be necessary in patients with significant premorbid cardiovascular, pulmonary, or renal disease.