Burns are classified either simply by partial or full thickness destruction or by depth of injury (first to fourth degree) (Table 38-1). In adults, the percentage of body surface affected by burn injury can be estimated by the “rule of nines.” This rule assigns percentages of the total body surface area (BSA) to the anterior and posterior surfaces of the head, limbs, and trunk (Fig. 38-1). As another useful measure, the palm of the adult patient’s hand approximates 1% of total BSA. The Lund-Browder chart is used for estimating the extent of the burn in children because their head contributes a disproportionally large proportion of body surface. Estimation of the total area involved by full thickness burns is useful in determining fluid requirements, need for specialized burn unit care, and expected mortality. However, precise grading of burn wound depth is not essential for the nonburn specialist; in most cases it is reasonable to regard any burn more extensive than superficial as serious, particularly if it covers ≥10% of the body. Adults with extensive or severe burns and most burned children require hospitalization. Commonly accepted criteria for hospital admission are given in Table 38-2.

Burns cause hypovolemia as a result of massive shifts of fluid from intravascular to extravascular compartments and by exudation through injured skin. Hemoconcentration (because of intravascular fluid losses) occurs early, but hemodilution is more likely after appropriate fluid resuscitation. Because the hematocrit can change in either direction, it should be checked regularly during resuscitation with a traditional target being ≥30%. Central venous catheterization is often necessary to deliver the volume of fluid required for adequate resuscitation, and whenever possible such catheters should be placed though unburned skin to minimize the risk of subsequent catheter or wound related sepsis.

Three major strategies (i.e., Evans, Brooke, and Parkland) have been advocated for replacing circulating volume (Table 38-3). A central feature of all plans is to administer substantial volumes of salt-containing (typically Ringers lactate) fluid during the initial 24 h. These strategies differ in their reliance on colloid and free water. The Parkland formula is probably the most widely used fluid-replacement strategy, according to which 4mL of isotonic crystalloid per kilogram per percentage of BSA burn is given in the first day. Customarily, one half of the fluid deficit is replaced in the first 8 h, with the remainder administered over the next 16 h. (For purposes of fluid replacement, burn area is usually capped at 50% BSA.) However, patients with concomitant inhalational injury may require more fluid for successful resuscitation (up to 6 mL/kg/% BSA burn). A traditional fluid replacement target has been to achieve 0.5 to 1 mL/kg/h of urine output. It is now recognized that in a substantial number, perhaps half, of cases too much volume is administered. One clinical clue to too much fluid infusion is a urine output consistently exceeding 2 mL/kg/h. Other risks of excessive fluid administration are limb and abdominal compartment syndromes (ACS). ACS is sufficiently common to warrant monitoring of bladder pressure in patients undergoing large volume resuscitation. All fluid-replacement strategies are associated with the development of tissue edema; however, newer strategies using hypertonic (250 mmol/L) saline solution may achieve hemodynamic resuscitation with less swelling. (There is some evidence that hypertonic resuscitation strategies may increase the risk of renal insufficiency.) Although poorly tested, in mass casualty situations where intravenous therapy may not be feasible, oral rehydrating solution may be useful. Following fluid replacement, vasopressors may be needed to maintain adequate cardiac output, blood pressure, and urine flow. High doses of α-adrenergic agonists (e.g., norepinephrine, phenylephrine) should be avoided, if possible, because of their tendency to decrease nutritive blood flow to already injured tissue.

After 24 h, sodium requirements decline and permeability of leaky vessels decreases. Free water and colloid are then administered in larger quantities to maintain circulating volume and electrolyte balance. After the first 24 h, evaporative water losses may be estimated by the following formula: hourly water loss (in mL) = (25 + % area of burn) × (total
BSA in m²). This formula predicts that a patient with a 25% burn and a 2 m² BSA will lose approximately 100 mL of water per hour. This formula provides the basis for the common recommendation in both the Brooke and Evans fluid strategies to administer roughly 2 L of free water daily. Within the first 12 h of the burn event, colloids offer little advantage over crystalloid because the newly injured vasculature fails to retain even the larger colloid molecules. Despite lack of proven benefit, many physicians still administer albumin to decrease the amount of crystalloid used, especially when the serum albumin concentration falls below 2 gm /dL.