Burn injuries affect all ages and have significant implications given the thousands of hospital admissions each year. Anesthesiologists must understand both the physiologic derangements and clinical issues that arise in this patient population.
Burns injuries are categorized according to depth of tissue destruction. A first degree or “superficial” burn damages only the epidermis and can heal without intervention. A second degree or “partial thickness” burn extends down to the dermis sparing hair follicles and sweat glands which allows for skin regeneration. A third degree or “full thickness” burn destroys all layers of the skin including glands and follicles. Fourth degree burns are deep thermal injuries that extend to the fascia, bone, or muscle. Surgical grafting is necessary to prevent scar formation in patients with third and fourth degree burns.
In addition to depth, extent of injury is also important. Total body surface area (TBSA) burned is expressed as a percentage and has prognostic value. TBSA can be estimated for adults based on the “rule of nines” (Figure 179-1). Children have a larger head in proportion to their lower body; therefore, estimation of TBSA is different.
Estimation of total body surface area. (Reproduced with permission from American College of Surgeons. ATLS: Advanced Trauma Life Support for Doctors [Student Course Manual]. 9th ed. ACS; 2012.)
The skin is the largest organ of the body and has many functions. It is a protective barrier against microorganisms, provides many sensory functions, and plays a role in key physiologic processes including thermoregulation, fluid and electrolytes homeostasis, metabolism, and vitamin production. Burns cause derangements of all these functions putting the patient at risk for infection, hypothermia, dehydration, electrolyte imbalance, and many other undesirable complications.
Not only do burns cripple our largest organ, large burns also cause profound physiologic impairment in many other organ systems:
Neurologic—Burn encephalopathy should be considered and a baseline neurologic exam should be completed prior to anesthesia.
Cardiac—Intravascular depletion and circulating myocardial depressant factor depress cardiac output, especially in the first 48 hours of the burn. Once resuscitated, the hypermetabolic state causes increased metabolic and cardiac demand and increased levels of circulating catecholamines. This leads to tachycardia and increased cardiac output.
Hematologic—Hemoconcentration occurs due to leaky capillaries, decrease in albumin, and third spacing fluid initially. Later patients become anemic due to hemolysis and blood loss. The patient may be hypercoagulable related to the direct tissue injury and/or rapid blood transfusion.
Pulmonary—Pulmonary edema can result from capillary leak syndrome. Other changes include decreased functional residual capacity, decreased chest wall and lung compliance, increased alveolar–arteriolar oxygen gradient, and increased minute ventilation (from normal 6 L/min to as much as 40 L/min with large burns).