Physiologic Considerations

Burn injuries present a complex pathophysiologic process with local and systemic effects. Systemic manifestations are typically seen in patients with burn injury at or greater than 30% total body surface area (TBSA) and are mediated by cytokine and catecholamine release. The initial phase, historically called “burn shock,” is due to third-spacing of fluid causing hypovolemia. Jeschke and colleagues demonstrated significantly elevated cardiac output at the time of injury that is sustained throughout the acute phase, but systolic dysfunction has also been documented in a small cohort of children in the perioperative period by transesophageal echocardiogram. Basic science models also demonstrate myocardial depression in the setting of burn injury. ^,

The hypermetabolic phase, initiated 24 hours after injury and mediated by cortisol, catecholamines, and cytokines, is characterized by hyperdynamic circulation, elevated oxygen consumption, and a catabolic state that can persist for years after burn injury. Changes in circulating volumes owing to large intravascular fluid shifts and variations in organ perfusion decrease circulating concentrations and cause resistance to drugs, namely opioids. Upregulation of extrajunctional acetylcholine receptors is hypothesized as the mechanism for well-documented resistance to non-depolarizing neuromuscular blockers and the contraindication to the use of succinylcholine for paralysis. ^, Furthermore, Jeschke has demonstrated infiltrative processes and resultant hepatomegaly in the acute burn phase that may impact drug metabolism.

The short- and long-term sequelae of burn injury portend significant morbidity and mortality, requiring careful attention during all phases of a child’s care. In subsequent sections, the authors discuss the implications of these physiologic changes on the perioperative care of these patients.

Evaluation and Extent of Injury

The initial evaluation and stabilization of a burn-injured child are critical given the propensity for rapid airway compromise, increased oxygen demands, and limited hemodynamic reserve. A full trauma evaluation is warranted because coexisting injury is common. Accurate evaluation of percent TBSA involvement of the partial- and full-thickness burn is the next essential step in planning for resuscitation, stabilization, and even prompt referral to a pediatric burn center. The traditional rule of nines estimation of burn injury is less accurate in children, who have relatively large heads and small limbs compared with adults. Lund and Browder reported an age-based algorithm that considers this difference ( Fig. 1 ). Despite the importance of TBSA estimation, there is still frequent miscalculation of burn injury in very young or obese children, particularly in centers less familiar with pediatric burn care.

Lund-Browder chart. Ant., anterior; FTL, full-thickness loss; Post., posterior; PTL, partial-thickness loss.

( From Lund CC, Browder NC (1944). “The estimation of areas of burns”. Surg Gynecol Obstet. 79: 352–8. Reprinted with permission from the Journal of the American College of Surgeons, formerly Surgery Gynecology & Obstetrics.)

Airway and Breathing

The pediatric anesthesiologist may be called on at any stage of the burn injury for assistance with airway management, whether it is during initial stabilization, during rescue of a failed or lost airway, during surgical procedures, or even during extubation planning. The presence of appropriately sized videolaryngoscopic or fiber-optic equipment should be confirmed before any airway manipulation because burn injuries may introduce profound anatomic changes, which can complicate airway management. High oxygen and metabolic requirements in these vulnerable patients further challenge these clinical situations.

The presence of inhalation injury significantly increases morbidity and mortality and should be an early part of the initial assessment. ^, Facial burns, carbonaceous sputum, singed nasal hairs, and hoarseness are indicators of possible inhalational injury and need for intubation. Serial fiber-optic bronchoscopy may be useful in the identification and monitoring of supraglottic and glottic injury. Soot and erythema in the upper airways indicate that thermal injury to the airway is present, and a scoring system has been proposed that grades and prognosticates inhalation injury and need for intubation. If inhalation injury has occurred, bronchodilators and mucolytic agents have proven useful for stabilization before intubation and may reduce the duration of mechanical ventilation. ^,

Even if inhalation injury is not present, there may be indications for intubation. Burn injury of the face or neck may predispose to airway swelling or compromise as the burn matures. Similarly, capillary leak exacerbated by fluid resuscitation may cause pulmonary edema, decreased compliance, and respiratory failure, which are particularly problematic in younger patients and patients with larger, scald burns. At the authors’ institution, if a large-volume resuscitation is anticipated after the initial evaluation, the team preemptively intubates ahead of respiratory compromise.

Nonetheless, the decision to intubate in the setting of burn injury represents a key area of practice variation between low- and high-volume centers. A review of all patients intubated at Parkland Hospital from 1982 to 2005 demonstrated that a significant number of these patients were extubated within 24 hours of arrival. This study was recently corroborated in a metaanalysis of preburn center care in which 31% of patients were described to be “unnecessarily intubated” before transfer to the burn center. Although neither of these studies addressed the pediatric population, the authors hypothesize that pediatric patients are at higher risk of prophylactic intubation because their smaller airways and higher metabolic needs predispose them to more rapid airway compromise and respiratory failure, respectively. An overly cautious approach seems to be chosen particularly when first responders and preburn centers are less comfortable with the pediatric burn injury.

At the authors’ burn center, like most large pediatric burn centers, cuffed tubes are used almost exclusively, because the safety and utility of cuffed endotracheal tubes to adequately match the high minute volume and positive-end expiratory pressure requirements of burn patients have been demonstrated in both the anesthesiology and the burn literature. Implications of burn injury and ventilation are discussed later in this article.

Fluid Resuscitation

Appropriate fluid resuscitation is the cornerstone of initial management of the burn patient, requiring careful consideration of resuscitation endpoints to avoid consequences of volume overload. Furthermore, early initiation of large-volume resuscitation decreases the incidence of sepsis, renal failure, and overall mortality. ^, Although the Lund-Bowder diagram provides a more accurate assessment of overall injury in pediatric patients, this algorithm is not perfect, and erroneous estimates of burn injury in children frequently result in overresuscitation or underresuscitation. ^,

The Parkland formula is the best known estimate of fluid resuscitation volumes in burn patients, but the Cincinnati and Galveston formulae were designed to be more specific to the fluid needs of pediatric patients, especially those less than 30 kg. These “two figure formulae” account for the insensible loss caused by the burn injury and also consider the hourly maintenance requirements. No studies to date compare the use of the Cincinnati formula to the Galveston formula with respect to overall outcome. No matter the formula used, it is important to remember that younger and smaller patients are less able to mobilize glycogen stores and maintain euglycemia, necessitating the use of dextrose-containing fluids for maintenance. Isotonic fluids, however, are the mainstay for resuscitation of insensible losses. Finally, colloid use and its role in burn resuscitation have been points of significant debate. Two studies have emerged that point to the potential benefit of early albumin administration in children. ^, In fact, adding albumin as early as within 8 hours of burn injury decreased overall volume administered, decreased length of stay, and reduced volume overload.

Despite the risk of respiratory failure, longer ventilatory requirements, compartment syndrome, and overall increased length of hospital stay, overresuscitation is a common occurrence. A metaanalysis revealed that burn centers routinely administer fluids greater than predicted by the Parkland formula, which was also confirmed by Nagpal and colleagues in the pediatric population. Urine output has been the traditional measure of end-organ perfusion with targets between 0.5 and 1.0 mL/kg per hour. Other physical examination findings, such as sensorium, warmth of extremities, pulse rate, and systolic blood pressure, have been cited but may be challenging to assess in the critically ill and hypermetabolic state of a burn-injured child. Lactate and base excess may be physiologic markers of perfusion and could be trended, but no prospective studies support any of these practices in an evidence-based way. One of the more compelling technologies that has recently been studied uses an invasive monitor to measure and trend transcardiopulmonary thermodilution as a surrogate for cardiac output. Kraft and colleagues demonstrated that its use decreased total fluid administration while maintaining cardiovascular parameters and reducing renal and heart failure in the postburn period.

The pediatric anesthesiologist may be asked to care for these critically ill and physiologically vulnerable patients at any stage of their burn injury. Whether for early assistance with a challenging airway or vascular access, monitor insertion for resuscitative efforts, or anesthesia services along the entire spectrum of hospitalization, the pediatric anesthesiologist should know the pathophysiological changes and their implications of care in order to adequately care for these patients.

Surgical Considerations

After initial stabilization, a pediatric burn patient may require a variety of procedures for which an anesthetic is necessary. Knowledge and understanding of relevant surgical techniques facilitate a team-based approach to care and empower the anesthesiologist in the perioperative management.

Often the first encounter with a burn-injured child, if not for airway management or general stabilization, is for emergent escharotomy or fasciotomy. Circumferential burns or electrical injury to a limb may require fascial release if signs of compartment syndrome are present. Full-thickness burns to the thorax and abdomen can compromise respiratory mechanics, can impair intraabdominal perfusion, and may mandate early escharotomy to provide life-saving improvement in oxygenation, ventilation, and perfusion. Similarly, secondary abdominal compartment syndrome, a result of large volume resuscitation and increased intraabdominal pressures, is relieved by decompressive laparotomy.

Early wound closure through tangential excision and split-thickness skin grafting lowers overall morbidity and mortality in children even though transfusion requirements may be greater. Xiao-Wu and colleagues identified the optimal time for early excision as 48 hours. Wound excision involves removal of nonviable, burned skin to facilitate an optimal environment for reepithelialization via split-thickness skin graft from a healthy donor site. These excisions exacerbate insensible fluid loss and bleeding from injured tissues and result in severe pain from the graft harvest site. The management of bleeding, volume resuscitation, and pain is an important anesthetic consideration of burn surgery, which will be discussed here in further detail.

The severe pain and risk for wound complications associated with split-thickness skin grafting prompted investigation into alternative approaches for reepithelialization. Cultured epithelial cells were first used in the 1990s to decrease the need for large donor sites and achieve improved cosmesis. Coleman and Siwy demonstrated benefit in a small number of children, despite increased time for preparation and significant cost. Recently, the use of an autologous skin cell suspension was reported as a viable alternative with improvement in pain control and wound-healing indices. This skin cell suspension is prepared in real time from less donor skin and does not require the time for cells to culture and grow in vitro.

Surgical strategies are used to minimize blood loss during burn surgery. Tourniquets can be used on limbs for excision or reconstruction surgery, yet can be a source of pain and metabolic consequences. In areas where tourniquets cannot be used, subcutaneous and subdermal tumescent solutions can decrease intraoperative bleeding and postoperative pain. Lidocaine is a safe additive to tumescent solutions with benefit for pain control, but studies investigating the safety and efficacy of other local anesthetics are currently lacking. Studies describe the use of both phenylephrine and epinephrine to decrease blood loss. ^, Although epinephrine-containing tumescent solutions have minimal hemodynamic consequences, the authors recently reported a high incidence of hypertension and reflex bradycardia in children receiving phenylephrine-containing tumescence. ^, In addition, the volume of tumescence used should be monitored because it can exacerbate volume overload when subcutaneous fluid redistributes to the central circulation compartment.

Long-term surgical management of burn injuries involves both contracture release and hypertrophic burn scar rehabilitation. Surgical or fractionated laser treatments result in improved function and cosmesis of the scar site. These treatments can be painful procedures and usually require general anesthesia, especially in children. A recent review identified as many as 26 unique approaches to anesthetics for these patients, underlining the need for further study consensus around anesthetic implication and appropriate anesthetic management.

Operating Room and Patient Preparation

The preparation of the environment and patient for a safe anesthetic in the setting of pediatric burns deserves discussion. Increased ambient temperature prevents the untoward effects of hypothermia, such as altered effects of intravenous and inhalation anesthetics, prolonged neuromuscular blockade, increased bleeding and need for transfusion, and impaired wound healing. Compromised thermoregulatory and hemostasis mechanisms leave burned children at high risk for hypothermia.

Monitor application and intravenous access may be challenging if the entire patient is sterilely prepared for burn surgery because this precludes application of adhesives for electrocardiogram leads and pulse oximetry probes. Similarly, intravenous access and invasive monitors may be prepared into the surgical field. Innovative and creative methods have been reported for securing electrocardiogram leads. ^,

Nutritional Support

Adequate nutritional support is vital. The hypermetabolic state precipitated by the systemic catecholamine and cytokine release, especially in burn injuries greater than 30% TBSA, increases energy expenditure up to 130% to 140% above predicted values and can result in protein catabolism and weight loss for up to 2 years after injury. Early initiation of nutritional support in the form of enteral feeds reduces hospitalization and mortality. The frequent interruption of feeds with the nil per os state for procedures may introduce nutritional deficits, which are correlated with mortality. Two recent retrospective analyses have advocated for continuing feeds perioperatively to mitigate these nutritional deficits and report no aspiration events in children with postpyloric feeds and established airways. ^, The authors’ burn center maintains full nothing by mouth precautions for surgery in the prone position.

Airway and Ventilation

Although the considerations and challenges of airway management in the setting of acute injury were previously discussed, repetitive anesthetics and airway maneuvers during the care of a burn patient often require modification and contingency planning as burn scars mature. Limited mouth opening, oropharyngeal swelling, neck contractures, and distortions of tracheal anatomy may evolve over the course of burn injury and are predictors of difficult intubation. Laryngeal mask airway placement may be limited by airway distortions and often requires unique approaches. If tolerated, fiber-optic oral intubation is optimal in order to maintain spontaneous ventilation. Surgeon involvement in airway planning is imperative in case of the need for a definitive surgical airway, but it is important to consider that anatomic distortions also create distinct operative challenges.

Respiratory status and ventilatory challenges should also be considered. Acute respiratory distress syndrome (ARDS) can occur in up to 50% of pediatric burn patients and may require special ventilatory modes to improve oxygenation. Low-tidal volume strategies, a mainstay in ARDS, have recently been challenged by the use of high-tidal volume in pediatric burn patients. Some centers report successful use of high-frequency oscillatory ventilation in both the intensive care unit and the operating room as a mainstay of ventilation for patients who develop ARDS. Provider unfamiliarity with this mode of ventilation is cited as an inhibitor to its widespread use in the perioperative setting. In the authors’ institution, these children are transported to the operating room with individuals who are expert in the management of these specialized ventilatory settings.

Extubation of a burn patient requires careful assessment of airway patency. Lack of an air leak and the mechanism of burn injury are key prognostic indicators of extubation failure. Steroids facilitate the reduction of airway edema before extubation, and high-flow nasal cannula may prevent reintubation. ^, Tracheostomy may be required to facilitate long-term ventilation with reduction in peak inspiratory pressures and improved chest wall compliance. The proper timing of tracheostomy is debatable. However, recent studies support early tracheostomy to help decrease the risk of subglottic stenosis. ^,