Anesthesia for patients with acute burn injuries

Introduction

Remarkable advances continue to be made in the care of patients with major burn injuries. Early aggressive fluid resuscitation has dramatically improved initial survival. The development of specialized burn centers has allowed the concentration and coordination of resources needed to provide a multidisciplinary approach from the time of admission with the goal of not just maximizing survival but optimizing functional recovery as well [1].

Anesthetic management is an important part of this multidisciplinary approach. Anesthesia providers have highly developed skills and experience in airway management, pulmonary care, fluid and electrolyte management, vascular access, and pharmacological support of the circulation. These areas of clinical expertise are all central to the care of patients with major burn injuries. However, the effective use of this clinical expertise for the care of burn patients requires knowledge of the pathophysiological changes associated with burns and an understanding of the multidisciplinary approach to burn care [2]. To be effective, perioperative management should be compatible with overall goals and especially with ICU care.

Patients with large acute burns present multiple challenges to anesthetic management (Table 1). Virtually all organ systems are affected by large burn injuries. These changes have an important influence

Marc G. Jeschke et al. (eds.), Handbook of Burns

on anesthetic management including not only drug selection and dosage but airway management, monitoring, fluid administration, sedation, and pain control. As a result, nearly every aspect of anesthetic care must include some adjustment to deal with pathophysiological changes due to the burns.

Preoperative evaluation

Care of the acutely burned patient requires knowledge of the continuum of pathophysiological changes from

Table 1. Perioperative challenges in the anesthetic management of acute burn patients

Airway compromises

Inhalation injury

Impaired circulation

Difficult vascular access

Mechanical difficulties with monitors due to cutaneous burns

Massive hemorrhage

Altered drug response

Sepsis/systematic inflammatory response syndrome

Altered temperature regulation

Co-existing diseases

Associated trauma

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Table 2.

Patient age

Extent of cutaneous burns (% total body surface area)

Burn depth and distribution

Mechanism of injury (flame, scald, etc.)

Airway exam

Inhalation Injury

Quality or Resuscitation

Associated Injuries

Coexisting diseases

Surgical plan

the initial injury through wound healing and resolution of metabolic changes. In conjunction with the standard features of a preoperative assessment, the anesthetist should focus on certain features associated with increased risk and technical challenge when planning perioperative care of the patient with acute burn injuries (Table 2). The mechanism of injury should be clearly identified since this determines the quality of burn injuries as well as the kinds of associated disorders the patient may present with. As an example, a person burned in the enclosed space of a house and a worker suffering electrical burns would present with very different associated injuries. Since fluid requirements and the pathophysiological response to injury are dynamic it is also important to know the time elapsed since injury.

Pathophysiological alterations in respiratory function are common in severely burned patients (Fig. 1). An airway exam is the first priority in patients with a history consistent with burns to the head and neck and/or smoke inhalation. Tissue distortion due to edema can make intubation by direct laryngoscopy difficult or impossible and these changes will increase with fluid resuscitation. As a result, it is very important to diagnose airway compromise early in the patient with acute burns. Pharyngeal burns can lead to lethal airway obstruction and pre-emptive intubation can be life saving when this threatens. However, facial burns are often not associated with pharyngeal edema and airway obstruction. In the absence of respiratory distress or other reasons for immediate intubation such as very extensive burns, shock, or inability to protect the airway, it may be safer to defer securing the air-

way until it can be accomplished in a controlled setting and when a clear indication for intuition can be identified [3]. This can reduce complications of unnecessary intubations such as exacerbation of laryngeal injuries or life threatening consequences of unintended extubation of a patient who has been intubated then heavily sedated or pharmacologically paralyzed. Initially, patients with smoke inhalation often present with good gas exchange and a normal chest radiograph. Inhalation injury often progresses over time as the inflammatory response develops and small airways are occluded by sloughed tissues, casts and stagnant secretions. Therefore, initial chest x-ray or arterial blood gas analyses usually serve more as a baseline for evaluating changes in pulmonary function. Impaired gas exchange and significant chest radiograph findings on admission are ominous observations. The diagnosis of inhalation injury is usually based on the history and physical exam and confirmed by bronchoscopy.

In addition to airway and pulmonary parenchymal pathology, pulmonary insufficiency can occur in patients with acute burn injury when a restrictive pulmonary deficit results from circumferential burns that limit chest wall expansion during inspirationorfromabdominal compartmentsyndrome when abdominal contents limit diaphragmatic excursion. Restricted chest wall mobility can be corrected by surgical release (escarotomies) and if abdominal compartment syndrome is diagnosed there

Smoke inhalation Thermal airway damage Laryngeal edema Airway Trauma Restrictive breathing defect from circumferential escar Bronchospasm Pulmonary edema Aspiration pneumonitis Pneumonia

Central nervous system injury/ impaired respiratory drive

Fig. 1. Common causes of respiratory pathology in burned patients

Table 3. Formulas for estimating fluid resuscitation of adult acute burn patients

are a number of interventions to reverse associated pathophysiological changes. It is important to be sure that the anesthesia ventilator in the operating room is capable of delivering support that the patient requires in the ICU.

Hemodynamic status, electrolyte balance, and renal function are affected by the extent of burn injuries and the quality of fluid resuscitation. It is important to understand the hemodynamic status of the patient. In the immediate post-injury stage, patients exhibit a syndrome known as burn shock in which increased vascular permeability causes transudation of protein rich fluid from the vascular compartment to the inerstitium leading to intravascular hypovolemia and tissue hypoperfusion. Delays in initiating fluid resuscitation or inadequate volumes administered during burn shock can result in hypoperfusion and damage to non-burned tissues as well as exacerbation of the burn wounds. Over resuscitation also can have deleterious effects such as pulmonary edema or compartment syndromes of extremities or the abdomen. Several formulae based on extent of cutaneous burns are available to estimate the volume of fluid and rates of administration needed to maintain

Table 4. Formulas for estimating fluid resuscitation of pediatric acute burn patients

intravascular volume (Table 3). On a ml/kg basis pediatric burn patients have been found to require more volume for resuscitation. In addition, pediatric patients may require resuscitation for smaller burns (e. g. 10–20% total body surface area). Separate formulas have been recommended for fluid resuscitation of pediatric burn patients (Table 4).

Formulas are a starting point for fluid therapy. A comparison of predicted volumes based on the resuscitation formula in use with the actual volumes administered can provide a quick assessment of the appropriateness of the treatment efforts. Resuscitation is then titrated to the patient’s response as judged by mean blood pressure and urine output. Many factors such as smoke inhalation, extensive deep burns, electrical burns, soft tissue trauma, and delay in resuscitation can increase the fluid requirements for resuscitation of burn patients. As a result, the needs of each patient are unique and volume resuscitation must be titrated to the patient’s response. The response to resuscitation can be evaluated by reviewing vital signs and urine output. However, these endpoints are often not good predictors of tissue perfusion. Blood gas analysis, either arterial or venous, can provide additional information regarding the metabolic status and adequacy of perfusion. Specific endpoints include hemoglobin, base deficit, and lactic acid concentration. If the patient’s response is poor despite what appears to be an appropriate volume of fluid administered, underlying pathology should be clarified and additional support such as an inotropic drug provided.

Large burn injuries often present in association with other coexisting diseases or additional trauma associated with the burn accident. It is important not to focus on the burn wound to the exclusion of other serious pathology. A closed head injury, for example, can significantly alter fluid and hemodynamic management choices. A careful history and physical exam should not ignore these other non-burn issues.

Effective perioperative care can only be accomplished within the context of the planned surgical intervention. The appropriate vascular access and level of monitoring depend on the anticipated physiological stress of surgery and expected blood loss. These can only be revealed by clear communication with the patient’s surgeons. Close consultation with the surgeons is an essential component of the preoperative assessment of the patient with acute burn injuries.

Monitors

Circulation

Arterial blood pressure Central venous pressure Pulmonary artery pressures Cardiac output

Pulmonary artery catheter Peripheral thermodilution catheter Pulse pressure variability

End tidal carbon dioxide Electrocardiography Urine output

Ventilation and Oxygenation

Pulse oximetry

End tidal carbon dioxide Airway pressures and volumes Inspired oxygen analyzer Blood gases

Temperature

Foley catheter probe Rectal probe Esophageal probe

Oro-nasopharyngeal probe

The choice of monitors in a burned patient depends ontheextentofinjuries,physiologicstate,andplanned surgery (Fig. 2). According to American Society of Anesthesiologists, the patient’s oxygenation, ventilation, circulation, and temperature should be continually evaluated. Standard monitors include electrocardiography (ECG), pulse oximetry, arterial blood pressure, temperature, capnography, and inspired oxygen concentration. Pathophysiological changes associated with major burns and the potential for massive surgical bleeding may require more invasive monitors and increased vigilance regarding certain physiological variables during the perioperative period. It should be noted that a change in monitoring or transient loss of monitoring may occur during washing, position change, or dressing application.

In many cases, accurate physiological monitoring can be challenging in burn patients. Topical ointments and skin destruction due to large cutaneous burns may prevent adherence of standard gel electrocardiography electrodes and pulse oximetry to the skin. In this case, metallic surgical staples and alligator clips are effective for ECG monitoring. Pulse oximetry during burn surgery can be difficult when extremities are either burned or in the operative

Fig. 2. Commonly used monitors for burned patients

field. The probes or connections may become wet and tendered useless during bathing or irrigation in the operating room. Transmission pulse oximetry probes may be applied or clipped to the digits, ear lobes, or lips as one solution to these challenges. Some clinicians have modified standard pulse oximetry probes for use on the tongue in conjunction with use of a plastic oral airway. Reflectance pulse oximetric technology has been developed to combat problems with signal transmission during hypoperfusion and when a transmission path is unavailable. Forehead probes are commercially available and may detect hypoxemia more quickly than the ear or finger probes [4].

If direct measurement of arterial blood pressure is not required, measurement of blood pressure using a non-invasive cuff has been found to be accurate even when the cuff is placed over bulky bandages [5]. In cases where blood loss is expected to be extensive, as well as in selected high-risk patients or in those failing resuscitation to clinical goals, invasive monitoring with an arterial catheter may be

helpful. An arterial catheter is also advised if clinically significant changes in the blood pressure are expected to occur more rapidly than the interval between non-invasive blood pressure measurements or if vasoactive infusions are needed. Although pulse oximetry and end tidal carbon dioxide measurements are adequate monitors of oxygenation and ventilation in patients with normal pulmonary function, these modalities may be inadequate for patients with significant pulmonary disease such as smoke inhalation injury, acute lung injury or the adult respiratory distress syndrome. Under these circumstances it is useful to have an arterial catheter to provide access to arterial blood samples for gas analysis in order to optimize ventilation. This may be very helpful in adjusting mechanical ventilation.

Perioperative management of patients with major burns is often facilitated by the presence of a central venous catheter. A central venous catheter can provide reliable and secure venous access for administering fluids and drugs (especially vasoactive drugs) and for obtaining blood samples when extensive cutaneous burns make peripheral venous access difficult or impossible. In addition, substantial losses of intravascular volume or limited cardiac function make it difficult to appropriately replace volume without some monitor of filling pressure. Although central venous pressure (CVP) is often used to manage intravascular volume, it is an unreliable indicator of preload [6]. Filling pressures interact in complex and unpredictable ways with ventricular compliance and contractility as well as intra-thorac or intra-abdominal pressures to influence cardiac preload. Cardiac function and response to volume loading have been found to correlate poorly with filling pressures. Preload is defined as end-diastolic myocardial fiber tension; however, this cannot be measured in a clinical setting. Although knowledge of the CVP is not reliable for fine tuning preload, it is important to monitor filling pressure while administering large volumes rapidly. If it appears that tissue perfusion is inadequate and the CVP is low, it is usually safe to administer a fluid bolus as both a diagnostic and therapeutic intervention. If the CVP is high, it is possible that a fluid bolus might cause harm due to intravascular volume overload.

A pulmonary artery catheter (PAC) can also be used to assess cardiovascular function in burned pa-

tients. In addition to the pulmonary artery occlusion pressure (PAOP), right ventricular cardiac output (CO) can be obtained with a pulmonary artery catheter from a thermodilution curve, in which the CO is inversely proportional to the area under the curve. Unreliable results will be obtained in those with rightsided regurgitant lesions or with septal defects. The systemic vascular resistance may also be calculated with PAC-derived information using the following equation:

SVR = MAP-CVP × 80

CO

Despite what appears to be an intuitive benefit of PAC-derived information, the clinical utility of this monitor has been increasingly brought into question. Neither PAOP nor CVP have been found to correlate with either end diastolic volume or stroke volume. Moreover, changes in these indexes of cardiac preload have not correlated with changes in either stroke volume or end diastolic volume in groups of normal volunteers or critically ill patients [6]. Some clinical investigators have found increased mortality associated with the use of a PAC. As a result of these observations and the higher cost of hemodynamic monitoring with the PAC, many clinicians are using this monitor less frequently. Newer volumetric monitors that are less invasive than the PAC have been found to be more reliable indicators of cardiac preload. A transpulmonary thermodilution technique that utilizes a central venous catheter and an arterial fiber optic thermister catheter inserted into the femoral artery can provide estimates of global end diastolic cardiac volume and total intrathoracic blood volume (ITBV). In contrast to CVP and PAOP, augmentation of ITBV has been used successfully to guide fluid resuscitation of severely burned patients. However, use of ITBV was associated with larger resuscitative fluid volumes that predicted by the Parkland formula [7].

Dynamic measures of cardiac preload, such as systolic pressure variation (SPV) and pulse pressure variation (PPV), have been found to be better predictors of volume responsiveness than static indicators such as central venous pressure or pulmonary artery occlusion pressure. These dynamic parameters can be used to discriminate patients who are intravascularly depleted and may benefit from volume loading from patients who are adequately fluid