Smoke inhalation and burn injury (Proceedings)

The emergency clinician is occasionally called upon to treat smoke and burn injuries resulting from house fires or other sources of thermal, chemical, or electrical injury. Most burn wounds seen in veterinary medicine are relatively minor, possibly because animals with severe burns and smoke inhalation are less likely to be rescued from the scene of a house fire. However, life threatening burns and inhalation injury are being seen with increasing frequency and the emergency clinician should therefore be familiar with their pathophysiology and management.

Smoke Inhalation

Smoke inhalation can be associated with a variety of problems, including respiratory irritation or distress, neurologic effects, and complicating factors such as the development of bacterial pneumonia.1 Additionally, the presence of toxic gases such as carbon monoxide, hydrogen cyanide, hydrogen sulfide and others may further contribute to respiratory irritation and impaired oxygen delivery. The majority of fire-related deaths in animals result from carbon monoxide poisoning rather than the fire itself.

Inhalation of superheated smoke particles has numerous adverse effects. Smoke is a potent respiratory irritant, resulting in bronchoconstriction. The larynx and glottis can become edematous as a result of thermal burns, leading to upper airway obstruction. Chemical and thermal damage to the cells lining the airways leads to sloughing of the tracheobronchial mucosa, impairment of the mucociliary escalator, and formation of cellular casts that may obstruct the lower airways and promote bacterial growth. Disruption of respiratory epithelium and vascular endothelium leads to exudation of proteinaceous fluid into the terminal airways and further contributes to respiratory compromise, impaired surfactant production, and bacterial growth.

Carbon monoxide poisoning frequently occurs in conjunction with inhalation injury. Carbon monoxide has approximately 200 times the affinity for hemoglobin that oxygen does, allowing it to displace oxygen from the hemoglobin and form carboxyhemoglobin instead. Oxygen delivery to the tissues is therefore decreased, and tissue hypoxia may occur, particularly to organs with high oxygen demand, such as the brain and heart. Clinical findings include cherry-red mucous membranes, dyspnea, vomiting, dizziness, headache, altered mentation, and loss of consciousness. Serious intoxications may lead to pulmonary edema, seizures, coma, and death. Delayed neurologic sequellae have also been reported in dogs.

Burn Injury

Skin burns significantly affect patient outcome and increase morbidity and mortality. Animals that have been close enough to the fire to sustain skin burns usually have the most severe pulmonary complications associated with smoke inhalation. Burns are commonly classified according to the extent of body surface involved and the depth of injury to the skin. Extent of injury is initially estimated in human burn patients using "the rule of nines". This rule divides the adult human body into areas corresponding to 9% of the total body surface area, or multiples of 9%. For example, each forelimb comprises approximately 9% of total body surface area; each hind limb, 18%; head and neck, 9%; chest and abdomen, 18%; back, 18%; and perineum, 1%. Body surface area percentages vary in children, and as such, the rule of nines is not typically used in children less than 10 years of age. Although the rule of nines has been cited in veterinary texts, it seems similarly unlikely that these percentages accurately describe the majority of veterinary patients. Other methods of estimating extent of injury include serial halving (Do burns cover more than half the patient's surface area? If not, do burns cover ¼- ½ the surface area? and so forth), or measuring the burn area in centimeters and using a chart to calculate meters2 from the patient's body weight in kilograms.

Depth of injury may be described as first-, second-, or third-degree, or using the more recent terms, partial- and full-thickness. First-degree burns involve only the epidermis (like a sunburn), and are bright red, non-blistered, and painful. First-degree burns typically heal within 5 days without scarring, and are therefore not included in the calculation of extent of burn injury unless they exceed 25% of body surface area. Second-degree, or partial-thickness, burns involve all epidermal layers and extend to various depths within the dermis. Superficial partial-thickness burns involve the epidermis and less than ½ of the dermis, and are characterized by blisters, pain, blanching in response to pressure, and intact hairs. The surface may appear moist, red, or mottled. Injuries of this depth typically heal without serious scarring within 2-3 weeks. Deep partial-thickness burns involve destruction of the deep dermal layers and may appear dry, or blistered and moist. As skin thickness is not uniform, partial-thickness burns may interdigitate with full thickness burns, appearing mottled-red intermixed with whitish areas. Deep partial-thickness burns do not blanch, lose hair easily, and heal more slowly, producing scarring and loss of function. They may easily progress to full-thickness injuries as a result of edema, infection, thrombosis, or mechanical injury. Third-degree, or full-thickness, burns involve destruction of the entire dermis, usually extending into the subcutaneous tissues. They are dry, leathery, lack sensation, and appear white or charred. Healing of these injuries can occur only by contracture and epithelial migration from the periphery, or through excision and grafting. Depth of injury can be difficult to assess initially, and usually requires repeated evaluation over the first 24 hours for accurate determination. Once this information is collected, burned patients may be divided into minor, moderate, or severe categories for the purposes of treatment planning.

Pathophysiology of Burn Shock

Following severe burns (>20% TBSA), a severe systemic inflammatory response may develop within minutes, leading to cardiovascular collapse and multiorgan system failure if not quickly addressed. These systemic manifestations are driven by loss of the protective skin barrier, as well as release of inflammatory mediators from within the damaged tissues. A diffuse "capillary leak" syndrome develops, resulting in marked decreases in effective circulating volume as well as the development of edema in injured and non-injured tissues. Extensive tissue edema leads to tissue hypoxia at the junction between burned and non-burned tissues (the "zone of ischemia"), and may have adverse effects on depth of burn injury. Cardiac output decreases within the first eight hours of burn injury secondary to hypovolemia and myocardial depression associated with release of inflammatory mediators. Arterial blood pressure may be misleading however, as burn patients may have normal or increased blood pressure despite significant hypovolemia due to vasoconstrictive substances released from the burn wound.

Following successful resuscitation, microvascular leak typically "seals" after 18-24 hours. Hypermetabolic response develops during this time resulting in weight loss, protein catabolism, and insulin resistance. The hypermetabolic response typically persists until all wounds are closed, and continues for some time afterwards.

Sepsis is one of the major causes of death among burn patients. In addition to wound infections, respiratory infections, and catheter-related infections, decreased gastrointestinal perfusion in the first 24 hours following burn injury leads to compromised integrity of the mucosal barrier and allows passage of bacteria and endotoxin.

Prehospital Treatment of the Burned Patient

The first consideration in treatment of the burned patient is to stop the burning process. Flames should be extinguished and any collars or harnesses that may become constrictive should be removed. Because the skin is slow to cool, the burning process may continue for some time after the patient is removed from the heat source. For this reason burned areas should be cooled with running water for up to 10 minutes. Alternatively, cool wet towels can be placed over the burn areas. Ointments should not be applied at this time as these may hinder the subsequent assessment of extent of injury. Cold water or ice should also not be used as this can rapidly decrease the patient's body temperature and may contribute to increased wound depth by inducing vasoconstriction. To avoid hypothermia during transport, the patient should be wrapped in several clean, dry sheets or blankets.

Primary and Secondary Surveys

A primary survey should be performed to determine the extent of injury and to institute treatment as needed. Ensuring a patent airway and supporting breathing should be the first priority, followed by shock resuscitation. 100% oxygen should be administered to any patient suspected to have smoke inhalation injury to hasten the elimination of carbon monoxide. Intubation or emergency tracheostomy may be required if airway edema is severe.

Vascular access may be difficult in hypovolemic, burned patients. Ideally, short peripheral catheters should be placed in non-burned areas, though burned areas may be used in the first 24 hours. If burned sites are used for catheterization, the catheters should be removed within 24-48 hours due to bacterial colonization of these areas. Intraosseous catheters are another good alternative for patients in whom vascular access is limited. Central lines may be required in patients with large burns, those needing parenteral nutrition, or those requiring central venous pressure monitoring, but their use should be avoided whenever possible due to the risks associated with hypercoagulability in burned patients.

Following initial stabilization, a secondary survey should be performed to identify concurrent injuries. The face, oral cavity, and pharynx should be examined for the presence of burns or particulate debris that may indicate inhalation injury. The eyes should be evaluated for the presence of conjunctivitis, particulate material, or corneal ulceration. Corneal ulcers are common secondary to thermal injury or abrasion by particulate material, so fluorescein staining should always be performed. A topical anesthetic such as proparacaine may be used to facilitate examination behind the third eyelids for foreign material, and the eyes should be copiously flushed with sterile saline. Corneal ulcers may be treated with triple antibiotic ophthalmic ointment and atropine ophthalmic drops.

Baseline radiographs should be obtained to evaluate for changes related to smoke inhalation or traumatic injury. Chest radiographs may be normal initially, or bronchial markings may be present. The development of pulmonary infiltrates or lobar consolidation may suggest pneumonia.

Complete blood count, serum biochemistry panel, and urinalysis should be obtained upon admission. The presence of myoglobinuria may indicate a need for higher fluid rates to avoid renal tubular damage. Coagulation testing should be performed, as burned patients may suffer from hyper- or hypocoagulable states. Blood typing may be indicated if surgery is anticipated for large burns, as these procedures frequently result in significant blood loss.

Fluid Therapy

The goal of fluid therapy in the burn patient is to restore and maintain perfusion to the tissues while keeping edema fluid to a minimum. The greatest amount of fluid loss in burn patients occurs during the first 24 hours as a result of "capillary leak". Fluids given during this time rapidly leave the vasculature, with colloids having no benefit over crystalloids due to the leakiness of the endothelium. Crystalloids, such as lactated Ringer's solution, are therefore usually the fluids of choice for the first 24 hours. Fluid requirements can be estimated based on percentage of body surface area burned using the Parkland formula. LRS is given at 4 ml/kg x % TBSA (total burn surface area), with one half of the calculated volume given within the first eight hours, and the second half given over the next 16 hours. Urine output should reach 0.5-1 ml/kg/hr within the first three hours. If it falls below 0.5 ml/kg/hr, more fluid is needed. Lasix should not be used to increase urine output, as this will further deplete effective circulating volume as well as invalidate the use of urine output as an indicator of shock resuscitation.

Many resuscitation formulas recommend adding colloids at 0.5 ml/kg/day x % TBSA after 24 hours, as colloids are more likely to be retained within the vasculature at that time. (Note: some formulas advocate colloid supplementation as early as 8 hours post-burn). Hetastarch, fresh frozen plasma, or albumin may be used, though it is interesting to note that albumin supplementation in burn patients has not been associated with decreased mortality nor mobilization of tissue edema within the first week.5 Crystalloids are continued only at doses needed to maintain urine output, approximately 1.5 ml/kg/day x %TBSA.

It is important to emphasize that these fluid formulas should be used only as guidelines, and should be frequently reevaluated and adjusted based on physiologic parameters. Additionally, because these formulas have been derived from experiences with human patients and experimental models in animals, they should be applied cautiously in clinical veterinary patients, and dose reduction may be appropriate in cats.

Wound Care

Patients with small burns rarely develop overwhelming wound sepsis, and medical management for several days usually allows better determination of wound depth and extent. Wounds should be gently clipped of hair and then rinsed or soaked in dilute povidone-iodine solution. Animals with thick coats may hide more extensive wounds than initially suspected, so liberal clipping should be performed in these cases. After the wounds are cleaned, topical agents may be applied to decrease pain, prevent desiccation, and delay bacterial growth. Silver sulfadiazine is used most commonly as it has broad antibacterial activity, is soothing, and has no systemic effects. Topical agents can be applied directly to wounds with a clean tongue depressor, or the burn can be covered with impregnated dressings. Gloves should be worn at all times during wound care to avoid spread of resistant organisms.

The choice of dressing is a much-debated topic. Of critical importance is the maintenance of a moist environment to promote rapid wound healing. This may be accomplished through the use of semi-occlusive dressings, or with various types of hydrogel shown to speed healing and to decrease scarring of partial thickness wounds. Bandages should be loose enough to avoid putting additional pressure on the wounds.

Patients with more extensive burns generally do better if full thickness wounds are excised within the first week, starting 24-48 hours following burn injury. Early wound excision has been shown to circumvent the development of wound sepsis and reduce morbidity and mortality, length of hospital stay, and pain in patients with large burn wounds. Burns >20% total body surface area may require staged procedures, and burns > 50% TBSA make closure with autograft impossible. Once autograft closure is no longer feasible, temporary closure may be performed using cadaver allografts, porcine xenograft, or synthetic skin substitute, though these procedures are not routinely performed in veterinary medicine. Research is currently underway to evaluate the use of synthetic membranes such as Integra (Integra Life Sciences, Plainboro, NJ) that mimic vapor transmission characteristics of normal skin and allow fibrovascular ingrowth from the host, ultimately undergoing biodegradation.

Prophylactic antibiotic usage is controversial as penetration of the eschar is unlikely and the potential for development of antibiotic resistance exists. As such, antibiotic therapy is generally reserved only for documented infections and should be based upon culture and sensitivity of full thickness eschar biopsies. Excision of eschar has been associated with bacteremia however, so intraoperative antibiotic administration has been recommended.

Inhalation Injury

Management of smoke inhalation is typically supportive. The head should be elevated and excessive fluid therapy avoided to minimize development of edema. Bronchospasm may be treated with systemic ( agonists such as terbutaline, or inhaled albuterol administered via spacer (Aerokat, Trudell Medical, London, Ontario). Prophylactic antibiotics have not been shown to reduce morbidity or mortality associated with smoke inhalation, and may contribute to resistant infections. Antibiotics should therefore be reserved for documented infections, and should be based on tracheal wash culture and sensitivity when possible.

Supplemental oxygen should be provided as needed. Carbon monoxide poisoning, if present, may be treated with hyperbaric oxygen therapy, but in most cases administration of 100% oxygen for 6 hours constitutes appropriate therapy without the increased risks and cost involved in transporting a critically ill patient to a facility with a hyperbaric oxygen chamber. Administration of 100% oxygen has been shown to shorten the half-life of carboxyhemoglobin from several hours to approximately 74 minutes (range 26 to 148 minutes).

Nutritional Support and the Hypermetabolic Response

Nutritional support is an important component of burn care, and should ideally be provided as soon after resuscitation as possible. Enteral nutrition using a nasogastric or esophagostomy tube is ideal, as this is believed to decrease gut atrophy, possibly decreasing bacterial translocation and subsequent sepsis. Resting energy requirements may be calculated using the formula [RER= Weight (kg) x 30 + 70]. Although the use of an illness energy requirement calculation (IER) has largely fallen by the wayside in veterinary medicine, multiplying resting energy requirements by an IER of 1.3-1.7 may be appropriate in the burned patient to compensate for the anticipated hypermetabolic response. Critically ill patients or those with very large burns may not tolerate their full nutritional requirements because of ileus or vomiting, and these patients may benefit from the supplementation of parenteral nutrition through a designated central line.

Pain Management

Pain can be reduced initially using cool compresses and soothing ointments such as silver sulfadiazine. Once burn shock has been adequately controlled, narcotics may be administered. Pure agonists such as fentanyl (CRI: 3-5 ug/kg/hr), hydromorphone (0.1-0.2 mg/kg IV q4h, or CRI: 0.025 mg/kg/hr), or morphine (0.5-1 mg/kg SQ q4h) are recommended for patients with moderate to severe pain. Ketamine can be useful for the relief of somatic pain, and may be used in conjunction with narcotics at a constant rate infusion of 0.15-0.6 mg/kg/hr. Lidocaine may provide adjunctive analgesia in addition to free radical scavenging properties, and may also be added at a rate of 1.5-3 mg/kg/hr. If using constant rate infusions, a loading dose equal to the hourly rate should initially be administered.

References

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