Thermal Burn Injury

Chapter 158 Thermal Burn Injury



Caroline K. Garzotto, VMD, DACVS



KEY POINTS



• Electric heating pads, motor vehicles with hot mufflers, and fire exposures are the most common sources of burn injuries seen in the veterinary patient.

• If the injury is from a fire exposure, the patient should be assessed for smoke inhalation (see Chapter 28, Smoke Inhalation).

• If more than 20% of the total body surface area is involved, cardiovascular shock, major metabolic derangements, and sepsis can occur. These patients will need both medical and surgical treatment.

• Burn wounds may take several days to “declare” themselves, because heat dissipates slowly from burned skin.

• The eschar should be removed early to help establish a healthy granulation bed and prevent infection.

• Silver sulfadiazine is the mainstay of topical treatment for burn wounds.

• Cost of treatment and prognosis, especially in animals with severe metabolic derangements needing critical care, should be thoroughly discussed with owners.


INTRODUCTION


Thermal burn wounds are relatively uncommon in veterinary medicine. The most common sources of burns in small animals include electric heating pads, fire exposures, scalding water, stove tops, radiators, heat lamps, automobile mufflers, improperly grounded electrocautery units, and radiation therapy.1 Most burn wounds can be managed the same as traumatic wounds (see Chapter 157, Wound Management). Like traumatic wounds, burn wounds can be labor intensive and expensive for the owner. In addition, numerous metabolic derangements can adversely affect the patient, prolong hospitalization, and complicate the recovery.



DEFINITIONS


Burn wounds are assessed using two major parameters: the degree of the injury and the percentage of body surface area involved. First, a review of skin anatomy is helpful.1 The most superficial layer of skin is the epidermis and the deeper layer of skin is the dermis. The dermis is comprised of a superficial plexus and a middle plexus, where hair and glandular structures arise. Below the dermis lies the hypodermis, which contains the deep or subdermal plexus and the panniculus muscle. The subdermal plexus brings the blood supply to overlying skin through the superficial and middle plexus. Capillary loops in the superficial plexus supply the epidermis; however, they are poorly developed in the dog and cat compared to humans, which is why these animals do not develop blisters.1


Although these are now considered older terms, many physicians still like to refer to burn wounds as first-degree, second-degree, and third-degree injuries (Table 158-1).1,2 First-degree burn wounds are superficial and are confined to the outermost layer of the epidermis. The skin will be reddened, dry, and painful to touch.



Table 158-1 Burn Wound Assessment and Healing

image

Second-degree burn wounds are partial-thickness injuries that involve the epidermis and a variable amount of the dermis. If only the superficial part of the dermis is affected, there will be thrombosis of blood vessels and leakage of plasma. The hair follicles are spared. In deeper partial-thickness burns, hair follicles are usually destroyed, the skin appears yellow-white or brown, and there is decreased sensation except to deep pressure.1


Third-degree burn wounds are full-thickness injuries that have destroyed the epidermis and dermis and can affect deeper tissues such as muscle, tendon, and bone. The skin is leathery and charred and lacks sensation. When burned, skin retains heat, so an accurate assessment of the degree of the wound may not be apparent initially.1 It can take up to 3 days for the burn to “declare” itself, and during that time thermal injury and circulatory compromise from thrombosed vessels can continue.


Patients with burns involing more than 20% of their total body surface area (TBSA) can have serious metabolic derangements. Patients with more than 50% of their TBSA involved have a poor prognosis, and euthanasia should be discussed with the owners as a humane alternative. TBSA can be estimated in animals using percentages allotted to body area using the rule of nines as described in Table 158-2.1-3


Table 158-2 Estimating Total Body Surface Area Burned































Area Percentage (%) Total %
Head and neck 9 9
Each forelimb 9 18
Each rear limb 18 36
Thorax 18 18
Abdomen 18 18
TOTAL 72 99

When skin is severely burned, it forms an eschar within 7 to 10 days. Eschar is a deep cutaneous slough of tissue composed of full-thickness degenerated skin.4 It appears as a black, firm, thick movable crust that separates from the surrounding skin, and purulent exudates often lie beneath it (Color Plate 158-1).



PATIENT ASSESSMENT AND MEDICAL MANAGEMENT


The patient should be assessed immediately for airway, breathing, and circulatory compromise as for all trauma patients (see Chapter 2, Patient Triage). Following a full physical examination, including inspection of the patient from head to foot pads, an assessment of the degree and TBSA of the burn wounds should be performed to help determine prognosis and the extent of treatment necessary. Blood should be collected for evaluation of packed cell volume, total solid and electrolyte levels, and blood gas parameters, minimally.



Metabolic Derangements


If more than 20% of a patient’s TBSA is burned or if the wounds are classified as second or third degree, hypovolemic shock should be anticipated. As a result of capillary thrombosis and plasma leakage, massive amounts of fluid are retained in the wound leading to burn wound edema.3 This results in the loss of fluid and electrolytes, with the most dramatic losses occurring during the first 12 hours. Systemic abnormalities should be anticipated, including anemia, hypernatremia or hyponatremia, hyperkalemia or hypokalemia, acidosis (metabolic and respiratory), oliguria, and prerenal azotemia. The course of the systemic abnormalities changes with time.2


Hemoconcentration will be noted initially because of the dramatic loss of plasma; however, red blood cell hemolysis also occurs simultaneously from both direct damage and destruction through the damaged microcirculation. The patient should be monitored for disseminated intravascular coagulation (DIC), upper airway edema and oliguria. Between days 2 and 6, the patient should be assessed for anemia, DIC, immune dysfunction, systemic inflammatory response syndrome, and early burn wound infection. From day 7 and on, the clinician should watch closely for hyperthermia, hyperventilation, pneumonia, sepsis, and wound demarcation.


Fluid losses can result in hypovolemic shock (see Chapter 65, Shock Fluids and Fluid Challenge). After initial shock resuscitation with isotonic crystalloids up to 90 ml/kg IV in dogs (50 ml/kg in cats) and synthetic colloids or blood products, if needed, total fluid delivery rate during the first 24 hours should be 1 to 4 ml/kg body weight × % TBSA burned.2 After 12 to 24 hours, when vascular permeability is stabilized, a constant rate infusion (CRI) of synthetic colloids (e.g.,hydroxyethyl starch, dextran-70) may be beneficial at a rate of 20 to 40 ml/kg/day. Plasma is given at 0.5 ml/kg body weight × % TBSA burned in humans, although this has not been investigated in dogs and cats. By 48 hours after injury, plasma volume is mostly restored, and thus patients are at high risk for generalized edema and fluid overload from the high initial demands for fluid replacement.3 Ideally, fluid therapy should be adjusted for the individual patient based on cardiovascular stability, central venous pressure (0 to 10 cm H2O), and urine output (>1 ml/kg/hr).

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Thermal Burn Injury

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