Luis H. Tello, DVM, MS
Small animals that have obtained burns are indeed trauma patients with extensive lesions in the skin and eventually also in the subcutaneous and muscular layers leading to many metabolic and organic complications. Thermal injuries represent severe mechanical damage to the cells of each layer of the integument. There are many different sources of burn lesions: electrical, chemicals, direct heat, fire, fireworks, etc. Common causes of burn in small animals are domestic accidents but there are also many reports about the use of electrical heating pads during surgery or in cage veterinary hospitals.
Severe burn patients are extremely complex medical cases, with compromise of respiratory, cardiovascular, electrolyte, metabolic, immune and energy pathways systems, and require a proper understanding and management of physiology, endocrinology, nutrition and immunology status of the patients, to give them appropriate treatment.
Burns affect primarily the skin, and the degrees of injury are related to the depth and extension surface affected. The skin has many different roles in the normal physiology of the body: it is the primary barrier against invasive infection; it helps to maintain the body temperature controlling the evaporation of fluids; and it adapts to aggressions or changes in the environment like pain, cold and heat. All these functions are impaired in burned animals and have been related as secondary cause of death.
Hypoproteinemia due to protein loss is a serious issue because, combined with the fluid shifts and hypovolemia, it sets the stage for severe shock and hypotension. Hypovolemia leads to a decrease in cardiac output and this leads to tissue and organ hypoperfusion and cellular hypoxia. Many organs and tissues are affected simultaneously, while the kidneys, in particular, are extremely sensitive to decreases in blood pressure. Acute renal failure is a common complication in severe patients, and its prevention and assessment should be included as part of the diagnosis and treatment protocol of burn patients. In addition, persistent hypovolemia and hypotension can lead to acute ischemic failure of other organs such as the lungs, liver, and gastrointestinal tract.
Burns are evaluated in general according to the severity of the damage in the body system, while the severity of insult is addressed according to the length of tissue exposure, the surface affected, the type of heat source and the previous condition of the patient: healthy, sick, old, young, etc.
Looking at the affected surface is part of the regular approach in human patients. While there are many standards to calculate affected surface in people, in small animals it is more complex due to the high variability in sizes and weights. One accepted proposal is to calculate the total surface by burned body segments: each forelimb means 9%, each rear limb means 18%, head and neck 9% and trunk and abdomen 18%.
Burn Depth Classification
Burn depth classification has been changed in veterinary medicine from the classic human medicine degrees system to another, according to the layers affected.
This type of burn affects only the outermost layer of the skin, the epidermis, and is the human equivalent of a first-degree burn such as sun burn which is common in white coated dogs and cats; skin redness is present and the skin is hyperesthetic. There are no blisters and normally these burns heal on their own with no scars.
Partial Thickness Burns
This is the veterinary equivalent to the human second-degree burn. The burns involve the epidermis and the superficial layer of the dermis; you can see blisters and sometimes the skin is denuded. The hair is well attached and usually mixed with yellow exudates. These lesions are very painful. Commonly, scarring occurs.
Full Thickness Burns
This is the human equivalent of third-degree burn, and it affects the epidermis, dermis and subcutaneous tissue. Generally, patients are presented in shock status; the skin looks dry and not perfused and does not bleed if it is cut. The hair coat can be easily epilated by just pulling, there is no pain sensation. A large scar will remain after a prolonged healing process.
Burns as an Emergency
The care of a burn patient will therefore be divided into 3 stages:
1. From arrival to 36 hours
2. Early period: 36 hours after arrival to 5 days
3. Inflammation-infection period: after the first week
Stage 1: From Arrival to 36 Hours
The initial assessment should start with the general physical condition, systemic compromise, the amount of body and surface affected, plus the degree of local injury. If the loss of skin is large enough, euthanasia can be recommended.
People involved in fires have respiratory injury due to the inhalation of air heated to a temperature higher than 150C°, resulting in burns into the mouth, oropharynx and upper airway. Pulmonary damage due to smoke inhalation is the major cause of mortality in human beings. Deaths are associated with the fall of oxygen concentration in the environment, inhalation of carbon monoxide and carbon dioxide during combustion and cyanide toxicity. This mechanism is rare in small animals, apparently because they walk almost at floor level.
Animals affected by smoke inhalation should be placed on 100% oxygen early after arrival to ICU. Inhaled heat produces upper airway obstruction due to airway edema. Early endotracheal intubation is crucial, and must to be performed if the physical exam shows signs of airway burn damage or if patient show respiratory distress. It is important to consider that pulse oximetry cannot evaluate the severity of hypoxia because of its inability to differentiate between oxygenated hemoglobin and carboxyhemoglobin.
The initial therapy is oriented to relief the pain with cold direct application in the burn area: chilly water, soak towels, cold tap water are good alternatives. Oxymorphine alone or combined with acetylpromazine in neuro-leptoanalgesia is indicated to pain control in dogs. Cats can be treated with diazepam plus ketamine.
Oxygen (at 100-150 ml/kg/minute) should be initiated as soon as possible, and a central catheter into jugular vein should be placed. Give fluid replacement at 4 ml/kg per hour in dogs and 2 ml/kg per hour in cats. An isotonic balanced electrolyte solution like lactated Ringer´s or normal saline is the first choice. Free glucose fluids must be avoided because hyperglycemia and glucosuria will occur after deep burns.
Potassium levels should be monitored as there will be a rise with severe hyperkalemia during the first 24 hours, associated with cell destruction in the burned tissue. Solutions with contents of 4-5 mEq/L of potassium are recommended during this phase.
Check out serum protein levels, urine production, hematocrit level, haemoglobin, electrolytes and blood gases. If total protein drops below 3 gm/dl, fresh plasma or colloids should be added. Acidosis can be corrected with sodium bicarbonate and 5 mEq/Kg of body weight may be given every hour or 30 minutes. If hematocrit falls below 20% or haemoglobin falls below 7 gm/dl, whole blood or washed red blood cells must be added to the treatment. A haematocrit above 30% is the goal.
After initiating analgesia treatment, the hair must to be clipped; burn wounds can be washed with antiseptic solutions as povidone iodine or chlorhexidine. Necrotic tissue, foreign material and debris must be removed.
First or second degree burn wounds should be topically treated with antibiotic medication, silver ointment is the first choice, and bandaged. With third degree burns, the eschar (a dry scab or slough formed on the skin as a result of a burn) must be removed quickly and on a daily basis. This is a very painful procedure, so anaesthesia or proper analgesia should be considered. Scar tissue removal will help expose the healthy underlying granulation tissue.
Systemic antibiotics do not penetrate the eschar, so topical therapy is always indicated with antibiotic ointments and creams. Gentamycin, polymyxin, neomycin and bacitracin are very effective against the contaminant flora in burn wounds, as well as fluoroquinolones. Some reports show that aloe vera has certain antiprostaglandin effects that can help to maintain normal dermal vasculature.
Stage 2: Early Period: 36 Hours After Arrival to 5 days
This period of time is transition from flow phase of shock to the hypermetabolic phase. The main challenges at this stage are:
Proper care of burn wounds
Pain and anxiety control
The main pulmonary problems arise from airway obstruction due to thermal or chemical burns of the airway mucosa. Adequate laryngoscopy is very helpful to assess the real damage. Long term intubation should be considered if mechanical ventilation is available.
A cough and increased mucus production, related to mucosal irritation, are very common in this period. However, the damage and impairment of the ciliary function leads to infections such as bacterial tracheobronchitis, pneumonia or bronchopneumonia. Proper antibiotic selection through culture of secretions is the first choice for these complications.
Evaporation is a major source of water loss within the burned areas. An estimate of this loss must be made to perform proper fluid therapy. Anemia is another complication--this is caused by red cell destruction and impaired bone marrow production.
Fluid therapy is a keystone during this period of time. Fluids with 5% glucose and a small amount of sodium are indicated because no major losses of sodium occur during this stage. No aggressive fluid therapy is indicated: a figure of 60-70 mmHg as mean arterial pressure and urine production around 1-2 ml/kg/ hour should be watched for. An albumin level of around 2.5 g/dL is the goal, and hematocrit should be kept over 30%. Whole blood transfusion can be considered.
It is important to remember that burns have major effects to an animal's immune system, associated with impaired cell mediated immunity, a decrease in neutrophil function and compromise of the humoral immune response. With all these factors, infection can be a major complication in wound care. Culture, biopsy analysis and antibiotic studies must be performed in order to gain control over infection. As wound cleaning, excision and escharotomy are regular procedures, they can be used to obtain proper samples for culture.
Careful handling of stress, anxiety and pain are extremely important for the small animal burn patient. Narcotics such as morphine, oxymorphine, butorphanol and low doses of benzodiazepines are indicated. Phenothiazines must be avoided because their extrapyramidal side effects in burn patients.
Stage 3: Inflammation-Infection Period, After the First Week
Sepsis, SIRS (systemic inflammatory response syndrome) and septic shock are common during this period. Adequate nutritional support is very important for clinical outcome and feeding tubes are first choices in starving animals.
Pulmonary infection and RADS (respiratory acute distress syndrome) remain major causes of mortality during this period. Partial ventilatory support can be useful, if necessary.
Treatment in human patients commonly includes anabolic agents, in order to attenuate catabolism during this phase. There is no information available in small animals patients to support this management.
Pain control in the burn patient is a very important issue due to the ethical and physiological impact that the pain can have. Technicians and vet nurses should be aware of the importance of decreasing a patient's discomfort. Small details in nursing care should not be overlooked. Allowing the patient room to move comfortably despite bandages, elevating wounded areas and maintaining a clean, dry environment in which to sleep will contribute significantly to a patient's comfort level.