Pathophysiology

Severity factors

There are five factors that need to be considered when determining the severity of a burn injury (Box 1).

i) Extent – The larger the area of the body burned, the more serious the injury. There are several methods available to accurately calculate the percentage of body surface area involved. The simplest and most easily recalled is the Rule of Nines (Fig. 1). However, it is only for use with the adult burn population. It cannot be used for persons under the age of 15 years. The Lund and Browder method (Fig. 2) is useful for all age groups, but is more complicated to use.

The chart assigns a certain percentage to various parts of the body and includes a table indicating adjustments necessary for different ages. For the pediatric population, there is a modified version of the Lund and Browder method (Fig. 3). If the burned areas are small and irregularly-shaped, the Rule of Palm can be used. One looks at the palmer surface of

Fig. 2. Lund and Browder method for estimating extent of burn

Fig. 3. Pediatric estimation of burn area using modified Lund and Browder

the burned person’s hand, which represents 1% body surface area, and uses that to calculate the scattered areas of burn. If approximately 10% or more of the body surface of a child or 15% or more of that of an adult is burned, the injury is considered serious. The person requires hospitalization and fluid replacement to prevent shock.

ii) Depth – Two factors determine the depth of a burn wound – temperature of the burning agent and duration of exposure time. In other words, the hotter the source and the longer it is in contact with the skin, the deeper the injury. Previously the terminology used to describe burn depth was first, second and third degree. In recent years, these terms have been replaced by those more descriptive in nature: superficial partial-thickness, deep partial-thickness and fullthickness. A description of each is included (Table 3).

Superficial burns, such as those produced by sunburn, are not taken into consideration when assessing extent and depth. The skin is divided into 3 layers, which include the epidermis, dermis and subcutaneous tissue (Fig. 4). The epidermis is the thin, outer, nonvascular layer. Its role is one of protection, heat regulation and fluid/electrolyte conservation. Below the epidermis lies the dermis, perhaps

Fig. 4. Anatomy of burn tissue depth

the most important layer of skin, where the depth of burn has a profound impact. The dermis contains connective tissue, blood vessels, hair follicles, sweat and sebaceous glands. Skin-reproducing cells are located throughout the dermis and the sufficient presence or absence of these epidermal cells determines whether the wound will re-epithelialize or require skin grafting.

The subcutaneous tissue lies below the dermis and contains fat, lymphatics, nerves and vascular networks. Below the subcutaneous tissue is the muscle and bone, which may be affected by deep flame and major electrical injuries.

iii)Age – In patients less than 2 years of age and greater than 50, there is a higher incidence of morbidity and mortality. The severity of the burn increases with age. Infants, toddlers and the elderly have thinner skin, making the risk of a deeper burn greater in these age groups. Persons of this age also have weaker physical resources to mount a resistance against the debilitating effects brought on by a burn. Sadly, the infant, toddler and elderly are at increased risk for abuse by burning.

iv)Part of the body burned – Burn location is an important factor to consider. Patients with burns to the face, neck, hands, feet or perineum have greater challenges to overcome and require the specialized care offered by a burn centre. The challenges are both functional and esthetic in nature. Specialized care can minimize loss of permanent function, reduce infection and maximize esthetic outcomes.

v)Past medical history – A person’s past medical history is an important variable to consider postburn. Someone with pre-existing cardiovascular,

pulmonary or renal disease will have a poorer predicted outcome than a previously healthy individual, since a burn injury will exacerbate pre-existing conditions. Persons with diabetes or peripheral vascular disease have a more difficult time with wound healing, a central factor in burn recovery, particularly if the burns are on the legs and/or feet. Burn patients with poor nutritional status pre-burn or with previous drug and/or alcohol abuse patterns have fewer physical reserves to draw from and, as such, require more resources for a prolonged period of time. Finally, burn patients, who have also sustained inhalation injuries, head trauma, fractures or internal damage, have a poorer prognosis overall.

Local damage

Burn wounds are produced when there is contact between a source of energy, such as heat, chemicals,

electricity or radiation, and body tissue. Local damage varies, depending upon the temperature of the agent, duration of contact time and the type of tissue involved. There are 3 zones of tissue damage in the burn wound. The deepest zone of coagulation (fullthickness) is the site of irreversible cell death, where blood vessels and re-epithelializing cells have been completely destroyed. These areas require skin grafting for permanent coverage. The middle layer, or zone of stasis, is the area of deep, partial-thick- ness injury where there are some skin-reproducing cells present in the dermis and circulation to the area is partially intact. Healing will occur generally within 14 – 21 days, as long as infection or desiccation are prevented. The outermost zone of hyperemia is the area of least damage. This superficial, partial-thickness wound has minimal cell involvement and will generally recover spontaneously within 7–10 days.

Coagulation necrosis occurring at the time of the injury damages or destroys tissues and vessels. During the inflammatory process post-burn, leukocytes and monocytes begin to appear at the site of the injury. Fibroblasts and collagen fibres gather within 6–12 hours after the burn to begin the task of wound repair. White blood cells begin the process of phagocytosis and necrotic burn tissue begins to slough. Fibroblasts and collagen fibres form granulation tissue. Areas of partial-thickness injury, devoid of infection and desiccation, will heal in by primary intention from the edges of the wound and from below. Full-thickness wounds require early excision and skin grafting. Over time, the collagen fibres and reepithelializing cells continue to heal and add strength to the newly formed tissue. The healed areas initially look pale and flat. However, as the blood supply increases to those areas over the next month or so, they become red and raised. In addition to these scars forming, there is a natural tendency for burned tissue to shorten and contractures to develop. Over the next year to 18 months, the burn scars will fully mature and become less red and less raised.

Fluid and electrolyte shifts

The immediate post-burn period is marked by dramatic circulation changes, producing what is known as “burn shock” (Fig. 5). Blood flow increases to the area surrounding the wound. The burned tissue then releases vasoactive substances, which results in increased capillary permeability. As early as 15 minutes post-injury, there is a shift of fluid from the intravascular compartment to the interstitial space, producing edema, decreased blood volume and hypovolemia. The hematocrit increases in response to the decrease in blood volume and the blood becomes more viscous. This increase in viscosity, coupled with a decreased blood volume, results in increased peripheral resistance. Hypovolemic or “burn” shock follows shortly thereafter. As the capillary walls continue to leak, water, sodium and plasma proteins (primarily albumin) move into the interstitial spaces in a phenomenon known as “second spacing”. Potassium levels rise initially in the extracellular spaces due to release from injured cells and hemolyzed red blood cells. When the fluid begins to

Fig. 5. Burn shock

accumulate in areas where there is normally minimal to no fluid, the term “third spacing” is used. This phenomenon is found in exudate and blister formation. As intravascular volumes are depleted, the edema increases and the body begins to respond to the hypovolemic shock ( pulse and blood pressure). There is also insensible fluid loss through evapouration from large, open body surfaces. A nonburned individual loses about 30–50 mL/hr. A severely burned patient may lose anywhere from 200 to 400 mL/hr.

Circulation is also impaired in the burn patient due to hemolysis of red blood cells. Hemolysis can be due to direct insult from the burn, circulating factors released post-injury and capillary thrombosis in burned tissue. The hematocrit is elevated secondary to hemoconcentration and returns to more normal levels once fluids shift back to the intravascular space. Following successful completion of the fluid resuscitation phase, capillary membrane permeability is restored. Fluids gradually shift back from the interstitial space to the intravascular space, bringing with them sodium to the vascular space and potassium to the cells. The patient is no longer grossly edematous and diuresis is ongoing.