Editing Burns (section)

== '''Pathophysiology''' ==

* Thermal burns cause damage to skin and occasionally underlying structures through abrupt temperature change that exceeds biological tolerance. This leads to membrane disruption, protein denaturation, and necrosis.  The injury extends from the skin surgace to deeper structures in a first-order logarithmic distribution depending on the temperature of the burning agent and duration of exposure.
* Flame, scald and contact burns induce injury by transfer of energy, causing coagulative necrosis. Chemical and electrical burns also cause direct injury to cellular membranes.
* Zones of injury - 'Jackson levels' - has been questioned recently, but helpful paradigm for now.
** Zone of coagulation - necrotic area of burn, where cells are directly disrupted. Irreversibly damaged at the time of injury.
** Zone of stasis - moderate degree of insult, with decreased perfusion. Can either survive or go on to coagulative necrosis.
** Zone of hyperaemia - vasodilation from inflammation surrounding the burn wound. Contains the clearly viable tissue from which the healing process begins. Generally not at risk for further necrosis.
* Systemic effects
** Systemic inflammatory response to the burn - many mediators involved
** Generalised oedema in both burned and unburned skin
** Pre-renal AKI common, sometimes bad enough to lead to ATN
** Global depression in immune function - higher risk of bacterial wound infection, pneumonia, and fungal and viral infections. With burns >20% TBSA, this impairment is proportional to burn size.
** Hyper-metabolism - tachycardia, increased oxygen consumption, and massive proteolysis/lipolysis. Can be sustained for months, leading to massive weight loss and decreased strength.
* Inhalation injury
** Damage primarily from chemical burns associated with inhaled toxins
** Heat is dispersed in the upper airways, so direct thermal lung injury is rarely seen, except in high-pressure steam inhalation
** Smoke inhalation leads to immediate airway oedema and increased lymph flow, which is high in inflammatory mediators, and may cause further lung injury. Also leads to exudate formation within airways, which eventually coalesces into fibrin casts, which are difficult to clear with standard airway suction techniques.
** Three phases of inhalation injury

**# Acute pulmonary insufficiency
**# Increased extravascular lung water, hypoxia, and diffuse lobar infiltrates (72-96 hours post-injury); similar to ARDS
**# Clinical bronchopulmonary pneumonia (generally 3-10 days post-injury). Early pneumonias are generally staph, whereas gram negative organisms come later.

=== '''Burn depth''' ===

* Depth of injury is best assessed by experienced practitioners, although imaging may help in the future
* If unsure whether superficial or deep partial-thickness, assume deep
*

{| class="wikitable"
|'''Depth'''
|'''Appearance'''
|'''Sensation'''
|'''Healing time'''
|-
|Superficial (epidermal)
|Dry, red

Blanches with pressure
|Painful
|3 to 6 days

Won't scar. Topical salves and NSAIDs.
|-
|Superficial partial-thickness (epidermis and papillary dermis)
|Blisters (within 24 hours)

Moist, red, weeping

Blanches with pressure

Retention of hairs to gentle pulling.
|Painful to temperature and air
|Spontaneously re-epithelialize in 7 to 21 days. May have some slight discolouration and textural differences. 
|-
|Deep partial-thickness (epidermis and reticular dermis)
|Blisters (easily unroofed)

Wet or waxy dry

Variable color (patchy to cheesy white to red)

Does not blanch with pressure
|Perceptive of pressure only. Painful to pin-prick.
|Re-epithelialize in 15-21 days from deep hair follicles and sweat gland keratinocytes, often with severe scarring.
|-
|Full-thickness (into subcutaneous fat)
|Waxy white to leathery gray to charred and black

Dry and inelastic

No blanching with pressure
|Deep pressure only
|Rare, unless surgically treated. No keratinocytes remain - so must heal by re-epithelialization from wound edges. Benefit from excision of eschar and autologous grafting to minimise contractures.
|-
|Deeper injury (ie, fourth degree) 
|Extends into fascia and/or muscle
|Deep pressure
|Never, unless surgically treated
|}