Mechanisms of injury
A burn is a tissue injury from thermal (heat or cold) application, or from the absorption of physical energy or chemical contact (Table 14.1). Each has its own distinctive features and management problems.
Hot water produces a particularly well-defined type of skin damage. The temperature of boiling water (1000C) or steam is constant and the major determinant of the severity of injury is the duration of contact. In the home, spills from kettles or cooking pots are common injuries of childhood. As with all burning accidents, those least able to protect themselves (the very young, the very old and the very drunk) are particularly vulnerable. Children reaching up to grasp the flex of an electric kettle, or a pot handle, can drench themselves in boiling water, and the larger the volume, the more severe the injury in terms of area and depth. Even a cup of tea or coffee can cause considerable scalding. In predicting the outcome, it is worthwhile knowing whether milk has been added, thus lowering the temperature, although this question may seem frivolous to an anxious parent. Common areas involved are the face, neck and upper trunk or limbs. Although spills cool rapidly, limiting the duration of damage, the worst injuries of this type may leave permanent scarring. Immersion in boiling water, or prolonged steam exposure as in some industrial accidents (where superheated steam may have a temperature above 1000C), are particularly dangerous and likely to cause deeper burns.
Cooking fat or oil has a much higher temperature (1800C) than boiling water and hot fat cools slowly on the skin surface. Spills therefore cause deep burns.
Flame burns have a varied aetiology: house fires, clothing fires, spills of petrol on the skin, butane gas fires. They often occur in confined spaces and may be associated with inhalation injury. It is important to know whether the clothing ignited and how the flames were extinguished (did clothing burn away?). Generally, deep burns will result. Some garments may protect, but if clothing ignites there is a prolonged flame contact with the skin. Flame-retardant clothing may burn under extreme conditions.
The passage of electric current through the tissues causes heating that results in cellular damage. Heat produced is a function of resistance of the tissue, the duration of contact and the square of the current. Bone is a poor conductor of electrical current, whereas blood vessels, nerves and muscles are good conductors. Bone can therefore become very hot and cause secondary damage to tissues near to the bone. Low voltage (<1000 V) such as from a domestic supply (240 V, 50 Hz) causes significant contact wounds and may induce cardiac arrest, but no deep tissue damage.Highvoltage burns (>1000 V) cause damage by two mechanisms:
flash and current transmission. The flash from an arc may cause a cutaneous burn and ignite clothing, but will not result in deep damage. High-voltage current transmission will result in cutaneous entrance and exit wounds and deep damage. Lightning strikes cause very high-voltage, very short -duration discharge. A direct strike has a high mortality. A side strike may cause superficial burns to the skin and deep exit burns to the feet. Internal damage is not common, but respiratory and then cardiac arrest occur. Some ‘electrical’ injuries are not associated with the passage of current through the tissues, such as bar-fire burns, where a child grasps a heated and unguarded element of an electric fire and the damage is due to heat alone.
Tissue damage from cold can occur from industrial accidents due to spills of liquid nitrogen or similar substances. The injuries cause acute cellular damage with the possibility of either a partial-thickness or full-thickness burn. Severe cooling can freeze tissues and ice formation is particularly likely to cause cellular disruption. Freezing injuries, however, seem to be less damaging to the connective tissue matrix than heat injuries. Frostbite is due to prolonged exposure to cold and there is often an element of ischaemic damage. Vasoconstriction reduces the resistance of the tissue to cold exposure as the warming effect of the circulation is reduced. There is therefore combined tissue damage from freezing, together with vasospasm. Such injuries occur after exposure to severe cold such as that encountered in mountain regions or arctic conditions. It is a rare injury in the UK.
The tissue damage in friction burns is due to a combination of heat and abrasion.
There is generally a superficial open wound that may progress to full-thickness skin loss. Friction burns may be associated with degloving injuries where the damage is judged to be deep. Early surgical excision and skin cover is the best means of management.
X-irradiation may lead to tissue necrosis. Such injuries are exceedingly rare if industrial and medical safety precautions are working. The tissue necrosis may not develop immediately. These injuries are generally limited in area and surgical excision, and flap reconstruction may be appropriate management. Of greater significance is the long-term cumulative effect of ionising radiation in the induction of skin cancers and other tumours.
Numerous chemicals in industrial and domestic situations can cause burns. Tissue damage depends on the strength and quantity of the agent and the duration of contact. Some agents penetrate deeply or may have specific toxic effects. Chemicals cause local coagulation of proteins and necrosis, and some also have systemic effects (e.g. liver and kidney damage with tannic, formic and picric acids). The harmful effect will continue until the chemical is diluted or neutralised. The most important initial treatment is dilution with running water. Some specific agents cause particular problems or have specific remedies .
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