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.
Scalds
Hot water produces a particularly well-defined type of skin damage (Fig.
14.2). 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 (Fig.
14.3).
Flame
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 (Fig. 14.4). 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.
Electrical
burns
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 (Fig. 14.5).
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
Cold
injury
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.
Friction
burns
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.
Ionising
radiation
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.
Chemical
burns
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
(Table 14.2).