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Section 7

HIGH TENSION ELECTRICAL BURNS

GENERAL PRINCIPLES

In order to understand electrical current injuries, familiarity with the terminology is necessary.

Terminology:

• Household Current                 =100 to 220 volts

• Residential Power Lines        =5000 to 10,000 volts

• High Tension Wire                  = up to 100,000 volts

The electrical current produces heat when it meets a resistance to flow. 

Electrical current produces tissue damage in two ways:

Voltage and Amperage:

Electrical injuries are classified into low voltage (less than 500 volts) and high voltage (greater  than 1,000 volts). Low voltage injuries occur characteristically in a home or residential environment. Electrocutions in bathtubs and by electric hair dryers are the most common causes of low-voltage deaths. High-voltage injuries characteristically occur in an outdoor environment near power sources  and lines. Electrical current can arc (jump) 1 inch from the power source or line for every 10,000 volts being carried, so that a person does not actually have to touch the source to sustain an injury. 

The severity of injury to tissues is dependent on the amperage, ie., the actual amount of current passing through the tissues. It is impossible to know the amperage amount because of the variability of the resistance and exposure time at the accident, but one can infer amperage from the voltage of the source, at least as high or low. A low-voltage source is capable of producing major cardiopulmonary complications and death if a sufficient current passes across the chest to initiate ventricular fibrillation.

A high-tension source is usually required to produce the tissue necrosis characteristically seen along the path of the current. The damage is caused by both heat and production of direct current damage. Local tissue damage at the contact site can be seen with low voltage as well, again due to local heat generation. A good example is the oral burn seen in the children biting on the electrical cord.

Resistance

The resistance of tissues to passage of current is in large part dependent on water content, water being a good conductor.  Tissue resistance in decreasing order from high to low is bone, fat, tendon, skin, muscle, vessels, nerve.  However, with high-voltage injuries, the current readily passes through all tissues indiscriminately.  Similarly, the heat produced as current enters and exits the body leads to coagulation necrosis of the skin and underlying tissues.

Pathway of Current

The pathway of current can be somewhat unpredictable, but, in general, current passes from a point of entry through the body to a grounded site, i.e., a site of lower resistance to flow compared with air, which is a poor conductor.  Extremely high voltage sources usually exit in multiple areas in an explosive fashion.

Type and Duration of Current

High-voltage AC injuries are more severe than DC, although both will lead to devastating injuries.  The severity of injury is also directly proportional to the duration of current flow.  However, even extremely brief exposures to high amperage will produce massive tissue damage.

Cutaneous Injury (Entrance and Exit)

The determination, that a current injury to underlying tissue may be present is the finding of entrance and exit sites.  Their presence is pathognomonic of an electrical injury beneath the skin.

The heat generated at the skin surface is dependent on the local resistance, which in the dry hand can be sufficient to generate heat in excess of 1000°C with high-voltage sources.  This will lead to local mummification at the entrance.  The skin appearance at the site of contact is that of a well-defined charred wound that is depressed due to loss of tissue bulk.  The latter is due to evaporation of local water content by the high temperature.

Tissue appearance at the site of current exit varies considerably.  With moderate exposures, the appearance is often that of small skin ulcerations with a depressed center and heaped up edges.  Look for the exit site where the patient was believed to be grounded.  With passage of a large current, multiple exit sites are frequently seen along the route of the current.  The

appearance is often that which would be expected from an explosion, since pieces of cutaneous tissue are often absent, having been blown out by the immense energy of the exiting current.

Remember: the exit wound is a deep wound and the magnitude of injury can easily be underestimated by the appearance on the surface.

Muscle Necrosis

Electrical burns more closely resemble a crush injury than they do a thermal burn.  The damage below the skin where the current passes is usually far greater than the appearance of the overlying skin would indicate.  The immediate damage to these tissues is caused by the heat destruction of cells, which is  usually patchy in distribution along the course of the current.  The second process is that of devascularization caused by injured blood vessels that progressively thrombose over a period of several days.  The third process is that of compartment syndrome with pressure necrosis especially prominent in nervous tissue and muscle enveloped by a non yielding fascial covering.

Myoglobin and hemoglobin released from damaged muscle and red blood cells can precipitate in the renal tubules, producing an acute tubular necrosis picture.  Myoglobin is colorless in circulating plasma, whereas free hemoglobin is, of course, pink to red.  Both, however, will produce pink to red urine.  Myoglobin precipitation is accentuated by an acid urine and decreased by an alkaline urine.  The low flow state caused by hypovolemia will simply aggravate the injury.  Direct renal vascular damage from the current can also result.

Neurologic abnormalities are common.  Acute central nervous system dysfunction with coma, seizure, motor, and, to a lesser extent, sensory deficits are well described.  Many of these abnormalities are permanent.  In addition, a number of delayed injuries occur, including both peripheral neuropathies and cord damage with paralysis.

Other Complications

Immediate cardiac arrest is the most common cause of death after electrical injury.  The process is due to both the direct alteration of rhythm by the current, leading to fibrillation or to the depression of respiration and subsequent hypoxia.  Both brain and chest wall muscle changes will occur, leading to impaired ventilation.  Hand to hand passage of a high voltage current has a reported immediate mortality of 60%.  

Renal failure is reported in 10% or more of injuries.

Within minutes to hours of injury, the damaged but still perfused muscles begin to swell.  The finite boundaries of the fascial envelope causes a rapid increase in tissue pressure, which, when exceeding the 20 to 25 mmHg microvascular hydrostatic pressure, produces further local tissue ischemia and local compressure nerve injury.  A tissue pressure exceeding 30 mmHg is clearly abnormal and must be decreased to avoid further damage.  Edema increases over the ensuring 24 to 48 hours.

The combination of these processes, the uneven nature of the necrosis, and, in particular, the damage to muscles and nerves closest to bone results in severe functional impairment and a high rate of amputation (30 to 40%)  

Multiple "exit" sites from a 15,000 volt exposure, including buttocks and extremities, thermal injury to back from local high heat production and clothes catching fire.

Besides heat injury, there is also tissue damage from the current itself.  The mechanism of tissue damage is complex and not completely defined.  There is clearly an injury to nerves, blood vessels, and muscle from the current itself, damaging the cells.  Endothelial cell injury due either to heat or current will result in loss of protection against local clotting and microvascular thrombosis, and tissue devascularization will result.