Electric shock and lightning injury

Introduction and epidemiology

Electricity is an integral part of our everyday world and electric shock is common. Patients may present to the emergency department (ED) with resulting injuries that range from trivial to fatal (termed electrocution). Although permanent disability can occur, it is reassuring to note that if the initial exposure is survived, subsequent death is unlikely. For each death caused by electricity, there are 2 serious injuries and 36 reported electric shocks.

There are approximately 20 electrical fatalities each year in Australia. Victims are predominantly male and young. Death is just as likely to occur at home as in the workplace, most often in summer. Electricians and linesmen are most at risk. The ratio of low-to-high-voltage deaths ranges from 3:1 to 7:1. The presence of water is associated with fatality. Electrical burns represent 3% to 5% of admissions to burns units.

Type of current

The vast majority of serious electrical injuries result from alternating current (AC), which is approximately three times as dangerous as direct current (DC). Alternating current can produce tetanic contraction of muscle such that the victim may not be able to let go of the current source. This is not a feature of direct current shock.

Human muscular tissue is sensitive to frequencies between 40 and 150 Hz. As the frequency increases beyond 150 Hz, the response decreases and the current is less dangerous. In Australia, a frequency of 50 Hz is used for household current because this is optimal for the transmission and use of electricity and also has advantages in terms of generation. As such, household current lies directly within the dangerous frequency range. It also spans the vulnerable period of the cardiac electrical potential and is thus capable of causing ventricular fibrillation.

Voltage

Voltage is the electromotive force in the system. In general terms, the greater the voltage, the more extensive the injury, but it must be remembered that the amount of current passing through the body will also be determined by resistance (Ohm’s law). High voltage is defined as greater than 1000 V. Household voltage in Australia is 240 V. Voltages less than 50 V (50 Hz) have not been proved hazardous. Survival has been reported following shocks of greater than 50,000 V.

Resistance

Different tissues provide differing resistances to the passage of electrical current. Bone has the highest resistance, followed by, in decreasing order, fat, tendon, skin, muscle, blood vessels and nerves. Skin resistance varies greatly according to moisture, cleanliness, thickness and vascularity. Moist skin may have a resistance of 1000 Ω, and dry, thick, calloused skin may have a resistance of 100,000 Ω. By Ohm’s law, dry skin resistance to a contact with a 240 V potential results in a current of about 2.4 mA, which is just above the threshold for perception. However, the resistance of wet or sweat-soaked skin drops to 1000 Ω, increasing the current flow to 240 mA, which is easily enough to induce ventricular fibrillation. Not surprisingly, moisture has been identified as a key factor in over half of electrocutions.

Current path

Prediction of injuries from knowledge of the current path is unreliable. Mortalities of 60% for hand-to-hand (transthoracic) and 20% for head-to-foot passage of current are quoted, but have not been verified. When current passes hand-to-hand (or hand-to-foot), only about 5% of the total current passes through the heart. If current passes leg-to-leg, no current traverses the heart.

Contact duration

The longer the duration of contact, the greater the potential for injury. Fortunately, most contacts are brief and frequently result in the victim being thrown back from the current source. This may result in a secondary injury, especially if the victim falls from a height.

Unfortunately, exposures to more than 10 mA of alternating current can induce sweating. Moisture decreases skin resistance and increases current flow, thereby reducing the ability to release the current source. This can progress to a fatal exposure.

Burns

As electricity traverses the skin, energy is converted to heat. The smaller the area of contact, the greater the current density, heat production and the consequent skin and adjacent tissue destruction.

Electrothermal burns are best characterized by arc burns, which result from the external passage of current from the contact point to the ground. These may be associated with extensive damage to skin and underlying tissue. Secondary flame burns may occur when the current arc ignites clothing or nearby combustibles.

Electrical burns may range from first degree to third degree. The typical appearance is of a central depressed charred black area surrounded by oedema and erythema. Single or multiple exit wounds may be present.