Introduction
Although severe electrical injuries are relatively uncommon, the true incidence remains unknown. Many electrocution victims fall from heights, present with dysrhythmias, or are simply found dead; the significance, and even the occurrence, of an electric shock may be unknown.
Electrical injuries tend to follow a bimodal age distribution. The first peak occurs in toddlers, who generally sustain electrical injuries from household electrical outlets and cords. The second peak occurs in adults who work with or around electricity for a living, such as miners, construction workers, and electrical utility workers [1].
The National Electronic Injury Surveillance System from the Consumer Product Safety Commission estimates that emergency departments treated 5,500–6,500 patients annually for product-related electrical shocks from 1992 to 2012 [2]. The majority of these incidents were minor, resulting in emergency department evaluation and subsequent discharge.
Most estimates place the annual death rate from electrical injury at 1,000–1,500 per year, with more than 60% occurring in adults 15–40 years of age. Electrocutions at home account for more than 200 deaths per year and are mostly associated with malfunctioning or misused consumer products [3]. As far as occupational exposure is concerned, according to the Electrical Safety Foundation International, there was an average of approximately 280 fatal electrical injuries per year in all industries from 1992 to 2008; one in 10,000 electrical utility workers in the United States dies from electrical injuries [4,5].
Basic concepts and pathophysiology
Electricity, a flow of electrons across a potential gradient from higher to lower concentration, requires both a complete path, called a circuit, to create continuous flow and a potential difference, measured in volts (V), to drive the electrons through the circuit. The volume of electrons flowing along this gradient is the current, measured in amperes (A). Resistance is the impedance to flow of the electrons and is measured in ohms (Ω).
In direct current (DC), electrons flow constantly in one direction across the voltage potential. Batteries are a common source of DC current, and high-voltage DC current is commonly used as a means for the bulk transmission of electrical power over long distances. Alternating current (AC) results when the direction of electron flow changes rapidly in a cyclic fashion. In the United States, standard household current is AC flowing at 60 cycles per second (Hz) and 110 V. In much of the rest of the world the standard household current is 220–240 V flowing at 50 Hz. Low voltage has been arbitrarily defined as less than 1,000 volts. As a general rule, high voltage is associated with greater morbidity and mortality, although fatal injury can occur with low voltage as well.
Six factors determine the outcome of human contact with electrical current: voltage, type of current, amount of current, resistance, pathway of the current, and duration of contact [6]. In many cases, the magnitude of only a few of these factors is known.
At the same voltage, AC exposure is considered to be about three times more dangerous than DC exposure. The differences in the two types of current have practical significance only at low voltages; at high voltages both currents have similar effects. AC current is more likely to produce explosive exit wounds, while DC current tends to produce discreet exit wounds. AC current is also more likely to cause muscular tetany than DC current. However, high-voltage contacts to both AC and DC current can produce a single violent skeletal muscle contraction, leading to the person appearing to be “thrown” from a voltage source.
The physical effects of different amounts of current vary. A narrow range exists between the threshold of current perception (0.2–0.4 mA) and the “let-go current” (6–9 mA) [7]. The let-go current is the level above which muscular tetany prevents release of subject’s grip on the current source. When AC current flows through the arm, even at the standard household frequency of 50–60 Hz, flexor tetany of the fingers and forearm can overpower the extensors. If the hand and fingers are properly positioned, the hand will grasp the conductor more tightly, leading to extended contact with the power source [8]. However, current flow through the trunk and legs may cause opisthotonic postures and leg movements if the person has not grasped the contact tightly. Thoracic tetany is also possible and can occur at levels just above the let-go current, usually at 20–50 mA, resulting in respiratory arrest. Ventricular fibrillation (VF) usually occurs at 50–100 mA.
Electrocution causes injury in several ways. As electrical current is conducted through a material, resistance to that flow results in dissipation of both energy and heat, leading to tissue damage from direct heating. The amount of heat produced during the flow of current can be predicted using Joule’s First Law, Q = I2Rt, where Q is the amount of heat generated, I is the current flowing through a conductor, R is the amount of electrical resistance, and t is the time of exposure. Using Ohm’s Law, I = V/R, the relationship between voltage and heat generation can be derived as Q = V2t/R. Therefore, if resistance and other factors remain constant, the heat from current flow through tissue increases proportionately to the duration of current flow, the square of the current intensity, and the square of the voltage differential. This conversion of electrical energy to thermal energy can result in massive external and internal burns.
In addition, electroporation, defined as the creation of pores in cell membranes by means of electrical current, can be caused by electrical charges insufficient to produce thermal damage but strong enough to cause protein configuration changes that threaten cell wall integrity and cellular function [9,10]. Finally, muscle contractions or falling can result in blunt mechanical injury from exposure to high voltage.
Because electricity requires a complete circuit for continuous flow, the path of electricity flow determines the tissues at risk, the type of injury, and the degree of conversion of electrical energy to heat. For example, current passing through the thorax might cause arrhythmias, direct myocardial damage, or respiratory arrest whereas cerebral current could cause seizures or motor paralysis. Nerves, blood vessels, mucous membranes, and muscles tend to have the least resistance because of their high concentration of electrolytes [11]. The tissues that have the highest resistance to electricity tend to increase in temperature and coagulate. In particular, bone, which has a very high resistance to electrical current, tends to generate a significant amount of heat and often causes damage to nearby muscles. Skin can have a wide range of resistance to electricity, with dry skin having a higher resistance than moist skin. As a result, a patient with dry skin may have extensive superficial tissue damage but more limited conduction of potentially harmful current to deeper structures. On the contrary, wet skin (e.g. electrocution of a person in a bathtub or swimming pool) offers almost no resistance at all, thus generating the maximal intensity of current that the voltage can generate [12].
