Narrative Review: Electrocution and Life-Threatening Electrical Injuries

Patients who experience electrical shock, including being struck by lightning, sustain a wide spectrum of injuries with unique pathophysiologic characteristics that require special management. We review the physics and mechanisms of tissue damage, typically encountered injuries, and the prognosis of patients with severe injuries. Epidemiology It is estimated that approximately 1000 people die of exposure to electricity annually in the United States (1, 2). The age distribution of patients who are electrocuted is bimodal; the first peak occurring in children younger than 6 years of age, and the second occurs in persons in young adulthood (3, 4). Electrocution in children usually occurs at home. Most deaths in adults due to electrocution are work related, and electrocution is a frequent cause of occupation-related death. Miners and construction workers account for most of these cases, with rates of 1.8 to 2.0 deaths per 100000 workers (5). Patients surviving electrical shock represent 3% of approximately 100000 patients admitted to specialized burn units annually (6). Physics Electricity is defined as the flow of electrons across a potential gradient from high to low concentration. This potential difference, expressed in voltage (V), represents the force driving the electrons. The amount or volume of electrons that flow along this gradient is called current and is measured in amperes (I). The impedance to flow is described as resistance (R). Ohm's law expresses the relationship among these factors as I = V/R (Figure 1). Using this equation, it is easy to see that current is directly proportional to voltage and inversely proportional to resistance. In alternating current, the direction of electron flow changes rapidly in a cyclic fashion, for example, standard household current of 110 V flows at 60 cycles per second (60 Hertz [Hz]). Direct current, on the other hand, flows constantly in 1 direction across the potential. Examples of entities that produce or carry direct current include batteries, automobile electrical systems, high-tension power lines, and lightning. Figure 1. Ohm's and Joule's laws. The primary determinant of damage caused by direct effects of electricity is the amount of current flowing through the body, which can potentially lead to fatal arrhythmias or apnea. Additional factors that determine damage include voltage, resistance, type of current, current pathway, and duration of contact with an electrical source (7). Joule's law describes the relationship of 3 of the aforementioned factors, with thermal energy generated as follows: Energy (thermal) = I2RT, with T representing the time of current flow. This equation demonstrates the relationship of the squared function of current and time and resistance to the amount of thermal energy delivered, which leads to tissue damage. Tissues that have a higher resistance to electricity, such as skin, bone, and fat, tend to increase in temperature and coagulate. Nerves and blood vessels that have low resistance to electricity (I = V/R) conduct electricity readily. The skin has a wide range of resistance to electricity and plays the crucial role of gatekeeper when the body is exposed to electricity. Dry skin, which has a higher resistance to electricity than moist skin, may have extensive superficial tissue damage but may limit conduction of potentially harmful current to deeper structures (8). Moist skin receives less superficial thermal injury but allows more current to pass to deeper structures, resulting in more extensive injury to internal organs. Mechanism of Injury Electrical shocks of 1000 V or more are classified as high voltage (9). Thus, low-voltage electrical shocks are less than 1000 V. Although this classification appears to be somewhat arbitrary, voltage is often the only variable known with certainty after exposure to electricity and therefore is the most reasonable marker for categorizing electrical shocks. High-voltage electrical shocks are expected to result in more severe injury per time of exposure. Typical household electricity has 110 to 230 V, and high-tension power lines have voltages of more than 100000 V. Lightning strikes are can produce 10 million V or more (8). There are 4 causes of electrocution: 1) direct effect of current on body tissues, leading to asystole, ventricular fibrillation, or apnea; 2) blunt mechanical injury from lightning strikes, resulting in muscle contraction or falling; 3) conversion of electrical energy to thermal energy, resulting in burns; and 4) electroporation, defined as the creation of pores in cell membranes by means of electrical current (10). Unlike thermal burns, which cause tissue damage by protein denaturation and coagulation, electroporation disrupts cell membranes and leads to cell death without clinically significant heating. This form of injury occurs when high electrical field strengths (defined as volts per meter) are applied (11). Direct current causes a single muscle contraction, often throwing the person receiving the electrical shock away from the source of electricity. Alternating current is considered more dangerous than direct current because it can lead to repetitive, tetanic muscle contraction. In the case of contact between the palm and an electrical source, alternating current can cause a hand to grip the source of electricity (because of a stronger flexor than extensor tone) and lead to longer electrical exposure. The amount of alternating current needed to cause injury varies with the frequency of the current. Skeletal muscles become tetanic at lower frequencies, ranging from 15 to 150 Hz (8, 12). Household electricity (60 Hz) is particularly arrhythmogenic and may lead to fatal ventricular arrhythmias. Alternating current is the most frequent cause of electrocution. Typical Injuries Skin Burn injuries are categorized into 4 groups: electrothermal burns, arc burns, flame burns, and lightning injuries. We discuss the last in greater detail in the section titled Special Circumstances. Electrothermal burns are the classic injury pattern and create a skin entrance and exit wound. Regardless of the mechanism involved, wounds due to exposure to electricity can be classified as partial-thickness, full-thickness, or skin burns involving deeper subcutaneous tissue. High-voltage injuries commonly produce greater damage to deeper tissues, largely sparing the skin surface. Thus, using estimation of surface burns to guide therapy may lead to critical errors because minor superficial injury may be associated with massive coagulation necrosis of deeper tissue (13, 14). Respiratory Respiratory arrest immediately following electrical shock may result from inhibition of the central nervous system respiratory drive, prolonged paralysis of respiratory muscles, tetanic contraction of respiratory muscles, or a combined cardiorespiratory arrest secondary to ventricular fibrillation or asystole. In the last case, respiratory arrest may persist after restoration of spontaneous circulation, presumably because of the inherent automaticity of cardiomyocytes, which cause quicker recovery of cardiac function. If respiratory arrest is not corrected promptly by ventilation, secondary hypoxic ventricular fibrillation may occur (15). Parenchymal lung damage is rarely seen in patients who have experienced electrocution or received an electrical injury. Cardiovascular Cardiac effects of electrical shock can be divided into arrhythmias, conduction abnormalities, and myocardial damage. The last category can be further separated into injury due to direct electricity exposure and secondary myocardial injury due to induced ischemia. These effects are not mutually exclusive. Arrhythmias Sudden cardiac death due to ventricular fibrillation is more common with low-voltage alternating current, whereas asystole is more frequent with electric shocks from direct current or high-voltage alternating current (16, 17). Experimental studies show alternating current to be more hazardous than direct current applied at the same voltage. In a dog model, ventricular fibrillation occurred 9 times more often with alternating current than with direct current shocks. Of interest, at voltages ranging between 50 to 500 V, the incidence of ventricular fibrillation was inversely proportional to voltage and the occurrence of ventricular tachycardia and atrial fibrillation were directly proportional to voltage (16). Potentially fatal arrhythmias are more likely to be caused by horizontal current flow (hand to hand); current passing in a vertical fashion (from head to foot) more commonly causes myocardial tissue damage (1820). Survivors of electrical shock frequently experience some form of subsequent arrhythmia (10% to 46%) (17, 21, 22). The most common arrhythmias are sinus tachycardia and premature ventricular contractions, but ventricular tachycardia and atrial fibrillation have been reported (17, 2123). Most arrhythmias occur soon after the electrical shock, but delayed ventricular arrhythmias (noted up to 12 hours following an incident) may occur (24). Most patients not experiencing sudden cardiac death have nonspecific STT-wave abnormalities on 12-lead electrocardiography (ECG) that usually resolve spontaneously (25, 26). Patients without ECG changes on presentation are unlikely to experience life-threatening arrhythmias (22). Conduction Abnormalities Sinus bradycardia and high-degree atrioventricular block have been reported following electrical shocks. Electrical injury caused by alternating current seems to have a predilection for the sinoatrial and atrioventricular nodes (27). The reason for this vulnerability is unclear. It has been hypothesized that the sinoatrial- and atrioventricular-node ion channels are the easiest to disrupt and that ischemia and infarction in the right coronary artery distribution (running closest to the chest surface and supplying both nodes) make the nodes more vulnerable to electrical current. Some of th