ROSENS EMERGENCY MEDICINE Chapter 134
Lightning and Electrical Injuries
Kelly P. O’Keefe, MD
Rachel Semmons, MD
PRINCIPLES
Background
Electrical injury
The first recorded death from electric shock occurred in 1879 in Lyons, France, after a carpenter sustained an injury from a 250V AC source. The first electrocution in the United States (U.S.) occurred in 1901 when an intoxicated gentleman made contact with a DC generator terminal in front of a World’s Fair crowd in Buffalo, New York. The patient’s death appeared to be quick and painless, prompting the suggested use of electrocution for capital punishment.
Electrical injuries have a trimodal age distribution. Toddlers and younger children experience low-voltage injuries in the household as the result of contact with electric sockets and cords. Adolescents and young adults more frequently experience high-voltage injury from contact with electric lines outside of houses. Another peak occurs in the third to fourth decade of life almost exclusively in men with occupational injuries due to high-voltage encounters with power lines, and to a lesser extent from electric tools. Electrical injuries comprise only a small percentage of workers’ compensation claims, but are the second highest source of indemnity and paid medical claims, are responsible for many days of lost work, and are significant causes of permanent disability.1
Electric burns comprise only a small percentage of admissions to burn centers, but these patients have a more prolonged hospital course, require more interventions (such as fasciotomies, escharotomies, and amputations), and have higher mortality rates than patients with thermal burns.
While electrical injuries from inadvertent household and occupational exposure have been on the decline, the use of electronic conduction weapons (e.g., Tasers and stun guns) is increasing. These weapons, which deliver brief bursts of high-voltage, low-amperage direct current, are favored by law enforcement because they incapacitate subjects with minimal morbidity and lethality. Industry manufacturers report over 1.5 million discharges since these devices were made available for commercial use, and they are largely believed to be safe. 2
Lightning
The incidence of lightning-related deaths in the U.S. has declined from a longstanding average of over 100/year to less than 40/year in recent times, perhaps as the result of American urbanization. Approximately 10% of lightning strike victims die, usually within the first hour from fatal arrhythmias or respiratory failure, and the majority of survivors suffer permanent disabilities. Victims are most commonly young males who work or play outdoors in the spring and summer months, often during thunderstorms. Lightning usually strikes single victims but injuries to multiple victims can occur when a bolt lands amid an outdoor group, often athletes or observers at sporting activities.
(H2) Physics of Electricity
Electrical Injury
The degree of injury from electrical shock depends on multiple factors including the type of circuit, current, resistance (measured in amperes), voltage, duration of contact, and pathway of flow (Box 134-1).
Type of Circuit
Electrical sources create current that either flows in one direction (direct current, or DC), or alternates direction cyclically at varying frequencies (alternating current, or AC). The few systems that utilize DC include batteries, automobile electronics and railroad tracks. Exposure to DC most frequently causes a single, strong muscular contraction. This may throw the subject back from the source in a way that limits duration of exposure but can result in other injuries. AC is more commonly used (e.g., household currents) because it conveniently allows for an increase or decrease of power at transformers. It is more dangerous than DC of similar voltage, as amperage above the “let-go current” will cause muscular tetanic contractions. Since the flexor muscles of the upper extremities are stronger than extensor muscles, these contractions pull the victim closer to the source and result in prolonged exposure. Box 134-2 shows the physical effects of different amperage levels at a common 60-Hz AC exposure.
Capacitors store electric charge in circuits, and discharge from these devices may result in sudden bursts of very large amounts of electrical energy. Injury from a capacitor may occur even when the electrical device is not energized (plugged in), often involving the unwary repairman.
Current
Current is the flow of electrons down an electrical gradient. It is measured in units of amperage. According to Ohm’s Law, current is directly proportional to the voltage of the source and inversely proportional to the resistance of the material through which it flows. Key laws regarding the physics of electrical injury are summarized in Box 134-3.
Resistance
Resistance is the degree to which a substance resists the flow of current. Resistance varies amongst body tissues. Neurovascular tissues are very good conductors of electricity, while tendons, fat, and bone are relatively poor conductors (see Box 134-4). Within a given tissue, resistance differs based on fluid and electrolyte content of cells. Dry skin offers the largest resistance, up to 100,000 Ohms in thick, calloused skin, but dermal resistance decreases to as little as 1,000 Ohms when wet. Current that is initially unable to pass through skin will create thermal energy and cause significant burns. As the skin blisters and deteriorates, its resistance decreases.
Current may jump across skin surfaces in a behavior called arcing, resulting in prominent burns across flexor surfaces. Internally, current follows the path of least resistance, and the degree of burns seen on the surface typically underestimates the damage occurring below the surface. As current strength increases, the relative resistance of tissues ceases to determine the pathway of current, and the entire body functions as a conductor.
Voltage
Voltage is the electrical potential, or difference in electrical energy, between two points. Joule’s Law, which describes the amount of thermal energy applied to tissues from electricity, is P = I2RT, where I is the amperage, R is the resistance, and T is the time that the electricity is applied. As the formula indicates, voltage is not the only factor responsible for damage, but it is often the only property that is known in cases of electrical injury. As a result, injuries are conventionally classified as being caused by high- or low-voltage sources, with 1000V as the dividing line. In the U.S. and Canada, household sources are low-voltage, typically 120V or 240V. High-voltage injuries are impressively characterized by partial to full thickness skin burns, deep tissue destruction, and frequent cardiac or respiratory arrest. Low-voltage exposure causes less surface damage, but may be equally lethal, particularly in cases where skin resistance is low.
Duration
The degree of tissue damage is directly proportional to the duration of exposure for all voltage levels. Exposure times greater than the length of one cardiac cycle tend to generate arrhythmias, likely in a manner analogous to the R-on-T phenomenon.
Pathway
The pathway followed by electrical current determines morbidity and mortality. The entrance and exit sites of the electrical current typically demonstrate greater evidence of skin damage, with full thickness burns commonly encountered. These sites are properly referred to as source contact point and ground contact point. A patient may have one or multiple source and ground contact points. The most common locations for these sites are the hands, wrists, and arms, but children also present with burns from oral contact with electric cords or sockets. The most common ground contact points are the heels.
Electric current passing through a limb causes greater local tissue damage than current passing through the trunkbecause the smaller cross-sectional area limits the ability to dissipate heat. However, current passing through the trunk results in greater mortality due to the involvement of more vital organs. Transthoracic pathways (arm-to-arm) are more likely to generate arrhythmias, and have higher mortality rates than vertical currents (leg-to-arm) or straddle pathways (leg-to-leg).3
Electronic Control Devices
The most commonly used device in this category is the Taser X26 (Fig. 134-1). The weapon consists of a handheld unit with two barbs that are deployed through connecting cables. The sharp portions of these barbs are 9mm thick (Fig. 134-2), and typically lodge in the skin, but can discharge current through clothing as well. The standard initial discharge is a five second burst of stimuli at 19Hz. The peak voltage delivered to the subject is 1200V, but the amperage is low at 2.1mA. The primary burst is followed by 100msec pulses intended to inhibit myoneurons, causing the subject to fall to ground. These weapons can also be used as a direct contact device in a technique called “drive stun.” When used in this way, the unit’s barbs are not deployed, and the subject experiences pain, but not muscular paralysis. Newer commercial devices designed for personal protection have the potential for increased amperage and time of exposure, thereby increasing the opportunity for more detrimental outcomes.2
Lightning Injury
While the same basic scientific principles of electricity apply to lightning, there are several major differences. Lightning strikes involve hundreds of millions of volts, significantly more than man-made electrical sources. In contrast, the duration of contact is drastically shorter, averaging 30 microseconds. As a result, current flow is altered, with a reduction in penetration causing less destructive tissue effects overall.
Lightning occurs as the result of a tremendous potential difference between positively and negatively charged particles. Water particles carried on warm updrafts rise from the earth and are cooled in the upper air, where cloud formation takes place. The phase change of the water from gas to liquid and even solid is associated with the generation of an electric charge. An intense electric field is created, with positive charges concentrated in the upper portions of the cloud, and negative charges inferiorly. The earth, which is normally negatively charged with respect to the atmosphere, becomes positively charged when the thunderstorm, with its negatively charged cloud base, passes overhead. A potential difference in charge as great as 100 to 300 million volts is created in this process. When the potential difference surpasses the resistance of the intervening air and other elements, a lightning bolt, like a huge spark, is generated. Initially small sparks are emitted and travel small distances from the ground upward, but these enlarge quickly as the distance between charged items shortens and the potential difference increases. Pilot strokes from the ground eventually meet a leader stroke from the cloud, a conducting pathway is formed, and a massive surge of electrical discharge follows. It takes approximately 20 milliseconds for a leader to reach the ground from the cloud, but only 20 microseconds for the return stroke from the ground up to be completed. Several discharges may take place in the same channel, with each discharge often exceeding 10 million volts. Thunder is a result of the superheating of the channel (estimated at 50,000 degrees F), which causes rapid expansion and compression of the local air, creating a tremendous sound pulse. Lightning takes various forms, described as streaked, forked, ribbon, sheet, or beaded. The most unusual form is ball lightning, which appears as a globe, rolls along structures, and may even pass through open doors or windows. Strikes occur from cloud to cloud, cloud to ground, or even ground to cloud.
Lightning may strike a person directly, may strike a tree or other object and injure anyone in direct contact with it (contact voltage), or may strike its target and then travel through the air to impact the victim (side flash, or splash injury). One’s chances of being struck are increased by wearing or carrying metal objects or other conductors. Side flashes may travel as far as 30 meters after striking the first object to impact another. Lightning may also strike the ground and be conducted to the victim. A greater distance from the ground strike lessens transmission of the charge. The risk of injury from a ground strike is increased when one contact point on the victim (eg., the right foot) is closer to the strike than a second contact point (e.g., the left foot), thus creating a potential difference. This is referred to as stride voltage, and is likely responsible for cattle deaths in a pasture after a thunderstorm. Hence, when out in the open during a storm, one is cautioned to place an insulating material, such as a raincoat, between the ground and their body, and to assume the “lightning position”- a squatting configuration with the feet together- or to curl up in a ball on the ground to reduce the number of contact points. Box 134-5 lists tips to avoiding lightning strikes.
Injury occurs from the force of a strike, from blunt trauma effects when the victim is thrown, from the superheating of metallic objects in contact with the patient, from blast-type effects and barotrauma, or from shrapnel.
Clinical Features
Electrical Injury
General Tissue Effects
At the cellular level, current causes damage to cell membranes and alters membrane solubility, leading to electrolyte abnormalities and cellular edema. This process, referred to as electroporation, eventually leads to irreversible cell damage and death.3 At the tissue and organ level, electric current produces damage when electrical energy is converted to thermal energy.
Skin
Most electrical injuries result in skin burns, which fall into one or more of four patterns as listed in Box 134-6. The relatively high resistance of skin in many cases leads to significant partial or full thickness burns at entrance and exit sites, most commonly seen over the upper extremities, particularly at the hand and wrist. The skull is another common source contact point. Visible burns may be insignificant in comparison with the damage that occurs beneath the surface, and as a result, are associated with significantly greater morbidity than simple thermal burns involving a similar surface area. Burns at entrance and exit sites will typically have a punctate appearance, with central depression and necrosis surrounded by a hyperemic border.
Arc burns, or “kissing burns” (Fig. 134-3), occur when electricity jumps from skin surface to skin surface, typically across flexed areas of the body. Temperatures may reach 3500°C and cause severe damage. Arc burns are noted most commonly across the volar forearm and elbow, and along the inner arm and axilla. In events where clothing catches fire, patients also experience typical thermal burns. Flash burns are skin burns caused by brief, intense flashes of light, electrical current or thermal radiation. Cutaneous burns across the chest and upper abdomen hint at transthoracic current and a worse prognosis.3
Cardiovascular System
Cardiac or respiratory arrest is the most common cause of death immediately following electrical injury. Traditionally, it is held that AC generates ventricular fibrillation, whereas DC results in asystole. In reality, exposure to either type of circuit is associated with both dysrhythmias. (3) Respiratory arrest may occur as a result of tetanic paralysis of the thoracic respiratory muscles or as a result of damage to the brainstem respiratory centers. Prolonged apnea in these situations leads to hypoxic cardiac arrest.
Dysrhythmias are seen immediately or in a delayed fashion following electrical injury. These include malignant arrhythmias, but more frequently involve sinus tachycardia or bradycardia, atrial fibrillation, and ectopic beats. A variety of ECG abnormalities may be present, including transient ST elevation or depression that does not correlate with myocardial ischemia or infarction. Injury to coronary arteries or directly to the myocardium may result in infarction, but this is rare. Non-specific cardiac biomarkers are frequently elevated in the period following electrical injury, but this is usually due to skeletal muscle injury and only rarely related to cardiac damage.
Head and Neck
Ocular involvement is common following current exposure, with cataracts the most frequent manifestation. Other forms of injury include vitreous and anterior chamber hemorrhages, retinal detachment, macular lacerations, and corneal or conjunctival burns. Injury to structures of the ear is less common, but sensorineural deafness can be seen as a result of nerve damage. Patients frequently develop vertigo, which may be transient or persistent. (4) Toddlers and young children sustain orofacialinjuries after chewing or sucking on electrical cords, or from lingual contact with sockets. Full-thickness burns may be sustained on the mucous membranes and lips, with destruction to the tongue and teeth as well. Injuries to the oral commissure produce cosmetic difficulties and, more significantly, the well-recognized complication of delayed labial artery bleeding, typically occurring two days after injury when the resultant eschar separates from the wound. (5)
Extremities
Neurovascular bundles have low resistance and are particularly prone to damage from electric current. Muscle necrosis occurs primarily, or secondary to compromise of the blood supply. Vascular injury is most prominent at the intimal and medial layers. Involvement of the intima results in immediate coagulative necrosis and thrombosis, while injury to the media causes aneurysmal dilatation and hemorrhage in a delayed fashion. Decreases in tissue perfusion lead to edema and tissue death. Areas of infarction may be distributed sporadically throughout the injured region, with areas of surviving tissue adjacent to necrotic tissue. Muscle that initially appears viable may deteriorate with time, especially in the periosteal regions. Endothelial and smooth muscle function is depressed for many weeks following the initial injury, contributing to a hypercoagulable state that increases the risk of delayed deep venous thrombosis.(6) The combination of tissue edema and perfusion defects makes compartment syndrome likely, with fasciotomy and even amputations required. Cyanosis or pulselessness may be transient, or may indicate permanent damage; in a similar fashion, limbs that appear initially well perfused may later necrose. 7