Type of Circuit

One of the factors affecting the nature and severity of electrical injury is the type of circuit involved, either direct current (DC) or alternating current (AC). High-voltage DC contact tends to cause a single muscle spasm, often throwing the victim from the source. This results in a shorter duration of exposure but increases the likelihood of traumatic blunt injury. Brief contact with a DC source can also result in disturbances in cardiac rhythm, depending on the phase of the cardiac cycle affected.

AC exposure of the same voltage tends to be three times more dangerous than DC. Continuous muscle contraction, or tetany, can occur when the muscle fibers are stimulated between 40 and 110 times per second. The standard frequency of electrical transmission in the United States is 60 Hz (cycles per second) versus 50 Hz in most other countries. This is near the lowest frequency at which an incandescent light appears to be continuously lit.

The terms entry and exit are commonly used to describe electrical injury patterns; however, the terms source contact point and ground contact point are more appropriate in referring to AC injuries. The hand is the most common source contact point with a tool that is in contact with an AC electrical source. The flexors of the upper extremity are much stronger than the extensors, causing the hand grasping the current source to pull the source even closer to the body. Currents greater than the “let-go threshold” (6-9 mA) can prevent the victim from releasing the current source, which prolongs the duration of exposure to the electrical current.

Voltage

Voltage is a measure of the difference in electrical potential between two points and is determined by the electrical source. Electrical injuries are conventionally divided into low or high voltage; 1000 V is the most commonly used divider. Although both can cause significant morbidity and mortality, high voltage tends to have a greater potential for tissue destruction and major amputation.

No fatalities are recorded from contact with the low voltages associated with long-distance communication lines (24 V) or telephone lines (65 V). Fatalities are reported, however, with exposure to 110-V household current, especially in special environmental circumstances decreasing resistance of skin.

Resistance

Resistance is the tendency of a material to resist the flow of electrical current. It varies for a given tissue, depending on its moisture content, temperature, and other physical properties. The higher the resistance of a tissue to the flow of current, the greater the potential for transformation of electrical energy to thermal energy. Nerves, designed to carry electrical signals, and muscle and blood vessels, because of their high electrolyte and water content, have a low resistance and are good conductors. Bone, tendon, and fat, which all contain a large amount of inert matrix, have a high resistance and tend to heat and coagulate rather than to transmit current. The other tissues of the body are intermediate in resistance (Box 142-3).¹¹

Skin is the primary resistor to the flow of current into the body. Skin on the inside of the arm or back of the hand has a resistance of approximately 30,000 Ω/cm². Thick, hardened skin can have 20 to 70 times greater resistance (Table 142-1).⁸ This high resistance may result in a significant amount of energy being expended at the skin surface as the current burns its way through deep callus, resulting in greater thermal injury to the skin. As the duration of contact increases, however, the skin begins to blister and offer decreased resistance. A surge of current internally can cause extensive deep tissue destruction. Moisture also lowers resistance. Sweating can decrease the skin’s resistance to 2500 to 3000 Ω/cm², and immersion in water causes a further reduction to 1200 to 1500 Ω/cm².

Amperage

Current, expressed in amperes, is a measure of the amount of energy that flows through an object. As defined by Joule’s law, the heat generated is proportional to the amperage squared. Amperage depends on the source voltage and the resistance of the conductor. The voltage of the source is often known. Resistance varies according to the involved tissues and may change markedly during the exposure, rendering predictions of amperage difficult for any given electrical injury.

The physical effects vary with different amperages at 50 to 60 Hz, which is the AC frequency used in European countries and the United States (Table 142-2). A narrow range exists between the threshold of perception of current (0.2-0.4 mA) and let-go current (6-9 mA). Thoracic tetany can occur at levels just above the let-go current and result in respiratory arrest. Ventricular fibrillation may occur at an amperage of 60 to 120 mA. Although the 120-V household source usually causes minimal injury across dry skin, amperage delivered to the heart when the resistance of the skin is decreased by sweat or submersion in water can result in current sufficient to cause electrocution with cardiac arrest but without apparent external injury.

Duration of Contact

In general, the longer the duration of contact with high-voltage current, the greater the electrothermal heating and degree of tissue destruction. As carbonization of tissue occurs, the resistance to current flow increases.

The physics differ with lightning. The extremely short duration and extraordinarily high voltage and amperage of lightning result in a short flow of current internally, with little if any skin damage and almost immediate flashover of current around the outside of the body.

Pathway

The pathway of low, high, or lightning voltages determines the tissues at risk, the type of injury, and the degree of conversion of electrical energy to heat. Current passing through the heart or thorax can cause dysrhythmias and direct myocardial damage. Cerebral current can result in respiratory arrest, seizures, and paralysis. Current with ocular proximity can cause cataracts.

Current density is the amount of current flow per area of tissue.¹⁰ As current density increases, any tendency to flow through the less resistant tissues is overcome. Eventually it flows through tissues indiscriminately, as if the body were a volume conductor, with the potential to destroy all tissues in the current’s path. Because the current is often concentrated at the source and ground contact points, current density and the degree of damage are greatest there. Nevertheless, extensive deep destruction of the tissues may exist between these sites with high-voltage injuries, and the surface damage is often only “the tip of the iceberg.” Damage to the internal structures of the body may be noncontiguous, with areas of normal-appearing tissue adjacent to burned tissue and with damage to structures at sites distant from the apparent contact points.

The pathway between contact points is a major determinant of the electrical field strength, which is the voltage per unit of length over which it is applied. For a given current, the shorter the distance between contact points, the greater the electrical field strength. Current from a 20,000-V power line passing from head to toe (approximately 2 m) results in an electrical field strength of 10,000 V/m. Approximately the same electrical field strength is created when “low”-voltage 120-V household current passes between two close contact points on the mouth of a child chewing on a power cord (120 V/0.01 m). Although the electrical field strengths are similar, there is a tremendous difference in the amount of at-risk tissue in the respective pathways.12,13

Although lightning is governed by the same physical laws as artificial electricity is, the rapid rise and decay of the energy complicate predictions of the extent of lightning injury more than with artificial electrical injury. The most important difference between lightning and high-voltage electrical injuries is the duration of exposure to the energy.¹⁴

Lightning is neither a direct current nor an alternating current but rather a unidirectional massive current impulse. The cloud-to-ground lightning impulse results from the breakdown of a large electrical field between a cloud and the ground that is measured in millions of volts. When connection is made with the ground, this voltage difference between the cloud and ground disappears, and a large current flows impulsively for a brief instant.¹⁴

Mathematical modeling of a lightning flow on the human body, substantiated in animal models, has included only direct strikes that account for only 3 to 5% of lightning injury to people.15,16 After a direct strike meets the body, current is transmitted internally for less than a millisecond before external “flashover” occurs.¹⁷ Although lightning current may flow internally for an instant and disrupt electrical systems, it seldom results in significant thermal injury or tissue destruction, and less than one third of lightning survivors have any signs of burns or skin damage. Muscle damage and myoglobinuria from lightning are rare. It is unknown whether the most serious manifestations of lightning injury, cardiac and respiratory arrest, vascular spasm, neurologic damage, and autonomic instability, result from induced electrical changes, current through highly conductive tissues, concussive injury, or other mechanisms.

Lightning tends to cause asystole rather than ventricular fibrillation. Although cardiac automaticity may reestablish a rhythm, the duration of the respiratory arrest may cause secondary deterioration of the rhythm to refractory ventricular fibrillation and asystole.14,18 Other injuries caused by blunt trauma or ischemia from vascular spasms, such as myocardial infarction and spinal artery syndromes, also occur.^19–22

Electrical Injury

The primary electrical injury is the burn. Secondary blunt trauma results from falls or being thrown from the electrical source by an intense muscle contraction or the explosive force that may occur with electric flashes from circuit box or transformer accidents. Electrical burns are classified into four different types (Box 142-4).

Heating of tissues secondary to current flow and tissue resistance causes electrothermal burns. Severe electrothermal burns most commonly occur when a person comes in contact with a high-voltage conductor. The prolonged flow of current can result in significant burns anywhere along the current path. Typically, the skin lesions of electrothermal burns are well-demarcated, deep, partial-thickness to full-thickness burns.

The most destructive indirect injury occurs when a victim becomes part of an electrical arc. An electrical arc is a current spark formed between two objects of differing potential that are not in contact with each other, usually a highly charged source and a ground. The temperature of an electrical arc is approximately 2500° C, and the electrical arc causes deep thermal burns at the point where it contacts the skin.¹² With electrical arcs, burns may be caused by the heat of the arc, electrothermal heating due to current flow, or flames that result from the ignition of clothing.

Burns may occur from radiated thermal injury when an electrical explosion occurs, similar to gas explosions. Instead of becoming part of the arc, current may appear to jump the gap by splashing across a large part of the body. These splash burns are generally only partial thickness because the person did not become part of the arc itself.²³

At the time of presentation it is often difficult to determine the mechanism of injury that caused an electrically injured patient’s burns. Electrothermal heating is the main cause of muscle damage and is seen almost exclusively in high-voltage accidents with prolonged (seconds) contact and current flow.²³

The histologic change in muscle injury that results from direct contact with an electrical source is coagulation necrosis with shortening of the sarcomere.8,11 Muscle damage can be erratic, so areas of viable and nonviable muscle are often found in the same muscle group. Periosteal muscle damage may occur even though overlying muscle appears to be normal. Similar to muscle damage, serious vascular damage usually occurs only after a high-voltage accident.

Vascular damage is greatest in the media, predisposing to delayed hemorrhage when the vessel eventually ruptures.¹¹ Intimal damage may result in either immediate or delayed thrombosis and vascular occlusion as edema and clots form on the damaged intimal surface of the vessel during a period of days. The injury is usually most severe in the small muscle branches, where blood flow is slower.²⁴ This damage to small muscle arteries combined with mixed muscle viability that is not visible to gross inspection creates the illusion of progressive tissue necrosis. Veins, having more sluggish flow, are more prone to thrombosis than arteries are, and significant distal edema can result.

The absence of a pulse on initial examination may be a result of immediate arterial thrombosis or transient vascular spasm. Pulselessness resulting from vascular spasm should resolve within a few hours. If pulselessness persists after this time, serious vascular injury is likely.

Damage to neural tissue also may occur by several mechanisms. An immediate decrease in neural conductivity occurs with coagulation necrosis similar to that observed in muscle. In addition, it may suffer indirect damage as its vascular supply or myelin sheath is injured or from pressure as progressive edema results in a compartment syndrome. Evidence of neural damage may develop immediately or be delayed hours to days.

The hands, feet, and skull are the most common contact points. Histologic studies of the brain reveal focal petechial hemorrhages in the brainstem, cerebral edema, and widespread chromatolysis (the disintegration of chromophil bodies of neurons).¹¹

Lightning Injury

Lightning injury may occur by electrical mechanisms and by secondary concussive or blunt trauma.16,25 Whereas direct strike is most commonly described as the mechanism of injury, studies show that it accounts for a very small proportion of injuries and deaths (Table 142-3).

Injury from contact occurs when the person is touching an object that is part of the pathway of lightning current, such as a tree, metal fence, indoor plumbing, or wiring. Side flash or splash occurs as a portion of lightning jumps from its primary strike object to a nearby person on its way to the ground.11,14,16,19,26 Step voltage, a difference in electrical potential between a person’s feet, may occur as lightning current spreads radially through the ground.^14,16 A person who has one foot closer than the other to the strike point has a potential difference between the feet so that a portion of the lightning current flows through the legs and body rather than the ground. This ground current is a common killer of large livestock such as cattle and horses because of the greater distance between their hind legs and forelegs, with the heart lying in the pathway.

People may also be injured by upward streamers.²⁵ Cloud-to-ground lightning approaches the earth as a downward stepped leader. As the leader approaches, the large electrical field induces opposite charges that surge through trees, buildings, people, and any other object near the thunderstorm. If one of these upward streamers connects with a downward leader, a completed lightning strike occurs. Individuals in the path of an upward streamer may be injured even in the absence of a completed lightning strike.

Ball lightning is a mobile, luminous, spheroidal, floating or bouncing ball that lasts a few seconds before suddenly vanishing or exploding. These glowing orbs are observed traveling down power lines and aisles of aircraft and even entering buildings through open doors.²⁷ Although ball lightning has been described by multiple reputable observers for many years, the origin, chemical makeup, and physics are still a mystery.

Blunt injury from lightning can occur from at least two mechanisms. First, the person may be thrown a considerable distance by the sudden, massive contraction caused by current passing through the body. Second, a concussive injury caused by explosive or implosive force occurs as the lightning pathway is instantaneously superheated then rapidly cooled after the passage of the lightning.¹⁴ This concussive injury or barotrauma is associated with tympanic membrane rupture, contusions of various organs, and pneumothorax.^28–33 Blunt or concussive injury can also accompany electrical injury.