Physics and Pathophysiology of Electrical Injury

The amount of electron flow (current) is proportional to an electron concentration gradient (voltage) and inversely related to the resistance of the connecting medium along the path of flow (138-140). This is expressed as Ohm's law: I = V/R, where "I" is current (amperes [A]), "V" is voltage (volts [V]), and "R" is resistance (ohms [Q]; see refs. 138, 139, and 141-145).

Electricity exerts two major effects on the body: cellular depolarization of nerves and muscle and heat production, the latter reflecting a longer duration of exposure (38,140,142,144,146-149). The long axes of most skeletal muscle cells and axons are oriented parallel to the direction of current flow, and transmembrane potentials are affected (147). Electrical injury of nerve cells and muscle alters membrane permeability by disruption of the lipid bilayer structure of plasma membranes, leading to formation of structural defects or "pores" (electroporation), and by denatura-tion of membrane proteins (142,145-147). Temperatures higher than 60°C (140°F)

Fig. 28. Burns caused by radiant heat. (A) Blister, broken (second degree). (B) Leathery consistency of lower extremity (third degree).
Fig. 29. Partial thickness burn. Epidermis sloughed; dermal alteration (D) (H&E, original magnification x25).

produce tissue destruction (150). Thermal injury denatures all tissues by coagulation necrosis (38,151).

Various factors determine the relative contribution of these mechanisms and the consequent pattern of electrical injury. They include the type of circuit, duration of contact, resistance of tissues, voltage and amperage of current, pathway of current, and surface area of contact points (i.e., current density inversely related to the area of contact; see refs. 38, 138, 141, 143, and 151-154 and Subheadings 7.3. and 7.4.).

7.1.1. Type of Circuit

Alternating current (ac), i.e., an electric field of alternating polarity, has a frequency of 60 Hz in the United States and Canada (50 Hz in Europe [138-140, 142,147,152,155,156]). AC is found in households (low voltage; 15 A in the United States and Canada) and utility lines (high voltage), which are the two most common sites of electrocution deaths (139,145,147,156). AC is the most efficient way to depolarize cells (140). The physiological muscle contraction-relaxation time is 20 to 40 ms (147). Muscle fibers, when continuously stimulated between 40 and 110 times per second (40-110 Hz), go into sustained contraction (tetany [152,157]). Sixty-Hz AC current changes direction 120 times per second (i.e., every 8 ms; [147]). Because tetany occurs at low amperage, AC is more dangerous than direct current (DC; refs. 139, 141, 142, 144, 145, 149, 152, and 158-160). If an AC electrical source is gripped by hand, the stronger flexor muscles of the hand and forearm go into tetanic contraction, which prolongs the grip and increases the duration of exposure to the current and the injury risk (139,141, 142,147,152,157,158). Rigor mortis can occur immediately after death in high-voltage electrocution and be localized to one extremity (Chapter 2, Fig. 8; refs. 140, 147, and 161-163).

In contrast, the electrons flow in one direction in a DC circuit (138,139). DC Tends to cause a single muscle contraction, throwing the victim and resulting in a shorter duration of exposure to the electrical source but increasing the chance of blunt trauma (139,141,142,145,147,152,157). DC occurs in lightning strikes and from contact with certain equipment (e.g., defibrillators [139,140,145,152,164-166]).

7.1.2. Resistance

Resistance (R), the inverse of conductivity, is the tendency of tissues to resist the flow of current and is dependent on the moisture content, temperature, and other physical properties of the tissue (142,152). The higher the resistance, the greater the transformation of electrical energy to thermal energy at a given current (38,142,144,145, 151,152,167,168). This is expressed as Joule's law: P = I2 X R X T (I = current; R = resistance; T = duration of contact).

The human body has a minimal resistance of 500 Q (139,159,163). Good tissue conductors include nerves, blood, mucous membranes, and muscle (139-141,152,158). Mucous membranes (mouth, rectum, vagina) have low resistance (range of 100 Q [139,141,169,170]). Tendons and fat offer more resistance, and bone is the most resistant (139,141,142,144,152,158,168). Skin has intermediate resistance and is the primary resistor to current flow, resulting in much electrical energy being dissipated at the skin surface contact (138,139,141,142,152,155). Moisture reduces skin resistance (sweating, 2500 Q/cm2; immersion in a bathtub, 1200-1500 Q/cm2 [139,141,142,152,171]). Moist skin does not burn, and increased current will flow into the body (152). In contrast, the heavily callused skin of the palms can have a resistance up to 2 X 106 Q/cm2 (141,147, 152,158). As the skin is burned, resistance drops, allowing increased current flow into the body (139,142,145,152).

The amount of heat produced is dependent on current density, which is inversely proportional to the cross-sectional diameter of the body structure, i.e., fingers and toes have more severe burns than an arm or leg (38,141,145,146,148,168). As current flow increases in the body, the current tends to overcome the resistance of less conductive tissues (152). As a result, the entire body becomes a volume conductor, with the exception of bone, and current flows through all tissues, resulting in variable electrical and thermal damage (38,139,147,152,155,168). Skeletal muscle, because it occupies the greatest volume of soft tissue in the extremities, carries the bulk of the current (147). Muscle not only is subjected to the direct thermal effects of current, but also continues to be heated by adjacent bone, which has a higher thermal capacity (38,139,147, 148,172).

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