As high current passes through the body, the tissues through which it passes are heated. This heating produces neural injury, vasospasm, vascular thrombosis, and myonecrosis. Current flow through a body region with a small cross-sectional diameter (e.g., a finger) may result in more tissue destruction than current flow through a body portion with a large cross-sectional diameter (e.g., the thorax).
Contact burns occur at entry and exist points where the victim makes contact with the electrical source. These lesions are typically charred in the center and surrounded by a zone of grayish-white necrosis, with only mild damage in the periphery of these wounds. Extensive contact burns usually indicate severe underlying pathology. However, not all serious injuries display cutaneous injury. A patient with low skin resistance (e.g., sitting in the bathtub) may sustain a low-voltage injury (e.g., radio falls in the bathtub) and not have a contact lesion.
Arc burns cause the most destructive indirect injury. An electrical arc is a spark of high-voltage current through the air between objects of differing electrical potential that are not in contact with one another. Usually a typical arc burn is from an electrical source through the air to the patient. Arcs can extend through the air for distances of approximately 25 cm per 100,000 V of potential difference. Ihe temperature of an arc ranges from 500° to 2500°C. Ihe arc may ignite clothes, causing thermal burns. Such high voltages produce falls and tetanic musculoskeletal injuries.
In addition to current flow, other factors affect the injury severity from electrical exposure ( Table 1.96-5). Prolonged contact leads to more severe injury secondary to heat production and the concomitant thermal burn. Ihe type of current, whether alternating or direct, contributes to the duration of the exposure. Electric current can be either continuous in one direction (direct current or DC) or with a periodic reversal of the direction of current flow (alternating current or AC). DC is used in medical devices such as defibrillators and pacemakers. AC is found in electricity supplied to homes and businesses. Ihe frequency of the AC is the time during which the current cycles through both positive and negative voltages. In the United States, most AC (including standard home current) is 60 cycles per second (hertz or Hz). At
60 Hz, household current has the same frequency response as skeletal muscle and can produce tetany beginning at 10 mA (Iable 19.6:6.). With muscular tetany in an extremity, the flexors overpower the extensors and the patient is drawn closer to the electrical source. Ihis tetanic phenomenon can cause injuries such as scapular fractures and shoulder dislocations. In contrast, a high-voltage AC or DC injury usually produces a single violent skeletal muscle contraction that tends to throw the victim away from the source.
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