injuries poisoning
Electrical Injuries
Pathophysiology of Electrical Injuries
Traditional teaching is that the severity of electrical injury depends on Kouwenhoven’s factors: Type of current (direct [DC] or alternating [AC]) Voltage and amperage (measures of current strength) Duration of exposure (longer exposure increases injury severity) Body resistance Pathway of current (which determines the specific tissue damaged) However, electrical field strength, a newer concept, seems to predict injury severity more accurately. AC changes direction frequently; it is the current usually supplied by household electrical outlets in the US and Europe. DC flows in the same direction constantly; it is the current supplied by batteries. Defibrillators and cardioverters usually deliver DC current. How AC affects the body depends largely on frequency. Low-frequency (50- to 60-hertz [Hz]) AC is used in US (60 Hz) and European (50 Hz) households. Because low-frequency AC causes extended muscle contraction (tetany), which may freeze the hand to the current’s source and prolong exposure, it can be more dangerous than high-frequency AC and is 3 to 5 times more dangerous than DC of the same voltage and amperage. DC exposure is likely to cause a single convulsive contraction, which often throws the person away from the current’s source. 500 V) currents tend to cause deep Burns , and low-voltage (110 to 220 V) currents tend to cause muscle tetany and freezing contact to the current’s source. The maximum amperage that can cause flexors of the arm to contract but that allows release of the hand from the current’s source is called the let-go current. Let-go current varies with weight and muscle mass. For an average 70-kg man, let-go current is about 75 milliamperes (mA) for DC and about 15 mA for AC. Low-voltage 60-Hz AC traveling through the chest for even a fraction of a second can cause Ventricular Fibrillation (VF) at amperage as low as 60 to 100 mA; for DC, about 300 to 500 mA are required. If current has a direct pathway to the heart (eg, via a cardiac catheter or pacemaker electrodes), Tissue damage due to electrical exposure is caused primarily by the conversion of electric energy to heat, resulting in thermal injury. Amount of dissipated heat energy equals amperage2× resistance × time; thus, for any given current and duration, tissue with the highest resistance tends to suffer the most damage. Body resistance (measured in ohms/cm2) is provided primarily by the skin, because all internal tissue (except bone) has negligible resistance. Skin thickness and dryness increase resistance; dry, well-keratinized, intact skin averages 20,000 to 30,000 ohms/cm2. For a thickly calloused palm or sole, resistance may be 2 to 3 million ohms/cm2; in contrast, moist, thin skin has a resistance of about 500 ohms/cm2. Resistance for punctured skin (eg, cut, abrasion, needle puncture) or moist mucous membranes (eg, mouth, rectum, vagina) may be as low as 200 to 300 ohms/cm2. If skin resistance is high, more electrical energy may be dissipated at the skin, resulting in large skin burns but less internal damage. If skin resistance is low, skin burns are less extensive or absent, and more electrical energy is transmitted to internal structures. Thus, the absence of external burns does not predict the absence of electrical injury, and the severity of external burns does not predict the severity of electrical injury. The absence of external burns does not predict the absence of electrical injury, and the severity of external burns does not predict the severity of electrical injury. Damage to internal tissues depends on their resistance as well as on current density (current per unit area; energy is concentrated when the same current flows through a smaller area). For example, as electrical energy flows in an arm (primarily through lower-resistance tissues, eg, muscle, vessels, nerves), current density increases at joints because a significant proportion of the joint’s cross-sectional area consists of higher-resistance tissues (eg, bone, tendon), which decreases the area of lower-resistance tissue; thus, damage to the lower-resistance tissues tends to be most severe at joints. The current’s pathway through the body determines which structures are injured. Because AC current continually reverses direction, the commonly used terms “entry” and “exit” are inappropriate; “source” and “ground” are more precise. The hand is the most common source point, followed by the head. The foot is the most common ground point. Current traveling between arm and arm or between arm and foot is likely to traverse the heart, possibly causing Overview of Arrhythmias . This current tends to be more dangerous than current traveling from one foot to the other. Current to the head may damage the central nervous system. Electrical field strength is the intensity of electricity across the area to which it is applied. It, along with Kouwenhoven’s factors , also determines the degree of tissue injury. For instance, 20,000 volts (20 kV) distributed across the body of a man who is about 2 m (6 ft) tall result in a field strength of about 10 kV/m. Similarly, 110 volts, if applied only to 1 cm (eg, across a young child’s lip), result in a similar field strength of 11 kV/m; this relationship is why such a low-voltage injury can cause the same severity of tissue injury as some high-voltage injuries applied to a larger area. Conversely, when considering voltage rather than electrical field strength, minor or trivial electrical injuries technically could be classified as high voltage. For example, the shock received from shuffling across a carpet in the winter involves thousands of volts but causes inconsequential injury. The electrical field effect can cause cell membrane damage (electroporation) even when the energy is insufficient to cause any thermal damage. Application of low electrical field strength causes an immediate, unpleasant feeling (being “shocked”) but seldom results in serious or permanent injury. Application of high electrical field strength causes thermal or electrochemical damage to internal tissues. Damage may include Hemolysis Protein coagulation Coagulation necrosis of muscle and other tissues Thrombosis Dehydration Muscle and tendon avulsion High electrical field strength injuries may result in massive edema, which, as blood in veins coagulates and muscles swell, can result in Compartment Syndrome . Massive edema may also cause hypovolemia and hypotension. Muscle destruction can result in 