Electrical injury is damage caused by manmade electrical current passing through the body. Symptoms may include skin burns, damage to internal organs and other soft tissues, cardiac arrhythmias, and respiratory arrest. Diagnosis is by clinical criteria and selective laboratory testing. Treatment is supportive, with aggressive care for severe injuries.

Although accidental electrical injuries encountered in the home (eg, touching an electrical outlet or getting shocked by a small appliance) rarely result in significant injury or sequelae, accidental exposure to high voltage causes about 400 deaths annually in the US.

Pathophysiology

Traditional teaching is that the severity of electrical injury depends on Kouwenhoven's 6 factors: type of current (direct [DC] or alternating [AC]), voltage and amperage (both are measures of current strength), duration of exposure (longer exposure increases injury severity), body resistance, and pathway of current (which determines the specific tissue damaged). However, electrical field strength, a newer concept, seems to predict injury severity more accurately.

Kouwenhoven's factors: 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-Hz) AC is used in US (60 Hz) and European (50 Hz) households; 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. Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source.

Usually, for both AC and DC, the higher the voltage (V) and amperage, the greater the ensuing electrical injury (for the same duration of exposure). Household current in the US is 110 V (standard electrical outlet) to 220 V (large appliance, such as a dryer). High-voltage (> 500 V) currents tend to cause deep burns, and low-voltage (110 to 220 V) currents tend to cause muscle tetany and freezing to the current's source. The threshold for perceiving DC current entering the hand is about 5 to 10 milliamperes (mA); for AC at 60 Hz, the threshold is about 1 to 10 mA. 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 mA for DC and about 15 mA for AC.

Low-voltage 60-Hz AC traveling through the chest for a fraction of a second can cause ventricular fibrillation 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), < 1 mA (AC or DC) can cause ventricular fibrillation.

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. 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; for moist, thin skin, resistance is 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, much electrical energy may be dissipated at the skin, resulting in large skin burns at entry and exit points but less internal damage. If skin resistance is low, skin burns are less extensive or absent, but more electrical energy may be dissipated in internal organs. 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.

Damage to internal tissues depends also on their resistance and additionally 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 most 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 arrhythmia. This current tends to be more dangerous than current traveling from one foot to the other. Current to the head may damage the CNS.