Chapter 75 Electrical safety and injuries
Patients suffering from the consequences of electrocution and associated burns occasionally require intensive care unit (ICU) management. Patients and staff in the ICU are at risk of electrocution from faulty electrical equipment. The necessity of direct patient contact with electrical equipment increases this risk, and when therapy involves an invasive contact close to the heart, microshock is an additional hazard. Faulty electrical equipment can also result in power failures, fires and explosions. The use of mobile phones and related devices nearby patient equipment can lead to malfunctioning.
PHYSICAL CONCEPTS
A current flowing in a circuit produces electric and magnetic fields, which induce currents to flow in neighbouring circuits. When this results in a current flowing between the two circuits, it is called coupling. With capacitive coupling, high frequency currents are most easily passed, and the size of the current is greatest when the circuits are close. Inductive coupling can result from the strong magnetic fields produced by heavy duty electrical equipment, such as transformers, electric motors and magnetic resonance imaging machines. The most common problem associated with coupling is electrical interference or ‘noise’. Monitoring equipment is designed to ‘filter’ out this noise. However, in certain circumstances, such as the use of high frequency surgical diathermy and magnetic resonance, sufficient amperage can be induced to cause microshock and burns.1,2 Smaller electromagnetic fields emitted by hand-held devices, such as mobile phones, can affect the programming of microprocessors. Cases of patient equipment malfunctions have been reported.3
Static electricity has no free flow of electrons. Insulated objects can become highly charged, usually by repeated rubbing. The charge is dissipated by electrons jumping onto another neighbouring object of a different potential. ‘Jumping’ electrons ionise and heat the air through which they pass, causing a spark which may ignite an inflammable liquid or gas. Lightning is a type of static electrical discharge. Direct currents of 12 000–200 000 A and voltages in the millions are involved; however, flow lasts only a fraction of a second.4
PHYSIOLOGICAL CONSIDERATIONS
For a current to flow through the body, the body must complete a circuit. Usually this involves the current flowing from its source to ground through the body. The pathophysiological effects depend on the size of the current, and this depends on the voltage and electrical resistance of the body, most of which occurs in the skin. Dry skin has a resistance in excess of 100 000 O.5 However, skin resistance is markedly reduced (to 1000 O)6 if the skin is wet, or if a conductive jelly has been applied. Hence, fromOhm’s law, dry skin in contact with 240 V mains supply will result in a harmless 0.24 mA current flowing through the body, whereas moist or wet skin will result in a potentially lethal 240 mA current.
ELECTROCUTION
Most cases of electrocution occur in the workplace (about 60%) or at home (about 30%), where misuse of extension cables is the main culprit.6 Pathophysiological processes involved in true electrical injuries are poorly understood. The extent of injury depends on (i) the amount of current that passes through the body, (ii) the duration of the current, and (iii) the tissues traversed by the current (Table 75.1).
Table 75.1 Origin and pathophysiological effects of different levels of electrical injury
| Current (A) | Source | Effects on victim |
|---|---|---|
| 10–100 μA | Earth leakage | Microshock (ventricular fibrillation) |
| 300–400 μA | Faulty equipment | Tingling (harmless) |
| > 1 mA | Faulty equipment | Pain (withdraw) |
| > 10 mA | Faulty equipment | Tetany (cannot let go) |
| > 100 mA | Faulty equipment | Macroshock (ventricular fibrillation) |
| > 1 A | Faulty equipment | Burns and tissue damage |
| > 1000 A | High tension injury | Severe burns and loss of limbs |
| > 12 000 A | Lightning | Coma, severe burns and loss of limbs |
TISSUE HEAT INJURY
Currents in excess of 1 A generate sufficient heat energy to cause burns to the skin and occult thermal injury to internal tissues and organs. Blood vessels and nervous tissue appear to be particularly susceptible.6
DEPOLARISATION OF MUSCLE CELLS
An alternating current of 30–200 mA will cause ventricular fibrillation.7 Currents in excess of 5 A cause sustained cardiac asystole, which is the principle used in defibrillation. Apart from ventricular fibrillation, other arrhythmias may occur. Myocardial damage is common and may result in ST and T-wave changes. Global left ventricular dysfunction may occur hours or days later, despite initial minimal ECG changes.8,9 Myocardial infarction has also been reported.10 Specific markers of myocardial injury, such as cardiac troponin, should be checked in all suspected cases of electrical injury to the heart.11
Tetanic contractions of skeletal muscle occur with currents in excess of 15–20 mA. The threshold is particularly low with alternating currents at the household frequency of 50–60 Hz. Tetanic contraction will prevent voluntary release of the source of electrocution, and violent muscle contractions may cause fractures of long bones and spinal vertebrae.6
