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Saturday, 2 July 2011

Electric shock

Electric shock


Electrical injury
Classification and external resources

A person who was affected by a nearby lightning strike. Note the slight branching redness travelling up his leg from the effects of the current.
ICD-10 T75.4
1700 volts burns a grounded hot dog
Safety rule #1 - Remove the voltage before any action at electrical wiring
Electric shock of a (human) body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable.[1]
The minimum current a human can feel depends on the current type (AC or DC) and frequency. A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest. 60 mA of AC (rms, 60 Hz) or 300–500 mA of DC can cause fibrillation.[2][3] A sustained electric shock from AC at 120 V, 60 Hz is an especially dangerous source of ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source. However, the potential seriousness of the shock depends on paths through the body that the currents take.[2] Death caused by an electric shock is called electrocution.
If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however. If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs.[4] The protection offered by the skin is lowered by perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.[2]
If an electrical circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established. This is known as a microshock. Currents of only 10 µA can be sufficient to cause fibrillation in this case.[citation needed] This is a concern in modern hospital settings when the patient is connected to multiple devices.

Signs and symptoms

Signs and symptoms

Burns

Second-degree burn after a high tension line accident
Heating due to resistance can cause extensive and deep burns. Voltage levels of 500 to 1000 volts tend to cause internal burns due to the large energy (which is proportional to the duration multiplied by the square of the voltage divided by resistance) available from the source. Damage due to current is through tissue heating. It is a relatively unknown fact that more electrical workers die from burns than from an electric shock. In fact, only around 20% of deaths are the result of electric shock.[5]

Ventricular fibrillation

A domestic power supply voltage (110 or 230 V), 50 or 60 Hz AC current through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60 mA. With DC, 300 to 500 mA is required.[2] If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other kind of electrode), a much lower current of less than 1 mA (AC or DC) can cause fibrillation. If not immediately treated by defibrillation, fibrillations are usually lethal because all the heart muscle cells move independently instead of in the coordinated pulses needed to pump blood to maintain circulation. Above 200 mA, muscle contractions are so strong that the heart muscles cannot move at all.

Neurological effects

Current can cause interference with nervous control, especially over the heart and lungs. Repeated or severe electric shock which does not lead to death has been shown to cause neuropathy. Recent research has found that functional differences in neural activation during spatial working memory and implicit learning oculomotor tasks have been identified in electrical shock victims.[6]
When the current path is through the head, it appears that, with sufficient current,[clarification needed] loss of consciousness almost always occurs swiftly. (This is borne out by some limited self-experimentation by early designers of the electric chair[citation needed] and by research from the field of animal husbandry, where electric stunning has been extensively studied.)[7]

Pathophysiology

Pathophysiology

Body resistance

The voltage necessary for electrocution depends on the current through the body and the duration of the current. Ohm's law states that the current drawn depends on the resistance of the body. The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."[10]
The International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents (the columns contain the distribution of the impedance in the population percentile; for example at 100 V 50% of the population had an impedance of 1875Ω or less):[11]
Voltage 5% 50% 95%
25 V 1,750 Ω 3,250 Ω 6,100 Ω
100 V 1,200 Ω 1,875 Ω 3,200 Ω
220 V 1,000 Ω 1,350 Ω 2,125 Ω
1000 V 700 Ω 1,050 Ω 1,500 Ω

Point of entry

  • Macroshock: Current across intact skin and through the body. Current from arm to arm, or between an arm and a foot, is likely to traverse the heart, therefore it is much more dangerous than current between a leg and the ground. This type of shock by definition must pass into the body through the skin.
  • Microshock: Very small, current source with a pathway directly connected to the heart tissue. The shock is required to be administered from inside the skin, directly to the heart i.e. a pacemaker lead, or a guide wire, conductive catheter etc. connected to a source of current. This is a largely theoretical hazard as modern devices used in these situations include protections against such currents.

Deliberate uses

Deliberate uses

Medical uses

Electric shock is also used as a medical therapy, under carefully controlled conditions:
  • Electroconvulsive therapy or ECT is a psychiatric therapy for mental illness. The objective of the therapy is to induce a seizure for therapeutic effect. There is no sensation of shock because the patient is anesthetized. The therapy was originally conceived of after it was observed that depressed patients who also suffered from epilepsy experienced some remission after a spontaneous seizure.[citation needed] The first attempts at deliberately inducing seizure as therapy used not electricity but chemicals; however electricity provided finer control for delivering the minimum stimulus needed. Ideally some other method of inducing seizure would be used, as the electricity may be associated with some of the negative side effects of ECT including amnesia. ECT is generally administered three times a week for about 8-12 treatments.
  • As a surgical tool for cutting or coagulation. An "Electrosurgical Unit" (or ESU) uses high currents (e.g. 10 amperes) at high frequency (e.g. 500 kHz) with various schemes of amplitude modulation to achieve the desired result - cut or coagulate - or both. These devices are safe when used correctly.
  • As a treatment for fibrillation or irregular heart rhythms: see defibrillator and cardioversion.
  • As a method of pain relief: see Transcutaneous Electrical Nerve Stimulator (more commonly referred to as a TENS unit).
  • As an aversive punishment for conditioning of mentally handicapped patients with severe behavioral problems. This method is highly controversial and is employed at only one institution in the United States, the Judge Rotenberg Educational Center; The institute also uses electric shock punishments on non-handicapped children with behavioral problems, and whether this constitutes legitimate medical treatment or abusive discipline is currently the subject of litigation.

Law enforcement and personal defence

Law enforcement and personal defence

Electroshock weapons are incapacitant weapons used for subduing a person by administering electric shock to disrupt superficial muscle functions. One type is a conductive energy device (CED), an electroshock gun popularly known by the brand name "Taser", which fires projectiles that administer the shock through a thin, flexible wire. Although they are illegal for personal use in many jurisdictions, Tasers have been marketed to the general public.[13] Other electroshock weapons such as stun guns, stun batons ("cattle prods"), and electroshock belts administer an electric shock by direct contact.

Torture

Electric shocks have been used as a method of torture, since the received voltage and current can be controlled with precision and used to cause pain while avoiding obvious evidence on the victim's body. Such torture usually uses electrodes attached to parts of the victim's body: most typically, while wires are wound around the fingers, toes, and/or tongue to provide a return circuit, the voltage source (typically a cattle prod) of precisely controllable pressure is applied to the testicles.
Electrical torture has been used in war and by repressive regimes since the 1930s:[14] The US Army is known to have used electrical torture during World War II;[15] Amnesty International published an official statement that Russian military forces in Chechnya tortured local women with electric shocks by connecting electric wires to their bra straps;[16] Japanese serial killer Futoshi Matsunaga used electric shocks for controlling his victims.[17]
Advocates for the mentally ill and some psychiatrists such as Thomas Szasz have asserted that electroconvulsive therapy is torture when used without a bona fide medical benefit against recalcitrant or non-responsive patients. See above for ECT as medical therapy. These same arguments and oppositions apply to the use of extremely painful shocks as punishment for behavior modification, a practice that is openly used only at the Judge Rotenberg Institute

Capital punishment

Capital punishment

Electric chair in Sing Sing.
Electric shock delivered by an electric chair is sometimes used as an official means of capital punishment in the United States, although its use has become rare in recent times. Although some original proponents of the electric chair considered it to be a more humane execution method than hanging, shooting, poison gassing, etc., it has now generally been replaced by lethal injections in countries that practise capital punishment. Modern reporting has claimed that it sometimes takes several shocks to be lethal, and that the condemned person may actually catch fire before the process is complete. The brain is always severely damaged and inactivated.
Other than in parts of the United States, only the Philippines reportedly has used this method, and only for a few years. It is legal in at least 10 states of USA.