Electrocution

Electrocution
Dr A McLeod

This mini-article explores electrocution as a cause of death - how it kills and what the autopsy findings are. This info is largely referenced from Knights Forensic Pathology with additional information from the US CDC

How electrocution occurs

There are several components to an electric 'shock'

Voltage - if the voltage is different in two places that are connected then current (see below) will flow between them. The more difference (higher voltage) the more current will flow. If the voltage is high enough then current can flow without direct contact via the air (arcing).
Current - you can think of this as the flow of electrons. Technically this is measured in coulombs (amperes x seconds) - time plays an important part in level of injury as even a low current can cause a lot of damge when exposure time is long. In practice amperes (or milliamperes) are used. Currents under 30 mA (milliamperes) rarely cause damage. Levels of 50 mA or above, over a few seconds are usually fatal.
Another danger of currant is that it can cause muscle contraction - in the hands the flexors are more powerful than the extensors so the hand clenches shut - often around the source of the current - this is the 'hold on' effect. Currents of 9-10 mA are enough for this to occur.
Resistance (impedence in alternating currant). The resistance must be overcome before currant can flow. Higher voltages have a higher ability to overcome resistance. Resistance varies over the body from 1-2 million ohms on the thick skin of hands and feet to a 500-1000 ohms elsewhere. The million or so on the hands can drop to a few thousand when the hands are sweaty and less than a thousand when wet thus explaining why electrocution is more likely with wet hands!

More on Voltage, current and resistance:
To calculate voltage (V) in volts: put your finger over V,this leaves you with I R, so the equation is V = I x R
To calculate current (I) in amperes: put your finger over I, this leaves you with V over R, so the equation is I = V/R. Note that damaging currents are in the milliampere range rather than amperes!
To calculate resistance (R) in ohms: put your finger over R, this leaves you with V over I, so the equation is R = V/I

It is the current that causes tissue damage.
You can see that (I=V/R) as either voltage increases, or resistance decreases the amout of current goes up - causing more damage.

Effects of Electrical Current on the Body (from CDC)

Current	Reaction
1 milliamp (mA)	Just a faint tingle.
5 mA	Slight shock felt. Disturbing, but not painful. Most people can "let go." However, strong involuntary movements can cause injuries.
6-25 mA (women) 9-30 mA (men)	Painful shock. Muscular control is lost. This is the range where "freezing currents" start. It may not be possible to "let go."
50-150 mA	Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.
1,000-4,300	Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely. 3A is the smallest fuse in common usage.
10 amps (A)	Cardiac arrest and severe burns occur. Death is probable.
13 A	The typical current at which the fuse blows in an item like a hairdryer or electric drill!

The flow of current through the body after bypassing the skin is fairly easy - it is well conducted by the (low elecrical resistance) organs and fluids of the body in a fairly straight line from point of entry to the earthing point. Certain entry and exit points are known to be more dangerous than others.

Entry right hand and exit left foot is the most dangerous - the current passes through the heart at the axis most likely to cause dysrhythmia.
Current from right hand to left hand is next most dangerous and then left hand to right hand. Both of these pass directly though the heart.

Mode of death

The commonest by far is cardiac dysrhythmia - usually ventricular fibrillation.
A small amount of electrocution victims suffer paralysis of the respiratory muscles in the chest and/or diaphragm.
A very small amount suffer damage to the brain stem - this presumably leads to either of the two above.

At higher currents (approx 4 A) clots may form in blood vessels.
In addition to direct damage electrical shocks may be responsible for fatal falls.

Circumstances of death

Usually accidental
Occasionally suicidal - becoming more common - especially in Germany.
Rarely homicidal

Indicators of electrocution as a mode of death

These can be very subtle - the best indicator is the circumstances surrounding the death. These may prompt the search for specific signs.

Typically electrical burns or blisters are confined to the site at which current enters the body - the earth point is usually lesion free but burns here may be seen sometimes.

There are two main type of electrical lesions:

The typical electrical burn lesion from direct contact occurs when heat generated by the electrical current turns fluid to steam, splitting tissue layers. The blister flattens again when cooling occurs and may burst if current is high enough or maintained long enough. These lesions are surrounded by a white area - this is almost pathognomonic for an electrical burn. This white area may be surrounded in turn by a hyperaemic area and may be in the shape of the contact point.
The other type of lesion occurs with arc burns. These occur when a spark of electricity makes contact with the body. A spark of 1000v can travel millimetres and 100,000v can travel 35 cm. At these massive voltages considerable heat can be generated. This melts the keratin of the skin leaving a hard nodule of keratin.

It is very important to look for these lesions - hands, feet and head must be carefully examined. Lesions may be concealed by the clasped fingers of rigor mortis and if necessary the tendons must be cut to release fingers for a proper examination.

Other important areas to check for lesions are:

In the mouth - especially in children where electrical devices may be put in the mouth.
On breasts where a sado-sexual component is plausible
Within the urethra, vagina and anus. Again especially where a sado-sexual component is plausible.

Electrocution in water:
This provides special problems - the entry point may be over a wide area and at low resistance - there may simply not be any electrical burn lesions. Here the context may be the only clue to electrocution as cause of death.

Severe burns:
These may result from either massive currents or lower currants over a long time. Even the current from an electrically powered vehicle may cause severe burns over the course of hours (see pic right).

Created February 2010