Severity and Type of Injuries

Electrically induced thermal injury results in a coagulative tissue necrosis. Electrical current may also alter the structure of proteins within the cell membrane. The resultant protein dysfunction alters homeostasis and can produce cellular damage and death. This phenomenon is known as electroporation. The homeostatic micro currents of nervous and muscle tissue are also affected. Initiation of muscular contraction can lead to tetany and fatal dysrhythmias.

The ability of an electrical current to cause morbidity and mortality is dependant upon six different factors:

1. The magnitude of the current

High current flow has a greater ability to induce thermal damage. Current is determined by the voltage of the electrical source, and also the resistance to current flow.

2. The resistance of tissue types involved in the current pathway

The internal body resistance is around 300 ohms whereas dry, calloused skin can have a resistance of up to 100000 ohms. Skin resistance can be effectively bypassed if there is skin breakdown from high voltage (>500V) or if it is wet or cut/broken. The flow of current will go along the path of least resistance; through the wet and salt rich environment of muscles/blood vessels.

70-100mA is the threshold for ventricular fibrillation. With an internal resistance of 300 ohms the voltage required to induce VF (if bypassing the skin when immersed for example) would be just 30V [IxR = 0.1 x 300]

3. The duration of exposure to electrical current

As duration of exposure increases the production of thermal energy also increases. With prolonged exposure greater thermal injury ensues.

4. The type of current – AC

In the UK, AC operates at a frequency of around 50 Hz. Human tissue responds particularly well to this frequency. Muscle contraction can be readily stimulated resulting in tetany.

The so-called ‘threshold of sensation’ for electrical current is around 1-4 mA. Currents of this magnitude are perceived as a tingling sensation. Currents greater than 6 mA cause tetany. Between 6-9 mA the ‘let go threshold’ is reached. In the forearm the flexor muscles are stronger than the extensors. Forearm tetany will cause a victim to grasp onto the electrical source and they may be unable to let go. This prolongs the exposure to electrical current. Tetany of the diaphragm and intercostal muscle can cause apnoea and hypoxia. This tends to occur with exposures in the magnitude of 20-50 mA. AC can also initiate ventricular dysrhythmias including ventricular fibrillation at currents greater than 50-70 mA.

5. The type of current – DC

DC exposure causes a violent single muscle contraction. Consequently, victims tend to be thrown away from the electrical source. This decreases exposure time but the violence of muscle contraction and the trauma of being thrown can result in significant injuries. DC is more likely to produce ventricular asystole, which may be transient.

6. The current pathway

As current flows from its source point to the ground contact point, any intervening tissue may be damaged. If the current pathway passes through or in close proximity to the heart, dysrhythmias are more likely.

Current concentration or current density is greatest at the entrance and exit points of the current pathway. Heat generation is greatest at these sites and consequently burns are more marked. Examination of surface burns may allow prediction of the current pathway.

Learning bite

Most morbidity and mortality results from thermal injury and dysrhythmia.