Typical Number in Hospital: Cost Bands: References: 3,4,6,9

To record the ECG, EEG, EMG, etc. electrodes must be used as transducers to convert an ionic flow of current in the body to an electronic flow along a wire. These are usually made of metal. Two important characteristics of electrodes are electrode potential and contact impedance. Good electrodes will have low stable figures for both of the above characteristics.

Electrode potential arises because a metal electrode in contact with an electrolyte (body fluids) forms a half cell with a potential dependent upon the metal in use and the ions in the electrolyte. One might expect that two electrodes of the same material would produce the same electrode potential which would cancel out in any recording, but the actual potentials do depend upon the conditions of contact; for instance, if two steel electrodes are placed in contact with the skin there may be a net contact potential between them of 100 mV. This might cause serious problems when amplifying signals in the microvolt region.

The most widely used electrodes for biomedical applications are silver electrodes which have been coated with silver chloride by electrolysing them for a short time in a sodium chloride solution. When chlorided the surface is black and has a very large surface area. A pair of such electrodes might have a combined electrode potential below 5 mV.

All electrodes suffer from variations in contact resistance due to movement, and the drying out of any coupling medium. This is improved by setting the electrode back slightly from the surface of the skin (floating electrode) on a quantity of coupling jelly (electrolyte paste). A further problem may arise if there is any direct current flowing through the electrode arising from faulty equipment or from small (microamp) bias currents in the measuring amplifier circuit. Over a period of time these currents cause chemical changes at the surface of the electrode causing polarization with consequent increase in the electrode potential. This may cause drift of the electrode potential and damage to the skin due to the chemical action. Many types of recording electrodes exist including metal discs, needles, suction electrodes, glass microelectrodes, foetal scalp clips or screws, etc.

Electrodes are also used to inject electricity into the body as in faradism, TENS, surgical diathermy and physiotherapy diathermy. Electrodes also exist in some analytical apparatus to measure the concentration of specific ions.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details