Surface electrodes for physiological measurement and stimulation
Smith, D.C.; Wace, J.R.
European Journal of Anaesthesiology 12(5): 451-469
1995
ISSN/ISBN: 0265-0215 PMID: 8542853 Document Number: 3566
For practical noise-free ECG and electromyographic monitoring, disposable electrodes of a rigid design with a relatively large effective area and deep gel cup are required. Although there is wide inter- and intra-individual variation in skin/electrode impedances, these are small in comparison with the extremely high input impedances of modern biomedical amplifiers and the absolute value of skin/electrode impedance is not particularly important for ECG and EMG recording. There will, therefore, be minimal benefit from aggressive skin preparation and attempts to reduce the skin/electrode impedance to very low values. Secure fixation of the electrodes and a small difference in impedance between the recording electrodes-the latter implying the need to use high quality electrodes with potassium chloride electrolytes and stable silver/silver chloride layers-will minimize the effects of movement-related artefacts and interference from common-mode signals arising from mains power supplies. For noise-free EEG recordings an electrode system with a low impedance is required, and aggressive preparation of the skin will improve the signal-to-noise ratio. This is not quite so important for processed EEG signals such as the auditory evoked response, because the computer averaging techniques used for signal processing tend to cancel out random noise potentials while enhancing the signal we want. High impedance electrodes can therefore be compensated for by increasing the number of periods of EEG activity which are averaged. At the high current densities passing through surface electrodes used for nerve and muscle stimulation, the low effective skin/electrode impedance resulting from the high stimulus current density eliminates the need for aggressive preparation of the skin.
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