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A typical electrode gel consists of a mixture of water, a thickener such as polyvinyl alcohol or carboxymethyl cellulose, and an electrolyte to increase conductivity. [1] [2] Some electrode gels have been shown to pose a potential fire risk when high-energy pulses are used, as for example when defibrillators are used. [3]
To perform the test, one set of electrodes are placed on viable tissue (e.g. the chest) as a control and a second set is placed around the tissue in question (e.g. legs or feet). The electrodes may mildly heat the skin to increase blood flow into the area. Oxygen may also be given to the patient to see if that increases oxygen levels in the tissue.
The electrodes are normally placed about an inch apart, and the resistance recorded varies according to the emotional state of the subject. Galvanic skin potential (GSP) refers to the voltage measured between two electrodes without any externally applied current. It is measured by connecting the electrodes to a voltage amplifier.
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In fact, firm application of the hands and feet against the electrodes blocks physiological sweating, and the active measure extracts electro-active ions (i. e., chloride near the anode, proton near the cathode) and pulls them towards the electrodes. The resulting conductance is then given for each foot and hand in μS (micro-Siemens).
With correct placement of the needle electrode and level of electrical impulse, there is often an involuntary toe flex or fan, or an extension of the entire foot. However, for some patients, the correct placement and stimulation may only result in a mild sensation in the ankle area or across the sole of the foot.
Severe burns (3rd degree) can occur if the contact with the return electrode is insufficient, or when a patient comes into contact with metal objects serving as an unintended (capacitative) leakage path to Earth/Ground. To prevent unintended burns, the skin is cleaned and a conductive gel is used to enhance the contact with the return electrode.
Until recently the common belief in the FES field has been that due to the electrode-skin contact impedance, skin and tissue impedance, and current dispersion during stimulation, much higher-intensity pulses are required to stimulate nerves using surface stimulation electrodes as compared to the subcutaneous electrodes.