- Surface Electrodes Definition: Surface electrodes measure electrical signals from the skin, including ECG, EMG, and EEG.
- Types of Surface Electrodes: Surface electrodes include metal plate, suction cup, adhesive, multipoint, floating, ear clip, and scalp electrodes.
- Metal Plate Electrodes: Made of nickel, silver, or German silver, these are used for ECG but can slip and cause errors.
- Floating Electrodes: These electrodes stabilize the interface using paste or jelly to reduce motion errors without direct skin contact.
- Multipoint Electrodes: With over 1000 contact points, these electrodes ensure low-resistance contact for accurate ECG readings.
What is a Surface Electrode?
Surface electrode measures the potential available from the surface of the skin. It senses the signal from heart, brain and nerves. Larger surface electrodes sense the ECG signals. Smaller surface electrodes sense the EMG and EEG signals. The types of surface electrodes are as follows.
Metal Plate Electrodes
ECG measurements use rectangular or circular metal plate electrodes made of nickel, silver, or German silver. These electrodes have a small contact area and do not fully seal on the patient. They are attached using electrolyte paste, but they can slip and shift, causing measurement errors due to their sensitivity.
Since it is suitable for application on four limbs of the body, they called limb electrodes. During surgical procedure since patient’s legs are immobile, limb electrodes are preferred. Chest electrodes interfere with the surgery, so not used for ECG measurement. At the same time for a long-term patient monitoring limb-electrodes are not used.
Suction Cup Electrodes or Welsh Cup Electrodes
Suction cup electrodes are used to measure ECG from different chest positions. They attach well to flat surfaces and soft tissue regions, providing good contact. Although large, only the rim touches the skin, leading to high contact impedance. They include a plastic syringe barrel, suction tube, and cables. Due to infection risks and cleaning difficulties, these are now less commonly used.
Adhesive Type Electrodes
Pressure from surface electrodes can squeeze out electrode paste. Adhesive electrodes solve this problem with a lightweight metallic screen and a pad for paste placement. The adhesive backing holds the electrode in place and prevents the paste’s electrolyte from evaporating.
Multipoint Type Electrodes
Multipoint electrodes are practical for ECG measurements, featuring over 1000 contact points. This ensures low-resistance contact with the skin, making them reliable under any environmental condition.
Floating Electrodes
Metal plate and limb electrodes often suffer from movement errors due to motion at the electrode-electrolyte interface. Floating electrodes stabilize this interface by using paste or jelly, without direct skin contact. This type offers improved mechanical stability.
Ear Clips and Scalp Electrodes
In the measurement of EEG, ear clip electrodes are used. Scalp electrodes provide EEG signals easily when placed on bare head. Generally,in 10 – 20 electrode system EEG measurement scalp electrodes are used. This type avoids measurement errors. During labor, fetal scalp electrode monitors baby’s heartbeat while still inside uterus.
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