Types of Surface Electrodes (Floating, Metal Plate & Multipoint)

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 are commonly used for ECG. Smaller surface electrodes are commonly used for EMG and EEG. The types of surface electrodes are as follows.

Metal Plate Electrodes

ECG measurements can 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, which then disturbs the trace.

Because a plate can sit on each of the four limbs, they are called limb electrodes. During some surgical procedures the legs stay still, so limb electrodes can stay out of the wound. Chest electrodes may get in the way of the surgical field, so they are skipped when the chest is open. For long-term monitoring, reusable limb plates are also a poor fit; disposable adhesive electrodes are used instead.

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. Infection risk and cleaning difficulty mean they are now less common than disposable adhesives.

Adhesive Type Electrodes

Pressure from surface electrodes can squeeze out electrode paste. Adhesive electrodes reduce that leak with a lightweight metallic screen and a pad for paste placement. The adhesive backing holds the electrode in place and slows evaporation of the paste electrolyte.

Multipoint Type Electrodes

Multipoint electrodes are a practical ECG pickup with a large number of contact points, sometimes quoted above 1000. Many points can lower contact resistance, but they do not guarantee a clean trace in every environment.

Floating Electrodes

Metal plate and limb electrodes often pick up movement artefact when the metal-electrolyte interface slides. Floating electrodes keep the metal off the skin and couple through paste or jelly. That recess gives the interface more mechanical stability.

Ear Clips and Scalp Electrodes

In EEG measurement, ear clip electrodes can be used as a reference. Scalp electrodes pick up EEG when placed on the scalp, usually in the 10-20 electrode system. Placement still has to follow that map; it does not cancel every artefact. During labor, a fetal scalp electrode can monitor the baby’s heartbeat while the baby is still in the uterus.