Current Affairs

Metallic conductors are materials made of metals such as copper, silver, and gold. The fixed atoms in these conductors are partially ionized, and the conduction electrons form an 'electron gas.' Electrical currents result from the movement of these conduction electrons, known as electronic current.

Ionic conductors include substances like saltwater. Ionic solids, such as Na+Cl−, dissolve in water to form an electrolyte, an aqueous solution of cations (Na+) and anions (Cl−). Electrical currents result from the movement of these free ions, known as ionic current.

The Ag/AgCl electrode is a non-polarizable electrode that works by exchanging electrons with metallic Ag and Cl− within the body via the reversible oxidation-reduction reaction: Ag+Cl- + e- ↔ Ag + Cl-. The majority of the contact potential is generated at the metal/electrolyte interface, so an electrolyte layer (e.g. NaCl hydrogel) is used to reduce movement artifacts. The electrode also passes a negative charge from the metal to the tissue and vice versa.

The interface problem is that electrons and ions cannot cross the interface between the metal conductor and the body to produce a continuous current. It is solved by using a polarizable electrode or a non-polarizable electrode with an electrolyte layer.

A contact potential is a d.c. potential difference established between a metal plate and an electrolyte. It affects ECG signals by swamping small signals and causing low frequency baseline shifts. It can be reduced by using differential measurement or a non-polarizing electrode with smaller contact potentials.

Skin preparation is important in ECG measurements because skin resistance (Rskin) is highly dependent on it and can fall from 100 kOhm to 1 Ohm with good preparation. This affects the electrode equivalent circuit and the input impedance of the amplifier.