ECG Instrumentation


In order to record the ECG, we need a transducer capable of converting the ionic potentials
generated within the body into electronic potentials which can be measured by conventional
electronic instrumentation. Such a transducer consists of a pair of electrodes, which measure
the ionic potential difference between their respective points of application on the body
surface. Electrodes may be classified either as polarisable, in which case they behave as
capacitors, or non−polarisable, in which case they behave as resistors. Common electrodes
have characteristics that lie between these extremes; the silver−silver chloride electrode
discussed below approximates more closely to a non−polarisable electrode.


Electrode placement
The most obvious way to record the ECG is between the Right Arm (RA) and the Left Arm
(LA) although another two combinations using the Left Leg (LL) are also used clinically
(RA−LL and LA−LL).

Another electrode is also used to connect the patient to the common ground of the
instrumentation. Usually, this ground electrode is attached to the right leg.



Silver−silver chloride electrode
Electrodes for recording biopotentials are composed of a metal (usually silver for ECG
measurement), and a salt of the metal (usually silver chloride). In addition, some form of
electrode paste or jelly is applied between the electrode (normally a flat silver disc) and the
skin. The combination of the ionic electrode paste and the silver metal of the electrode forms
a local solution of the metal in the paste at the electrode−skin interface (also referred to as
the electrode−tissue or electrode−electrolyte interface). Hence, some of the silver dissolves
into solution producing Ag+ ion


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Movement artefact
If the electrode is moved with respect to the elctrolyte, this mechanically disturbs the
distribution of charge at the interface and results in a momentary change of the half−cell
potential until equilibrium can be re−established. If a pair of electrodes are in contact with
an electrolyte and one moves while the other remains stationary, a potential difference
appears between the two during this motion. This potential is referred to as moverment
artefact and can be a serious cause of interference in the measurement of ECG (or any other
biopotential).

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