What do you mean by an indifferent electrode in ECG?

What do you mean by an indifferent electrode in ECG?

The electrode connected to the right leg is usually taken as the indifferent electrode in ECG (electrocardiogram). Other electrodes form either the positive or negative pole of each lead, while the indifferent electrode forms the reference electrode and may be connected to the equipment ground. The indifferent electrode connected to the right leg is color coded black. This is for bipolar recordings like the standard limb leads. Though the indifferent electrode is presumed to be electrically neutral, it is not exactly the situation. Anyway it is an electrode most distant from the heart.

In the past various points on body surface far from the heart have been tried as indifferent electrodes like back, left arm, right arm, and left foot [1]. Choice of distant electrode has significant influence on the  potential being recorded depending on the distance from the heart, especially for unipolar recordings. Wilson’s central terminal was designed to reduce this effect on precordial electrogram [2]. It may be noted that Wilson’s central terminal is formed by connected all three extremities through 5000 Ohms resistors to a central point. Even though the potential at the central terminal may not be always zero, potential variations of central terminal may be very small compared to extremity electrodes.

How good a particular limb used as indifferent electrode is, can be shown by recording a unipolar limb lead from that extremity. Maximum influence is by the limb recording largest potentials. But the extremity which is least desirable as a reference electrode varies between individuals depending on the direction of the electrical signals produced by the heart [1].

References

1. Hans H. Hecht. The influence of the indifferent electrode upon the precordial electrocardiogram: I. The normal electrocardiogram. Am Heart J.  1942; 24:529-544. https://doi.org/10.1016/S0002-8703(42)90970-0.
2. Frank N. Wilson, Franklin D. Johnston, A. Garrard Macleod, Paul S. Barker. Electrocardiograms that represent the potential variations of a single electrode. Am Heart J.  1934; 9:447-458. https://doi.org/10.1016/S0002-8703(34)90093-4.