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Electrograms - Recording Modes

The Different Recording Modes

There are two primary recording modes used to generate electrograms. The most commonly used mode is bipolar. For years this has been the standard type of electrogram used in EP. There are numerous physicians who understand the value of unipolar electrograms and have begun using them on a regular basis. It is important to understand how these two modes are related. Those who learn how to use both will be able to accomplish much more than those who believe that bipolar is all you need.

They All Have Two Electrodes

To start the learning process, we must first define what these two modes represent. All electrograms are based off of unipolar signals. A bipolar EGM is in fact, a derivative of two unipoles. To fully appreciate what this indicates, we must determine how a unipolar EGM is created.

All electrograms utilize two electrodes. One is positive and one is negative. All electrograms used in EP are derived from subtracting the information at the negative electrode from the information at the positive electrode. As shown in the image below, a bipolar electrogram uses information from both electrodes. A unipolar electrogram uses the same process, but the negative electrode is generated in such a manner that it has a zero potential. In essence, there is no electrical activity at the location of the negative pole. A unipolar electrogram represents the view from one pole.

 As we explore this statement further, we look at the process of subtracting the information from one electrode from the other. In the image below, we see that the unipolar waveforms documented at the positive and negative electrodes have been superimposed over each other. The bipolar pair represents an electrode spacing of a short distance, similar to the standard 2mm used in most EP catheters today. Remember that the negative electrode of the unipole was at a point of zero potential. As such, there was no information from that location and the unipolar only has information from the positive pole.

We now subtract the information from the negative electrode from the positive electrode. In the case where the unipoles at all electrodes are represented by a standard sine wave, the results of this process may be visualized below the overlapped electrograms.

The unipole remains unchanged. The bipolar electrogram has decreased in amplitude and the morphology has changed to that displayed in the lower portion of the image. It is relatively easy to set up a spreadsheet using graphs of numeric values to explore how different waveforms will look when one is subtracted from the other. The key point is that the unipolar electrogram remains unchanged as there is no information at the negative electrode to subtract from the electrogram from the positive electrode while the bipole provides an electrogram that shows only the difference in electrical activity between the two electrodes.

The difference between unipolar and bipolar will be explored a bit further on (lower on this same page). Before we proceed however, it is important to point out that the information at each of the electrodes involved in the process of electrogram creation is a unipolar electrogram itself. As we have indicated, a unipolar electrogram is achieved by subtracting the information at a negative recording electrode at a point of zero potential from the information documented at a positive recording electrode. So where is the zero potential electrodes for the all the unipoles documented at the electrodes used in this process?

The Zero Potential Reference

A zero potential may be derived using a couple of methods. One of the most common methods put forth was that of Wilson's Central Terminal, or WCT. The idea behind WCT was to take an average of the 3 primary leads, I, II and III which would create a zero potential.(1) This approach was similar to contributions from Goldbereger who, in 1942 introduced the augmented limb leads of aVR, aVL and aVF.(2) Goldberger used only two of the three alternate leads to come up with an averaged potential to use as the negative pole.(2) Neither of these systems actually represent a true zero potential as skin resistance generates small signal potentials.(3)

The Driven Reference

A driven reference may also be used to create a zero potential. This process takes the input at one electrode and introduces an inverse of that waveform. The input and output are combined resulting in a zero potential.

Regardless of the technique used to generate a zero potential, the ability to do so is essential to cardiac electrophysiology. The ability to create unipolar and subsequently bipolar EGM's is based upon the ability to create a zero potential electrode.



(1) Wilson FN, Johnston FD, Macleod AG, Barker PS (1934): Electrocardiograms that represent the potential variations of a single electrode. Am. Heart J. 9: 447-71.
(2) Goldberger E. A simple, indifferent, electrocardiographic electrode of zero potential and a technique of obtaining augmented, unipolar, extremity leads. Am Heart J 1942;23:483-92.
(3) Madias J. Point of View On Recording the Unipolar ECG Limb Leads via the Wilson's vs the Goldberger's Terminals: aVR, aVL, and aVF Revisited. IPEJ (ISSN 0972-6292), 8 (4): 292-297 (2008)
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