Index
Ep Defined | Getting Started | Working in the EP Lab
Right Atrium | Right Ventricle | Left Atrium | Left Ventricule | Cardiac Conduction | Cardiac Cell Properties | Action Potential | Sympathetic or Not | Med Page
Electrograms Defined | Recording Modes | Electrode Spacing | Filters | EGM Interpretation | Arrhythmia Analysis
The Physical Lab | Tools of the Trade
Setting Up | Catheter Placement | Baseline Measurement | SNRT | Conduction Study | Arrhythmia Induction | Pacing Protocols | Ablation | Tilt Table | Secrets to Success
Bradycardia | Atrial Tach | Atrial Flutter | Atrial Fibrillation | AVNRT | AVRT | Ventricular Tachycardia
Surface ECG's | Intracardiac Questions | Med Challenge | Advanced

Electrograms - Electrode Spacing

2mm, 5mm, 10mm or more?

         The placement and spacing of the electrodes used to record cardiac activity can have a profound affect on bipolar signals.1 For unipolar signals, electrode spacing has no meaning. The negative reference electrode is placed in a location of zero electrical activity and all the user sees is the electrical signal as visualized by the positive electrode. To help the reader understand this concept, picture youself standing on a hill with an unobstructed view in all directions. This is a similar perspective to the unipolar electrogram. There is good visualization in all directions.

          Now let us examine bipolar electrograms and how the viewing perspecitve is affected when we switch from unipolar to bipolar. Remember that bipolar electrograms represent the difference between two unipolar electrograms, each recorded at either the positive or negative electrodes. By using very tight spacing between the electrodes, bipolar signals filter out most far field signals. They also filter out information on the amplitude or voltage of the signal. To help demonstrate this, try this next exercise in the lab.

         Set up a decapolar or duo-decapolar catheter. This exercise works best when catheters with uniform spacing such as 2-2-2 or 5-5-5 are used. Program a single electrogram using the distal electrode as the positive recording pole and make electrode #2 the negative electrode. Record your signal and measure the amplitude using the horizontal voltage calipers available on most systems. Print out this recording. Now change the negative electrode to electrode #3. If you have uniform spacing on your electrodes, you will have doubled the spacing between the electrodes. If you have variable spacing, such as 2-5-2, you will still see an effect as you will have increased you electrode spacing from 2 to 7mm. Once again, record your signal, measure the voltage using the horizontal calipers. Compare it to the first signal. You will find that the voltage you are recording has increased substantially.

          Continue to change the negative electrode to use electrodes further down the catheter without changing the positive pole. Record the different settings until you have utilized all the different electrode combinations. Take your recordings and set them out in front of you in sequence from the first to the last. The results of electrode spacing should now be clear to you. As electrode spacing increases, the amplitude and width of the electrograms recorded increases. The small, narrow bipolar signal begins to look more and more like a unipolar electrogram. Imagine what would happen as you continue to increase the space between the recording electrodes.

The Surface ECG Link....

          The example given above using different electrode spacings is taken to it's furthest conclusion when you look at the surface electrograms. We know that leads I, II and III are bipolar leads. The remaining leads are all unipolar. Take a good look at a 12 lead electrocardiogram. Visually, can you detect any difference in the bipolar leads that would set them apart from the unipolar leads? Do you analyze them any differently? Of course the answer to both these questions is no. All surface leads are read using the same techniques. The electrode spacing on the bipolar leads completely encompasses the electrical activity that is being documented. When the electrode spacing is increased on a bipolar recording, it begins to take on aspects of a unipolar signal. This is why all surface electrocardiograms are read as unipolar signals.

          Now let us take this principal back to the intracardiac electrograms. Suppose your physician wants to set up a unipolar electrogram on his distal ablation catheter. To make matters worse, he wants to be able to see his bipolar signal also. You have never done this before and are stuck for a solution. If that should happen, try this technique.

          Jumper your distal RF electrode to any open channel. Do the same thing with any electrode in the heart, but choose one that is farthest from where the ablation catheter is being used. For this example, we are mapping a crista tachycardia on the lateral wall of the right atrium. The physician has applied several applications of RF energy without success. He suspects that he may be able to find an earlier location using unipolar mapping than he found using bipolar mapping. In this situation, we would use the distal Coronary Sinus electrode as our negative reference and jumper that to the open channel next to where you jumpered the RF electrode. For purposes of this example, we will say that on a Cardiolab system, you used B block 31(RF) and 32 (dCS), for the Bard, D15(RF) & D16(dCS) and for the EP Med Workmate you used 49(RF) & 50(dCS).

          Open the setup screen and create a catheters using the RF channel as positive electrode and the dCS channel as the negative electrode. Now the key part of this exercise. Adjust the high pass filter setting on this channel only from 30Hz to 2Hz. You can even use 0.1Hz (unfiltered) though this may cause a lot of baseline drift. You now have an effective unipolar signal from your distal RF catheter that you can display right above you bipolar signal from the distal RF electrodes.

          When you do this, notice how the unipolar signal can locate activation preceding the earliest acitvation that can be found when using bipolar to map. This is because unipolar electrograms utilize a "larger viewing window" and thus can see more than bipolar. You should also note that unipolar electrograms are much more likely to be affected by far field activity such as ventricular depolarization. Always compare your intracardiac signals to the surface electrocardiogram to determine if you are picking up any far field ventricular activity.

Source 1:  Cardiac Electrophysiology – From Cell to Bedside 3rd Edition; Douglas P. Zipes, MD and Jose Jalife, MD ISBN # 0-7216-7811-4           Cell to Bedside has long been one of the definitve resources for those who work in the electrophysiology labratory. This book is not for beginners as it presents a depth of knowledge that is often difficult to grasp without extensive experience. If you have been working in EP and are seeking to expand your knowledge beyond the basics, the this book is a must.
Source 2:  Cardiac Pacing - Edited by Kenneth A. Ellenbogen; ISBN 0-86542-184-6           This book comes is one of the early pacing texts. I found information in here regarding unipolar and bipolar sensing that was worth adding to this section.  This topic will be expounded in the section on intracardiac electrograms.
Source 3:  Clinical Electrocardiography, a Simplified Approach - 7th Edition / Ary L. Goldberger ISBN           This is a good resource for those who are new to interpretting the surface electrocardiograms. This was one of the few books where I found information on the specific electrode hookups for unipolar and bipolar.
Order these books at Cardiotext.com            If you are looking to add any of these texts to your library, click on the link above to visit the Cardiotext website. Select the section on Electrophysiology to see some of the latest titles available.
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