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 - Signal Filtering

Too much information or not enough?

         Each electrogram collected is passed through a series of filters that are each designed to remove specific type of information. The filtering used on each electrogram will have a significant impact on what information is removed, and thus what information is displayed, on each electrogram visualized.

          The most common filter is the noise filter. A noise filter is designed to remove specific high and mid range frequency signals while allowing other lower frequencies to pass. This design works to eliminate electronic source noise or power line noise from most signals. Electronic source noise is produced internally by circuits within the system during operation. As this type of noise is impossible to prevent, the noise filter can be an invaluable piece of equipment in a computerized recording system.

          Another filter used is the band pass filter which uses a combination of high pass and low pass filters to remove unwanted information from the recorded electrogram. A high pass filter blocks low frequency signals and allows the higher frequencies to pass. This type of filter is designed to remove physiologic waveforms such as the low frequency myopotential emitted by the diaphragm or the slow far field signals from cardiac chambers outside the chamber we are mapping. The low pass filter allows lower frequencies to pass and helps to remove high frequency environmental noise that comes from outside the recording system.

Insert Diagram of Band Pass Filter

          During any procedure, you may use different combinations of signal filters to display specific information. The noise filter is optional and can be applied as needed. The table below provides some of the typical band pass filter settings that are commonly used for different types of electrograms.

Signal Type High Pass Filter Low Pass Filter
Surface ECG
0.5Hz
100Hz
Intracardiac Bipolar
30Hz
300Hz
Intracardiac Unipolar
1-2Hz
300Hz
Unfiltered Unipolar
0.1Hz
(or no high pass)
300Hz

          As you can see from this table, most high pass filtering is minimized except for bipolar intracardiac signals. This allows for the bipolar IC recordings to focus on signals with higher frequencies, or signals with a faster conduction velocity. This also prevents bipolar IC signals from seeing slower moving signals. This will be discussed in more detail in the section on Intracardiac Electrograms. The low pass filter on the intracardiac signals is consistent. The surface ECG uses a lower setting to filter out a bit more of the higher frequencies that may add noise. This is due to the fact that the surface electrodes are on the outside of the body and may be subject to more environmental noise than the electrodes that are placed inside the patient’s heart. An example of differnet types of electrograms and their filter settings is included in the table below.

  • In this image, we see three different electrograms along with the filter settings that each was recorded at.
  • The top line is a surface recording of Lead I. Note that the high pass filter is at 0.5Hz.
  • The second line is an intracardiac bipolar with an electrode spacing is 2mm. The high pass filter here is 32Hz.
  • The third electrogram is a unipolar recording with a high pass filter set at 2Hz.
  • Each of these filter settings represents the nominal used for this system. Note that the nominal setting on most EP recording systems for intracardiac bipolars is 30Hz. The sweep speed is set at 200mm/sec. Note how the intracardiac bipolar shows a narrow, discrete signal. In contrast, the surface bipolar shows a broad signal that more closely resembles the intracardiac unipolar recording. Which of these three provides the user with the most information?

How adjusting the filters affects the signals...

         For the most part, you will not need to adjust the filter settings on any of your electrogram recorders, though it is valuable to understand what happens when you do adjust them. Filters are used to remove frequencies from the recorded electrograms. To remove 60Hz noise (electrical noise in the US and Canada), a low pass filter of 50Hz could be applied to the recorded signal. Any frequency above 50Hz would be removed from the corresponding signal. Any amplitude associated with these frequencies would also be removed resulting in, hopefully, a straight line with little or no background interference.

          Often notch filters are used to remove specific frequencies such as noise from the electricronics found in all equipment. In the US and Canada, electrical power at the plug arrives at 60Hz. Any recording system that operates on this power is susceptible to 60Hz noise. A notch filter, often labeled as a noise filter is commonly used to eliminate this frequency without removing any other desired frequencies. The downside of this approach is that filter ringing may often be visualized after a signal that is not removed out by the notch filter is displayed. Filter ring appears as a sinusodial waveform of progressively decreasing amplitude that appears on the back side of the displayed waveform. (Insert Image - Filter Ring)

          To help understand how the high pass filters used in cardiac electrophysiology affect the signals we see, a series of waveforms were recroded at multiple high pass filter settings. A surface electrocardiogram, a contact bipolar intracardiac electrogram and four non-contact unipolar intracardiac electrogram are shown in the following sequence. The following waveforms were collected using an EnSite 3000 mapping system.  As you view each of these signal sets, note how both the width and amplitude of the signals are being affected. It is important to understand the differences in how filters affect the different signals. The differences documented here are one reason why unipolar waveforms at lower high pass filter settings are often used to find the earliest activation of an arrhythmia. Electrograms recorded in this manner can visualize signals that are filtered out by bipolar waveforms with the nominal high pass filter setting of 30-32Hz.      Signal Filter Examples 0.1 Hz - 32.0

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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