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 - Electrograms Defined

It all starts here...

This section covers the basic information regarding what an electrogram is. By understanding this fundamental aspect of electrograms, we will be able to derive the most information from these signals. For electrograms are the language of cardiac electrophysiology. If we can not read this language, then we are limited in what we can do in the lab. Once we do learn to read these signals, the doors to greater understanding of what we do in the lab open for us.

A Graph of Voltage over Time

An electrogram may be defined as a recording that shows changes in electrical potential, usually displayed over a period of time. Simply put, an electrogram is a graph of voltage over time. If we look at the image above, we can see a timeline running along the top of image border. This timeline starts at zero and proceeds to a little over 3000. The unit of time is milliseconds (ms) as indicated just after the 200ms mark near the left side of the image making this recording a little over 3000ms or 3 seconds.

What we do not see is a voltage scale. The exact scale is determined by the recording system used to document this electrogram and since it was not provided, we do not have the ability to determine what the voltage was at any place where the voltage extends above or below the baseline. Note that we do know what the baseline value is. In all electrograms, the baseline corresponds to a value of zero millivolts (mv).

The Isoelectric Line

This zero line is referred to as the isoelectric line. Iso stands for "same" and "electric" indicates potential. Thus, an isoelectric line is one that shows where the electrical potential is remaining the same, or unchanging. This indicator of zero change gives us the base by which we can measure changes when they do occur. Analysis of the recording above shows three distinct regions where there are changes in the voltage. These changes are indicated as deflections from the isoelectric line.

If we examine the image to the left, we see that the three regions that show a change of voltage have been labeled as A, B and C. The first change, indicated at A is a relatively small change when compared to B. We know this because the amount of change, or deflection from the isoelectric line, at point A is less that the deflection that occurs at B. When we look at C, we note that the deflection here is similar in magnitude to A.

We can also look at the time over which these changes in voltage occurred. Both A and B seem to occur in a similar time period. This deduction is based upon the observation of the time that transpires on the timeline at the top of the recording. Analysis of the voltage change at C indicates that this took substantially longer than A or B.

This information makes perfect sense since most of us know that this recording is of a single beat of what appears to be a normal cardiac cycle. This process of analysis that we used to evaluate this sinus beat is one that may be used to evaluate any electrogram and is highly dependant upon having a stable isoelectric line. Without a stable "zero point reference", it would be almost impossible to make accurate assessments of how the voltage was changing. The isoelectric line provides us with this stable reference because it is grounded.

Grounded Electrograms

First and foremost, a ground provides an electrical safety function. Should the patient be exposed to a potentially lethal electrical current, the groung provides a conduit that the electrical energy will travel to. The resistance of the ground is less than the resistance in the tissue of the paitent, so the electrical current will travel to the ground first. This is why it is always important to connect the ground for any medical system to the patient before making any other electrical connections.

When we discuss the ground in reference to monitoring of electrograms, the ground plays a secondary role as weel. The establishement of the isoelectric line is achieved by grounding the patient in reference to the medical equipment that is being used to record the electrograms. Think of this in terms of establishing a common point of reference. The machine exists in one realm and the patient exists in a different realm. The only way the machine becomes aware of the patient is when the electrodes and wires connect the two. At this point, the machine will begin to document the information it receives by way of these connections.

The information is presented in an orderly fashion with the isoelectric line representing zero change in the electrical potential being picked up from the electrodes attached to the patient's skin. The image at the top of the page provides a good demonstration of what a recorded electrogram looks like. The isoelectric line provides a clear indication of the time periods when there is no change in electrical potential. This information is available becuase the recording system was able to generate a zero point. To do this, it uses the ground. This is an essential part of displaying electrograms for interpretation. Without a gound, no definative zero point may be defined. The electrogram below is an example of an ungrounded recording.

 

 

Without a proper ground, the recording device can not establish a zero point to use as the isoelectric line. This makes it difficult to determine what actually represents changes in electrical activity. When a ground is attached, we achieve the following rhythm.

 

Now it is easy to determine where the point of zero activity is. The flat periods between depolarizations are indicators of no change in the patient's electrical activity. As the atria depolarize, the first smaller deflection occurs. This is followed by the strong depolarization of the ventricles. Finally repolarization of the ventricles concludes the cardiac cycle. Note that atrial repolarization is masked by ventricular depolarization and is usually not visualized on the electrocardiogram.

 

The Driven Reference

The process by which the zero point of the ground is established is often accomplished by using a driven reference. When the ground is attached to the patient, it begins to process the information that it picks up. For most surface twelve lead recorders, the right leg electrode acts as the system reference and ground. The information that is recorded at this electrode is negated to create an artificial zero. The images below help to demonstrate how this process occurs.

The Process of creating a zero potential using a driven reference.

An electrogram is recorded at the system reference electrode.
The system generates an equal electrogram that is inverted in polarity. This image shows the two seperately for demonstration purposes.
The two electrograms superimposed appear like this.
The electrograms are added to together creating a persistant zero potential. This potential becomes the isoelectric line for all other electrograms displayed using the same system. Note that this zero potential is also used for the creation of unipolar electrograms. See Recording Modes.

 


 
   
     
 
 
About Us | Site Map | Privacy Policy | Contact Us | Disclosure