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Electrograms - Intracardiac Electrogram's

Intro to Intracardiac

          One of the big challenges in making the switch to working in the EP Lab is learning to read intracardiac electrograms.  The intracardiac electrogram can be very confusing for someone who has never had to decipher one before. It takes time and practice to understand what you are looking at. In this section, I will provide recordings that show both surface and intracardiac electrograms. As with reading any surface electrocardiogram, it is helpful to take a systematic approach to reading the intracardiac tracings. When you look at the examples provided, use the following steps to help you decipher the signals that are presented.

          It is important to understand that all electrograms are recorded in a similar manner. If you have not yet reviewed the section on Recording Modes, I would encourage you to do so in conjunction with your review of this section. Understanding how electrograms are recorded will provide some insites on the similarities and differences between the surface and intracardiac recordings.

          There are a few simple steps that will help the novice get a grasp on what they are seeing during an EP Study. This section provides a step by step approach to reading bipolar intracardiac electrograms by focussing on activation sequence, or what signals come first and what order the signals occur in.  The concept of activation sequence is one that must be learned in order to learn to read EGM's.

Start with the examples below and work through each step.  Be sure to pay attention to what each step is asking you to do, for these are the steps that you will use in the EP Lab.

1) Look and see how each line of signals on the page you see is labeled.

Usually the labels are located on the left side of the paper. While there is no set method to setting up and labeling the various channels, you will find that most labs follow a common sense approach. There are often two or three surface leads at the top of the page. The intracardiac channels take up the middle to lower portions of the page. The first intracardiac channels are usually from the high right atrium as this is where the signal is first seen when a patient is in normal sinus rhythm. Below the atrial channels I usually place the HIS recordings, followed by the CS and then the right ventricular channels. Proximal is placed above distal in most of my setups. When configured this way, you will see a natural progression of atrial and ventricular signals in normal sinus rhythm. If pressure lines are being recorded, they are usually found at the bottom of the page.

This image has three surface leads and 9 intracardiac leads.  When you look at the side labels, you will see the following channels in order; surface leads II, AVF and V1. Note that it would be more appropriate to use Leads I, AVF and V1. That configuration would provide visualization of right to left activation (Lead I), high to low activation (Lead AVF), and activation of the bundles (V1).

There are also intracardiac leads Right Atrium Proximal (RA Prx), RA Distal (RA Dst), His Bundle Electrogram Proximal (HBE Prx), HBE DST, Coronary Sinus Proximal (CS Prx), CS 7-8, CS 5-6, CS 3-4, CS DST, Right Ventricular Apex Proximal (RVA Prx) and RVA DST.  Remember that the catheter hookups start with distal being plugged into 1-2 and proximal being plugged into the higher numbers.

If you examine the labeling on the CS catheter, you will see this system used on a decapolar catheter.  CS Prx uses pins 9-10 and CS DST uses pins 1-2.  Between the proximal and distal poles are intermediate channels using pins 3-4, 5-6 and 7-8.

 The A and B in the channel labels refers to specific cables being used to hook up this system. The recording system used will determine how each intracardiac input is labelled.

The sweep speed has been increased to 100mm/second to allow for easier visualization of the intracardiac signals.

This may make identification of P waves more difficult as they are broader and harder to identify.

R waves are fairly easy to identify, though they appear somewhat strange.  

Do not be afraid to adjust the sweep speed and amplitude of the electrograms as needed to identify specific signals.

 It is often helpful to take some time to look at a single sinus beat at various sweep speeds.  

This allows time to adjust to the variable displays.

If you have difficulty in isolating the P waves, slow the sweep speed to 25mm/sec.

One you have become familiar with the surface signals, increase the sweep speed in steps.

At each point, make yourself familiar with how the P and R waves appear.

In this diagram, the P waves and R waves have been identified for you.

Finding these surface markers allows you to correlate with the intracardiac channels and makes it easier to begin your interpretation.

2) Check your first A signal.

When examining intracardiac recordings, it is helpful to start out by locating known landmarks such as P waves and R waves. This is where the surface leads are helpful.

3) Follow the A through consecutive channels.

If your configuration is set up similar to mine, it is easy to follow the A wave signal as you drop down from one intracardiac lead to the next. This is the first example of activation sequence, or which signals come first and in what order. This concept will be expanded upon further as we progress through the next several sections. To begin your interpretation of the intracardiac channels, start at the P wave from one of the surface electrograms.

From here, follow the page down to the right atrial channel.

Preceding the surface P wave should be an intracardiac signal.

This is the atrial depolarization.

It appears on the intracardiac channels earlier than the surface leads because the signal must traverse the tissue of the chest cavity from the heart out to the skin before it can be picked up by a surface lead.

Remember when I mentioned that setting up the channels from proximal to distal allowed a natural progression of signals when a coronary sinus catheter was used?

In this image, it is easy to see what I was referring to.

Each consecutive channel has an atrial signal that comes just after the A on the preceding channel.

The earliest A is on the right atrial proximal channel and the last A is the tiny signal seen on the distal coronary sinus channel.

4) Find the HIS.

After you have identified the A signals, locate the HIS channels. Find the A signal on these channels and then look a short distance to the right. You should see a small narrow deflection on at least one of these two channels. This deflection is the HIS signal and indicates depolarization of the HIS bundle fibers.

You can see the small HIS deflection on the distal HIS Bundle channel, (HBE Dst).

This indication of depolarization of the fibers of the HIS bundle is usually seen on only one of the channels from the HIS catheter.

5) Locate the V signals.

After you have identified the HIS, look for the earliest Ventricular signal, usually found on the RVA DST channel at the bottom of the page. Using a technique similar to following the A signals from top to bottom, you can follow the V signals from the right ventricular channels at the bottom of the page to right atrial channels at the top.

The ventricular signal for a normal sinus beat appears first on the right ventricular catheterout at the right ventricular apex.

It shows up next on the coronary sinus channels and then appears on the HIS channels.

The HIS catheter sits just below the AV Node and the CS sits between the left atrium and left ventricle.

Knowing how these signal appear in a normal beat will help you to determine the nature of non-sinus beats and thus, the mechanisms of arrhythmias.

The sequence of events represented by the electrograms is referred to as activation sequence.

This is one of the more powerful tools used in EP.

In essence, activation sequence is a review of what comes first, second, third and so on.

Finding the first signal on an intracaridac recording and knowing where that signal originates from will help you determine a great deal of information about the nature of the rhythm being analyzed.

This tool is discussed further in the section on Arrhythmia Analysis.

Be sure to look at the example of the PVC shown below.

  You should always remember that what you are seeing is all part of one signal. Think of it as an electronic wave that starts at any given point in the heart and expands outward. Eventually, this wave will touch all areas of conductive tissue within the heart, but it will reach some sooner than others. The order in which this signal reaches the various areas within the heart, as well as the catheters that are located throughout the heart, provide valuable information about the nature of the rhythm that has generated the signal being recorded. This can be easily demonstrated using one normal beat and one abnormal beat.

           Take a beat that is generated in the sinus node and travels through the normal conduction system to the ventricles. This beat will have a signal that will first be visible on the electrodes closest to the sinus node. This is usually one set of electrodes on the right atrial catheter. Whether the signal first shows up on the proximal or distal channel will be determined by catheter placement. The signal for this beat will then be detected on the HIS channels, the coronary sinus channels and finally, the channels of the right ventricular catheter. If you reverse this process and look at the signals from a PVC, you will see a sequence that is almost the opposite of what occurred with the normal beat. The signal will be seen first on the right ventricular catheter, then on the CS or HIS catheter and finally on the right atrial catheter.
In this image, the blue vertical line indicates the start of a beat that originates from the sinus node.

The process of dropping a vertical line is often used to determine on which channel a signal first appears.  Here, it is on the right atrial catheter which is close to the sinus node.

The second beat in this image is a PVC.  Onset of the PVC is marked by the red line.

Note that the channel with the earliest activation is the RVA Distal channel.

This indicates that the origin of this beat is closest to the RVA Distal channel somewhere out in the right ventricular apex.

Note once again how the intracardiac signal closest to the origin preceeds the onset of surface ECG.

Note also how we are able to determine the approximate location of the onset of this beat by looking at the activation sequence of the recorded electrograms. The first signal occurs near the RV apical catheter, so the origin must be close to that catheter.

          Understanding how to read an intracardiac electrogram is essential for diagnosis of arrhythmias during an EP study. Learning to read them can be a long process with a steep learning curve. Don't get discouraged. With patience and practice, you will finally come to the point where these signals zipping by on the screen will begin to reveal their secrets to you. This is one of those areas where experience is priceless. Each case that you do is another piece in the puzzle, and after you have enough pieces together, you will begin to see the big picture. That's when the excitement sets in, and electrophysiology starts to become fun. At this point, people will begin to think you are crazy. That's OK, cause crazy is just a state of mind.

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