It all starts here...

          The key to finding a successful solution in diagnostic cardiac electrophysiology is tied to three basic sources of information; the patient’s history, our own experience in the EP lab and our ability to decipher the information provided by cardiac electrograms. The later is by far the most important. Without the ability to read electrograms, the best you can hope to achieve in the lab is that of a glorified button pusher. While there are some who can perform at that level and believe that they are doing “EP”, the rest of us know from experience, that reading electrograms is, pardon the pun, the very heart of EP.

          With little formal education available for those who are starting out in the cardiac EP lab, many of us learn the button pushing approach as a start in the lab. This, at least, gives us the capacity to operate the equipment, one less thing for the physician to worry about. Once we are comfortable with the equipment, most of us will move on to gain a basic understanding of this field and it is here where we begin the efforts to decipher the code of EGM’s. A fair percentage of people starting in that lab are already fairly comfortable with surface electrocardiograms (ECG’s) due to previous experience with 12 leads. This is a good asset to have if you are starting in EP as the basic tools used in generating surface electrocardiograms are exactly the same one used to create intracardiac electrograms.

          An electrogram is a graph of voltage over time. In the case of cardiac electrograms, we are graphing the change in electrical voltage that occurs inside the heart. This change in voltage is directly associated with the cardiac action potential, which is also a graph of voltage over time. To record these voltage changes, we need two electrodes, one positive and one negative. The positive electrode is called the cathode and the negative electrode is the anode1. Generally, you will not need to know the terms anode and cathode unless you are studying for the HRS exam. They will throw these terms at you during this test.

          There are three primary concepts that must be understood to gain a true understanding of electrograms and the information that they provide. The first, and most important of these concepts, is that of recording mode. Electrograms may be recorded in either unipolar or bipolar mode. I know that many people who are starting in the EP lab will invariably balk at the thought of having to confront the polarity problem. What many of them do not realize is that they have probably been working with both of these types of electrograms for some time. If you can read a 12 lead, you are already working with both unipolar and bipolar electrograms. All that remains for you to master the different types of electrograms is to impart understanding of how the different polarities work. For those who are not yet proficient at reading a twelve lead, the information covered in the Electrogram sections will get you started on the road to understanding.

Electrogram Defined...

          An electrogram is a graph of voltage over time. In cardiac electrophysiolgoy, we examine both surface and intracardiac electrograms. As both of these types of recordings are displayed using the same information, we will discuss them both in this section under the term electrograms, or EGM's. When we are looking at EGM's, the horizontal line represents time and is displayed in milliseconds. The verticle line represents voltage and is measured in millivolts. This generally holds true for both surface and intracardiac electrograms.

         The first step in learning about electrograms is understanding how they are recorded and displayed. Many of the people who start out working in the medical field learn how to hook up a patient and use the ECG Cart's controls to record a rhythm without ever truly understanding the exact process that is involved. It is the goal of the author to help provide this basic information to anyone who is interested in learning about this process. Understanding the background of how electrograms work will give any medical professional a greater range of capabilities in the field.

Graphing Voltage Over Time

           The process of recording an electrogram starts by sampling electrical information at specific moments in time. How often this occurs is called the sampling rate. The sampling rate is specific on the type of equipment you are using. The more frequent samples are collected, the more precise or accurate your information will be. If you collect information once every second, or once each 1000ms (there are 1000ms in each second), you may not get an accurate picture of what is going on inside the heart. The total time it takes for the entire heart to depolarize and repolarize can be less than half that time, so you may end up getting little or know information. (input example here). Many of todays recording systems sample at rates of over 1000 times per second, or once every millisecond.

          Using what we know, we can now start to construct an electrogram. We start the process by sampling a single point of information. We then plot the voltage level we have measured as a point on our graph. A second measurement is taken one second later. For this example, we will use a sampling rate of once per second. We move along the horizontal time axis one second further and we put a point directly above that location at the level of the voltage we have measured. Now we connect the two points of information by drawing a line from our first voltage level to the second point on our graph. We repeat this process every second and we begin to see a graph that shows how the voltage changes over time. What we have is a very crude form of an electrogram.

Sampling Rate - More is Definately Better

          If we take our electrogram and start taking samples more frequently, we can begin to get a clearer picutre of what is goind on. The basic rule is that the more samples you take, the more accurate your information will be. So let us try our measurements again, this time taking a measurement every 10th of a second.

An example of this process is under development and will be available soon.

Grounding the Signal

          A ground helps to keep the displayed information from oscillating up and down due to background electrical activity. The Driven ground records basic information about background electrical noise at the point of the ground and feed an inverse signal back in to the patient. This is done at low amperage to prevent the signal from adversely harming the patient. By feeding in a signal that is an opposing waveform to the background electrical activity, two things are achieved.

1. All background noise is eliminated providing a clean signal.
2. A point of zero potential has been defined. This point of zero potential provides two very important functions:
   1. It provides a zero line or base line for the recorded electrogram to be anchored to.
   2. It provides a zero potential for recording unipolar electrograms.