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Electrograms - EGM Interpretation

Unipolar Electrogram Interpretation

          Electrogram interpretation involves three key steps that begin with unipolar recordings. If we think back to what we know with surface ECG's, we will find that much of what we know about reading electrograms has been put into practice each time we review a 12 lead. In the previous sections, we have discussed how all the surface 12 leads are read as unipolar regardless of the mode of recording used. For this reason, we will start reviewing EGM interpretation by using the surface ECG.

          Unipolar recordings shows three specifc aspects of the electrogram, signal strength, velocity and direction. If we examine a single cardiac cycle, these properties become readily apparent.       

1)  Signal strength: The strength of a given signal is determined by how far the signal deflects from the baseline, or isoelectric line. The isoelectric line is the "zero line" provided by grounding the signal. Many recording systems provide some means of viewing the isoelectric line of a recorded signal.

          Note the difference in the deflection of the P wave, the QRS and the T wave. Each of them represents a different phase in the cardiac cycle. Because of this, each portion of the recorded electrogram shows a different strength or amplitude.

- P Wave: The P wave indicates combined right and left atrial depolarization. As the chamber size and myocardial mass of the atria are considerably smaller than that of the ventricles, the signal strength is less and thus the deflection from the baseline is much smaller.

- QRS:  Next we view the QRS. This portion of the electrogram represents the combined depolarization of both the right and left ventricles. Because of the increased myocardial mass, the signal strength is shown to be considerably higher than the atrial electrogram.

- T Wave:  Finally, we come to the T wave. This represents the repolarization of the ventricles. Repolarization represents the phase of the cardiac cycle where the tissue returns to its base resting voltage. Because this process is gradual and occurs much more slowly throughout the ventricles, the amplitude of this aspect of the electrogram is not as strong as depolarization. For this reason, the amplitude of the deflection from baseline is smaller than that of the QRS, or depolarization. Note that atrial repolarization is not strong enough to be visualized on a surface electrogram.

2) Signal Direction:  Einthoven estblished the convention where by an electrical wave front moving towards a positive recording electrode would be displayed as a positive deflection and a wave front moving away from the positive recording electrode produces a negative deflection from baseline. This concept allows us to determine the direction an electrogram is moving. By analyzing the same three surface electrograms, we can determine much about the conduction through this patient's cardiac chambers.

The image displayed shows an isoelectric line for each of the three electrograms. This line indicates the zero line for amplitude as well as the point which differentiates between positive and negative deflection. Any electrogram that shows a deflection that extends above this line provides documentation that the electrical wave front is moving towards the location of the positive recording electrode of that lead. Let us examine each of these three leads to determine what information may be derived regarding the direction of the depolarization wave front displayed.

1) Lead I: The positive electrode for lead one is located on the left shoulder or arm with the negative electrode placed on the right shoulder or arm. This lead provides excellent visualization of right to left activity. Knowing this, we can look at the different components of the surface ECG and derive the following information;

- P Wave: The P wave in Lead I is positive. This demonstrates that the electrical wavefront is moving towards the positive recording electrode which is located on the left shoulder. Atrial activation is right to left.

- QRS: The predominate vector of the QRS, which represents the combined sum of ventricular depolarization, is right to left, but not completely so. We know this because there is a strong positvie component which indicates that the ventricular electrical activity is moving from the right towards the left. We know that the direction of the activity is not completely right to left as there is a negative component to the QRS in Lead I also.

- T Wave: Ventricular repolarization, which is indicated by the T wave, occurs much slower than depolarization. It also occurs globaly and therefor does not provide as much specific information as the QRS. We can state that repolarization is generally showing right to left activation.

2) Lead aVF: Lead aVF is one of the augmented limb leads derived from Einthoven's triangle using Wilson's central terminal as the negative electrode. The positive pole is located at the "foot" of the body giving us the phrase "augmented Vector Foot" or aVF. This lead is useful in demonstrating high to low activity.

- P Wave: In aVF, the P wave is predominately positive indicating that the combined activation of both atria is from high to low. Note that the P wave in aVF displays a significant notching between the first and second half of the recording. This suggests that the electrical activity in the left atrium, as indicated by the second half of the P wave, is showing a greater negative direction, or more activity from high to low, that the right atrium is.

- QRS: While the QRS in Lead I was predominately positive, we see that in Lead aVF the electrical activation is predominately high to low. This is indicated by the primarily positive deflection of the QRS in aVF. Note that the ventricles usually depolarize from the apex to the base providing a "low to high" pattern. This recording demonstrates that the pattern of ventricular conduction is abnormal in this patient.

- T Wave: Ventricular repolarization is shown to travel in an overall low to high direction.

3) Lead V1: The positive electrode for Lead V1 is in the 4th intercostal space to the right of the midline of the chest. The first of the six precordial leads, V1 shows activation from anterior to posterior or front to back. A positive deflection in V1 indicates activation travelling from back to front. With that in mind, let us analyze the electrogram shown in V1.

- P Wave: The P wave in V1 is predominately positive indicating that the origin of this electrical event may be found near the posterior aspect of the atria. Given that Lead I has also indicated that the activity was right to left, we may surmise that the origin is in the posterior aspect of the right atrium. By combining the information we discerned from Lead aVF, we now know that the origin was high in the right atrium, from the posterior aspect. Putting it together, we have an atrial depolarization from the superior / posterior aspect of the right atrium. This is the general location of the sinus node so we may infer that there is a good possibility that this atrial event originated from the sinus node. To verify this, we would need to evaluate all 12 of the surface leads.

- QRS: In V1, the QRS is predominately negative which indicates ventricular activation occurring in a front to back vector. Normal vectricular depolarization occurs from anterior to posterior so we know that the abnormality identified in the QRS from the aVF lead is did not alter this apsect of the ventricular conduction.

- T Wave: Lead V1 shows that repolarization is predominately a posterior to anterior process as indicated by the positive deflection.

3) Conduction Velocity:  The third piece of information that may be derived from unipolar electrograms is conduction velocity, or how fast the electrical wave front is passing through the cardiac tissue. Conduction velocity is indicated by the slope of the electrogram; the steeper the electrogram, the faster the signal.

Using the skills involved with analyzing electrograms as demonstrated in the previous examples, see if you can answer the following questions.

1) Which of the three vectors shows the slowest atrial conduction velocity?

- Right to Left
- High to Low
- Anterior to Posterior

2) Which event is slower, ventricular depolarization as shown by the QRS or ventricular repolarization as shown by the T wave?




If you answered that right to left atrial conduction velocity was the slowest you are correct. The slope of the atrial electrogram is substantially lower than that of the other two leads. This indicates that the signal moving right to left in the atrium is travelling relatively slower than the activity in the alternate leads.

This same process is used to determine that ventricular repolarization is significantly slower than depolarization. For this reason, the T wave shows a much more gradual slope than the QRS in every lead.

Bipolar Electrogram Interpretation

During the process of generating a bipolar electrogram from two unipolar electrograms, much of the information contained in the source unipoles is lost. The first step in the transition from unipolar EGM's to a bipolar electrogram is imposing a limit on the viewing window. Unipolar recordings have an unlimited viewing window and display i nformation on the electrical activity in every direction around the positive recording electrode. This is due to the fact that the negative electrode is at a location of zero electrical activity (the zero potential). This means that nothing is removed from the source unipolar electrogram.

When a bipolar electrogram is generated, a second unipolar electrogram at the negative electrode is subtracted from the unipole at the positive recording electrode. The effect of this process limits the information represented by the bipolar electrogram to the electrical activity that occurs between the two poles. The viewing window goes from unlimited to 2-5mm. All information from outside this viewing window is excluded from the bipolar electrogram.

The second step that removes information during the transition from bipolar to unipolar is the filtering process. An intracardiac unipolar electrogram is filtered with a band pass filter of 1-2Hz to 300Hz. An intracardiac bipolar electrogram is filtered at 30Hz to 300Hz. The high pass filter of the bipolar is 15 times higher than that of the unipole recording. All the frequencies between 2 and 30Hz are removed from the bipolar electrogram.

These two events remove a sufficient amount of information to make the steps of analyzing unipolar electrograms functionally useless when evaluating bipolar electrograms. The most specific information that may be derived from a bipolar recording is that an electrical event occurred between the electrodes used to record a bipolar electrogram at a given moment in time. The bulk of information regarding voltage, directionality and conduction velocity are lost.

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