Intracardiac electrogram basics of investing

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Unipolar electrograms use an active electrode within the heart and an indifferent electrode outside the heart. Most of the electrograms recorded during an EP study are bipolar while occasionally unipolar electrograms are also recorded. Unipolar electrograms have more localizing value and can also indicate if there is excessive pressure due to catheter contact with the ventricular wall which produces ST segment elevation due to an injury current.

This principle is also useful in confirming good contact of a pacemaker or defibrillator lead with the myocardium. The size of the electrograms will depend on the proximity of the electrode to the structure from which it originates.

For example, good atrial and ventricular electrograms can be recorded from a location on the mitral or tricuspid annulus. Electrograms should be correlated with the corresponding surface electrocardiograms, though certain potentials like those originating from the bundle of His do not have their counterparts on the surface ECG.

Low amplitude IEGMs could be either due to poor contact of the electrode with the myocardium or the absence of local electrical activity due to scar formation. The bipolar electrode which is a combination of two poles record potentials generated by various cells in its field of measurement.

In electrophysiological studies, various bipolar and unipolar recordings are used to identify the sequence of activation. This can lead to the identification of critical areas of an arrhythmia circuit which can then be the target for ablation. Electrodes are often placed in definite locations within the heart, usually under fluoroscopic guidance, for mapping arrhythmia circuits.

The morphology and amplitude of the intracardiac electrograms depend on the type of depolarization and local myocardial characteristics, orientation of activation wavefront in relation to myocardium, distance from the recording electrodes, size, configuration and interpolar distance of the recording electrodes, orientation of the bipole as well as the conducting medium. Recorded potentials are due to summation both high and low frequency waveforms. High frequency components are the sharp and multicomponent parts of the electrogram like catheter contact artifacts and electromagnetic interference.

Low frequency components are the dull sine wave like components and include the far field potentials. Each recording pole represents the local extracellular potential beneath them in case of unipolar recording.

When there are two unipolar electrodes adjacent to each other the potentials look similar, except for the time delay. In contrast, a bipolar electrogram is the difference between two unipolar recordings. Since the recorded signal voltages are usually less than ten millivolts, they have to be amplified by an amplifier capable of modifying gain as needed. The signals are then digitized with an analog to digital converter with a sampling rate Hz. The band pass filter settings are also different for intracardiac recordings.

While the low pass filter for surface electrocardiograms permit signals below Hz, that for intracardiac recordings is set at below Hz. The high pass filter for surface electrocardiogram is at 0.

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Electrograms denoting the electrical activity of structures near the electrode are termed near field electrograms while those from a distance are termed far field electrograms. For example, a ventricular electrode will record a near field ventricular electrogram and a far field atrial electrogram.

During an electrophysiology study EP study multiple channels display recordings from various intracardiac electrodes and surface ECG leads. Electrode pairs on multi electrode catheters are numbered from distal to proximal. For example, CS represents the distal most pair of electrodes of a multi electrode catheter placed in the coronary sinus. Electrograms recorded between two poles located within the heart are known as bipolar electrograms. Unipolar electrograms use an active electrode within the heart and an indifferent electrode outside the heart.

Most of the electrograms recorded during an EP study are bipolar while occasionally unipolar electrograms are also recorded. In electrophysiological studies, various bipolar and unipolar recordings are used to identify the sequence of activation.

This can lead to the identification of critical areas of an arrhythmia circuit which can then be the target for ablation. Electrodes are often placed in definite locations within the heart, usually under fluoroscopic guidance, for mapping arrhythmia circuits. The morphology and amplitude of the intracardiac electrograms depend on the type of depolarization and local myocardial characteristics, orientation of activation wavefront in relation to myocardium, distance from the recording electrodes, size, configuration and interpolar distance of the recording electrodes, orientation of the bipole as well as the conducting medium.

Recorded potentials are due to summation both high and low frequency waveforms. High frequency components are the sharp and multicomponent parts of the electrogram like catheter contact artifacts and electromagnetic interference. Low frequency components are the dull sine wave like components and include the far field potentials.

Each recording pole represents the local extracellular potential beneath them in case of unipolar recording. When there are two unipolar electrodes adjacent to each other the potentials look similar, except for the time delay. In contrast, a bipolar electrogram is the difference between two unipolar recordings. Since the recorded signal voltages are usually less than ten millivolts, they have to be amplified by an amplifier capable of modifying gain as needed.

The signals are then digitized with an analog to digital converter with a sampling rate Hz. The band pass filter settings are also different for intracardiac recordings. While the low pass filter for surface electrocardiograms permit signals below Hz, that for intracardiac recordings is set at below Hz.

The high pass filter for surface electrocardiogram is at 0. Notch filter is used to filter out the alternating current line voltage interference, which is set at 50 Hz or 60 Hz depending on the frequency of the supply voltage in the locality.