Myocardial cells generate the cardiac action potentials. 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 extra cellular 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 1000Hz. The band pass filter settings are also different for intra cardiac recordings. While the low pass filter for surface electrocardiograms permit signals below 200 Hz, that for intracardiac recordings is set at below 500 Hz. The high pass filter for surface electrocardiogram is at 0.05 Hz while that for intra cardiac is 30 to 50 Hz. 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.
The normal ventricular electrogram has multiple rapid components with an amplitude over 3 millivolts in ninety five percent of areas. Mean amplitude may be around 6.7 millivolts and mean duration around fifty four milliseconds. In over ninety five percent of cases the duration is less than seventy milliseconds.
The His bundle electrogram is a sharp biphasic or triphasic signal designated as H, normally seen between the atrial (A) and ventricular (V) signals, when the electrode tip is near the bundle of His. Once the His potential is identified, AH and HV intervals are measured. Measurements are made from the proximal His electrode signal. The position should be confirmed by fluoroscopy and the HV interval should be normally more than thirty five milliseconds, and associated with a good atrial electrogram. AH is measured from the earliest reproducible rapid atrial deflection to onset of H deflection in the His bundle catheter. HV interval is measured from the onset of H activation to onset of surface V (QRS) in any lead.
Abnormal intracardiac electrograms include fragmented potentials, double potentials and mid diastolic potentials. Low voltage potentials are those with an amplitude less than half a millivolt in the atrium and less than one and a half millivolts in the ventricle. Low voltage potentials could be due to myocardial infarction, fibrosis or poor contact of the electrode as well as due to far field effect.
Pulmonary vein potentials have a rapid high frequency initial deflection and short duration. They follow the far field atrial electrograms during left atrial pacing and precedes far field electrogram during pulmonary vein pacing.
A successful electrophysiologial study depends on the perfect understanding of normal and abnormal intracardiac electrograms recorded. The abnormal electrograms are used to identify critical regions of focal or re-entrant tachycardias, which can be the targets for ablation.