Intracardiac electrograms (IEGMs)

Intracardiac electrograms are the recordings of electrical activity within the various chambers of the heart using multipolar electrodes placed inside the heart. While conventional electrocardiograms (ECGs) from the body surface are recorded at a paper speed of 25 mm/second, IEGMs are recorded at a much higher paper speed of 100 to 200 mm/second. Filtering of IEGMs are also different from that of surface ECGs to reduce noise and interference. 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 1-2 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. 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. A fragmented signal may indicate a region of slow conduction.

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