Intracardiac Electrograms

Intracardiac electrograms are recordings of the electrical activity within the heart obtained directly from electrodes placed inside the cardiac chambers or on the endocardial surface. These signals provide detailed information about local myocardial depolarization and conduction properties, essential for diagnosing and treating arrhythmias. Unlike surface electrocardiograms (ECGs), which reflect the summation of electrical activity through the body, intracardiac electrograms offer high spatial and temporal resolution, allowing precise mapping of cardiac electrical events.

Origin of Intracardiac Extracellular Potentials

Intracardiac electrograms represent extracellular potentials generated by the transmembrane ionic currents during myocardial depolarization and repolarization. These potentials arise from the flow of ions across cardiac cell membranes, creating local voltage gradients in the interstitial space. The recorded signals depend on the relative position of the electrode to the active myocardial cells, the direction of the electrical wavefront, and the conductivity of the surrounding tissue and blood.

The measured potential at an electrode is a weighted sum of near-field (local) and far-field (distant) electrical sources. Near-field components correspond to electrical activity immediately adjacent to the electrode, reflecting the local myocardial activation, while far-field components represent distant cardiac events and can obscure or modify the local signal.

Unipolar and Bipolar Electrograms

Unipolar Electrograms

Unipolar electrograms are recorded by a single electrode referenced to a distant indifferent electrode, often placed outside the heart or at a remote intravascular site. This configuration captures the absolute extracellular potential at the electrode site relative to the reference.

Unipolar electrograms typically display large amplitude signals with distinct deflections corresponding to depolarization and repolarization phases of the myocardium. However, they include both near-field and far-field components, making interpretation complex without careful analysis.

Bipolar Electrograms

Bipolar electrograms are obtained by recording the potential difference between two closely spaced electrodes on a catheter. This differential measurement emphasizes local electrical activity while attenuating far-field signals common to both electrodes.

Bipolar recordings generally have lower amplitude than unipolar ones but provide better spatial resolution of local activation. The morphology of bipolar electrograms depends on the electrode spacing, orientation relative to the wavefront, and tissue properties.

Near-Field and Far-Field Components

Intracardiac electrograms consist of a combination of near-field and far-field electrical activity. Near-field signals arise from myocardium in direct vicinity of the recording electrode, representing local activation timing and amplitude. These signals are essential for identifying sites of earliest activation or abnormal conduction.

Far-field signals originate from remote cardiac regions and propagate through the conductive medium. They can appear as additional deflections or baseline shifts and may interfere with accurate interpretation of local activity. Differentiating near-field from far-field components is critical for precise electroanatomic mapping and ablation targeting.

Local Activation Time

Local activation time (LAT) is the precise moment when a specific myocardial site undergoes depolarization, as identified by the initial rapid deflection in the intracardiac electrogram. LAT is typically marked at the steepest negative slope of a unipolar electrogram or the peak deflection in a bipolar signal.

Accurate determination of LAT at multiple intracardiac sites enables construction of activation maps that delineate electrical conduction pathways, identify conduction delays, blocks, and arrhythmogenic foci.

Electrogram Amplitude and Local Voltage

Electrogram amplitude reflects the magnitude of the extracellular voltage generated by local myocardial depolarization. Higher voltages generally correspond to healthy, viable myocardium with preserved cellular coupling, whereas low-voltage areas may indicate fibrosis, scar, or diseased tissue.

Voltage mapping using intracardiac electrograms assists in identifying arrhythmogenic substrates such as scar tissue or regions of slow conduction that contribute to arrhythmia maintenance.

Electrogram Duration and Fractionation

Electrogram duration is the time interval during which local electrical activity occurs, measured from the onset to the end of the deflection. Prolonged electrogram duration can indicate slow or heterogeneous conduction.

Fractionated electrograms contain multiple low-amplitude deflections or split potentials within the activation interval. These features suggest areas of conduction block, anisotropy, or complex tissue architecture, often associated with arrhythmogenic substrates.

Electrogram Morphology and Wavefront Direction

The shape and polarity of intracardiac electrograms depend on the direction of the electrical wavefront relative to the electrode orientation. Wavefronts approaching an electrode produce positive deflections, while wavefronts moving away generate negative deflections.

Electrogram morphology analysis aids in discerning conduction patterns, identifying reentrant circuits, and differentiating near-field from far-field signals.

Electrode Size, Spacing, and Orientation

Electrode characteristics influence the quality and resolution of intracardiac electrograms. Smaller electrodes provide higher spatial resolution but may have lower signal amplitude and higher noise. Larger electrodes capture broader signals but may average electrical activity over larger areas.

Spacing between electrodes on bipolar catheters affects the sensitivity to near-field signals; closer spacing enhances local signal detection but reduces the field of view.

Orientation of electrodes relative to the wavefront direction alters bipolar electrogram morphology and amplitude, necessitating careful catheter positioning during mapping.

Electrode-Tissue Interface and Contact

Effective coupling between the electrode and myocardial tissue is essential for accurate electrogram acquisition. Poor contact or presence of interposing blood or fibrotic tissue can attenuate signal amplitude and distort morphology.

Contact force sensing technologies help optimize electrode-tissue interface, improving signal quality and procedural outcomes in catheter ablation.

Intracardiac Signal Filtering and Sampling

Intracardiac electrograms require appropriate filtering to remove noise and baseline drift while preserving relevant signal components. Typical bandpass filters range from 30 to 500 Hz, balancing elimination of muscle and motion artifacts against retention of critical electrical features.

High sampling rates (≥1 kHz) are used to capture rapid deflections and subtle electrogram details necessary for precise activation mapping.

Atrial Electrograms

Atrial intracardiac electrograms differ from ventricular signals in amplitude, duration, and morphology due to thinner atrial myocardium and complex fiber orientation. Atrial electrograms often exhibit fractionation and low voltage, especially in diseased or fibrotic tissue.

Mapping atrial electrograms is fundamental for diagnosing atrial arrhythmias such as atrial fibrillation and flutter.

His Bundle Electrograms

His bundle electrograms are specialized intracardiac recordings obtained near the atrioventricular (AV) node region. They capture electrical activity from the His-Purkinje conduction system, characterized by sharp, high-frequency deflections preceding ventricular activation.

Identification of His potentials is critical for assessing AV conduction properties and guiding ablation procedures near the conduction system.

Ventricular Electrograms

Ventricular intracardiac electrograms display higher amplitude signals reflecting the thicker ventricular myocardium. They provide detailed information on ventricular activation sequences, conduction delays, and scar-related arrhythmogenic substrates.

Ventricular electrograms are essential in the diagnosis and treatment of ventricular tachycardia and other ventricular arrhythmias.

Intracardiac Conduction Intervals

Intracardiac conduction intervals are time measurements between specific electrogram components recorded at different intracardiac sites. Examples include the AH interval (atrial to His bundle activation), HV interval (His bundle to ventricular activation), and local activation time differences.

These intervals help evaluate conduction system function, identify blocks, and localize arrhythmia circuits.

Intracardiac Activation Sequence Analysis

Activation sequence analysis involves mapping the temporal order of myocardial depolarization across multiple intracardiac sites. By marking local activation times on electrograms, detailed activation maps can be constructed, revealing normal and abnormal conduction pathways.

This analysis supports the identification of arrhythmia mechanisms, reentrant circuits, and targets for catheter ablation.

Multicomponent and Double Potentials

Multicomponent electrograms contain multiple deflections separated by isoelectric intervals, often indicating areas of slow conduction or heterogeneous tissue. Double potentials consist of two distinct deflections closely spaced in time, usually representing activation on either side of a conduction barrier or scar.

Recognition of these complex electrogram patterns is crucial in identifying arrhythmogenic substrates and guiding targeted therapy.