Origin of Intracardiac Extracellular Potentials

Origin of Intracardiac Extracellular Potentials refers to the biophysical processes by which electrical signals generated by cardiac myocytes are conducted through the heart tissue and captured by electrodes placed within the cardiac chambers. These extracellular potentials represent the changes in voltage in the interstitial space surrounding cardiac cells during the cardiac cycle, recorded as intracardiac electrograms (EGMs). They arise primarily from transmembrane ionic currents and the resulting local variations in the extracellular electric field.

Biophysical Basis of Intracardiac Extracellular Potentials

Transmembrane Action Potentials and Ionic Currents

The fundamental source of intracardiac extracellular potentials is the action potential generated by cardiac myocytes. When a cardiac cell depolarizes, ion channels open, allowing the flow of ions such as sodium (Na⁺), calcium (Ca²⁺), and potassium (K⁺) across the cell membrane. This movement of ions produces transmembrane currents, which consist of inward and outward ionic flows.

The transmembrane current density, denoted as I_m, creates a dipole because of the separation of charges across the cell membrane. This dipole effect generates electric fields in the extracellular space surrounding the cells. The time-varying nature of these currents during depolarization and repolarization phases leads to changes in extracellular voltage.

Volume Conductor Properties of Cardiac Tissue

The cardiac tissue acts as a volume conductor composed of intracellular space, extracellular space, and cell membranes. The extracellular space, filled with conductive fluids, allows the propagation of electrical signals from the active cells to distant recording electrodes.

The distribution and magnitude of extracellular potentials depend on:

• The geometry and orientation of activated cells.
• The anisotropic conductivity of myocardial tissue, which differs along and across fiber directions.
• The proximity of recording electrodes to active regions.

The extracellular potential φ_e at a point in space can be modeled as the superposition of contributions from multiple transmembrane currents distributed throughout the myocardium.

Spatial and Temporal Characteristics of Intracardiac Extracellular Potentials

Dipole and Multipole Sources

At the cellular level, the active region of depolarization can be approximated as a current dipole source. The extracellular potential generated by a dipole decreases with distance approximately as 1/r², where r is the distance from the source.

When multiple cells activate simultaneously in complex patterns, the resulting extracellular potential is a summation of numerous dipoles and higher-order multipole sources, leading to the complex shapes observed in intracardiac electrograms.

Wavefront Propagation and Potential Morphology

As the depolarization wavefront propagates through the myocardium, the spatial distribution of active transmembrane currents changes, dynamically altering the extracellular potential field.

The recorded EGM morphology reflects the direction, velocity, and pattern of wavefront propagation. For example:

• Bipolar EGMs show a rapid biphasic waveform as the wavefront passes between two closely spaced electrodes.
• Unipolar EGMs display a slower, monophasic potential representing the cumulative activity of a larger myocardial region.

Temporal features such as amplitude, duration, and polarity of extracellular potentials depend on these propagation dynamics.

Influence of Electrode Configuration and Position

Electrode Type and Size

Intracardiac electrodes vary in size and configuration, influencing the spatial resolution and amplitude of recorded extracellular potentials. Smaller electrodes detect signals from smaller tissue volumes, resulting in higher spatial resolution but lower signal amplitude.

Electrode Orientation and Distance

The relative position of the electrode regarding the wavefront direction critically affects the recorded potential. Electrodes aligned parallel to the wavefront propagation detect signals with different polarity and amplitude compared to those oriented perpendicularly.

Distance from the active myocardium also attenuates signal amplitude due to the volume conduction effect and tissue conductivity.

Summary of Key Factors Governing the Origin of Intracardiac Extracellular Potentials

Mathematical Model of Extracellular Potentials

The extracellular potential φ_e at a location r outside cardiac cells can be mathematically described by the integral of transmembrane current sources distributed in the myocardial volume:

Where:

• φ_e(r) is the extracellular potential at position r,
• σ is the extracellular conductivity,
• I_m(r') is the transmembrane current density at source location r',
• |r - r'| is the distance between the source and recording point,
• The integral is taken over the volume of active myocardium.

This equation reflects the volume conductor theory, describing how distributed transmembrane currents give rise to extracellular potentials measurable by intracardiac electrodes.

Clinical and Electrophysiological Relevance

Understanding the origin of intracardiac extracellular potentials is fundamental for interpreting intracardiac electrograms during electrophysiological studies and catheter ablation procedures. It aids in:

• Localizing arrhythmogenic foci by analyzing potential morphology and timing.
• Differentiating near-field (local) from far-field (distant) signals.
• Assessing conduction velocity and tissue viability.
• Guiding therapeutic interventions based on the electrical activity mapping of the heart.

Accurate interpretation depends on knowledge of the underlying biophysical mechanisms generating these extracellular potentials.

Summary Diagram

This diagram illustrates how transmembrane ionic currents in cardiac cells generate extracellular potentials that spread through the conductive extracellular space and are detected by intracardiac electrodes.

Understanding these principles is essential for accurate mapping and interpretation of intracardiac electrograms during cardiac electrophysiology procedures.