Wavebreak and Fibrillatory Conduction

Wavebreak and fibrillatory conduction are fundamental electrophysiological phenomena underlying the initiation and maintenance of cardiac arrhythmias, particularly atrial and ventricular fibrillation. Wavebreak refers to the disruption or fragmentation of a propagating electrical wavefront in cardiac tissue, while fibrillatory conduction describes the resulting chaotic and irregular propagation of these fragmented wavelets, leading to disorganized electrical activation and loss of effective mechanical contraction.

Mechanisms of Wavebreak

Wavebreak occurs when a normally continuous excitation wavefront encounters conditions that cause it to fragment or split into multiple wavelets. This disruption typically arises from spatial heterogeneities in electrophysiological properties such as conduction velocity, refractory period, or tissue excitability. Key factors contributing to wavebreak include:

Refractory Heterogeneity

Differences in refractory periods across adjacent cardiac regions create zones where tissue is excitable next to refractory tissue. When a wavefront reaches such a boundary, it may be forced to divide or slow, leading to conduction block in some areas and continued propagation in others, producing wavebreak.

Conduction Velocity Dispersion

Variability in conduction speed due to anisotropic tissue structure, fibrosis, or altered gap junction coupling can cause uneven wavefront propagation. Slowed conduction in one region adjacent to faster conduction in another promotes wavefront curvature and fragmentation.

Tissue Structural Discontinuities

Anatomic obstacles such as scar tissue, fiber direction changes, or areas of ischemia generate lines of conduction block or slow conduction. These discontinuities facilitate wavefront fragmentation by forcing the electrical impulse to detour or split.

Source-Sink Mismatch

Wavefront propagation depends on the balance between the depolarizing current source (excited cells) and the downstream cells' electrical load (sink). If the sink demands exceed the source's capacity, conduction fails locally, causing wavebreak.

Wavebreak Dynamics and Spiral Waves

Wavebreak often initiates reentrant circuits by generating spiral waves or rotors. These spiral waves rotate around a central core of unexcited or refractory tissue, sustaining localized rapid excitation that emits wavelets into surrounding myocardium. The core's size and stability depend on tissue properties and contribute to arrhythmia persistence.

The curvature of the wavefront is critical: tight curvature at the wavebreak site reduces conduction safety, increasing the probability of wavefront fragmentation. This curvature-driven instability underlies the generation of multiple competing wavelets.

Fibrillatory Conduction

Following wavebreak, multiple wavelets propagate through the atrial or ventricular myocardium in a disorganized fashion, a process termed fibrillatory conduction. This results in:

• Rapid, irregular activation sequences with varying conduction velocities and directions.
• Fragmentation of wavefronts into smaller wavelets that continuously collide, merge, or extinguish.
• Spatial and temporal electrical heterogeneity that prevents organized contraction.

Fibrillatory conduction sustains arrhythmias like atrial fibrillation by maintaining multiple simultaneous reentrant circuits. The myocardium becomes electrically chaotic, with no dominant activation pattern or coordinated contraction.

Electrophysiological and Structural Substrates Promoting Fibrillatory Conduction

Shortened Action Potential Duration and Refractoriness

Short refractory periods allow rapid re-excitation and support high-frequency wavelets, increasing the likelihood of wavebreak and fibrillatory conduction.

Increased Tissue Heterogeneity

Variations in ionic channel function, fibrosis, or ischemic injury create a patchy substrate favoring wavefront fragmentation.

Anisotropic Conduction

Myocardial fiber orientation causes direction-dependent conduction velocity, favoring wavefront curvature and wavebreak.

Clinical and Experimental Implications

Understanding wavebreak and fibrillatory conduction provides insight into arrhythmia mechanisms and therapeutic targets:

• Antiarrhythmic drugs may reduce wavebreak by homogenizing refractoriness or slowing conduction.
• Catheter ablation targets regions critical for wavebreak initiation or rotor maintenance.
• Mapping techniques identify regions of electrical heterogeneity to predict fibrillation sources.

Comprehending these phenomena is essential for developing strategies to prevent or terminate life-threatening arrhythmias by stabilizing wavefront propagation and preventing fragmentation.

Summary of Key Concepts

Mathematical Description of Wavefront Curvature and Conduction Velocity

The relationship between conduction velocity (CV) and wavefront curvature (κ) is a fundamental principle in wavebreak:

Where:

• $\mathrm{CV}$ is the conduction velocity at curvature $\kappa$
• $\mathrm{CV}{0}_{}$ is the conduction velocity of a planar (zero curvature) wavefront
• $D$ is a diffusion coefficient related to tissue conductivity
• $\kappa$ is the local curvature of the wavefront; positive curvature corresponds to convex wavefronts

As curvature increases, conduction velocity decreases. When curvature becomes too high, conduction may fail, causing wavebreak.

Illustration of Wavebreak and Fibrillatory Conduction