Myocardial Fiber Effects on Arrhythmia Patterns

The Lack of Fiber Data

The heart model needs to be patient-specific. Therefore, clinical data are processed and fed into the heart model to tune its parameters. Clinical data can be transformed into the components of the heart model. However, one component is not available: the myocardial fiber orientations. Introducing new equipment or procedures to the current ablation system is undesirable. A method is needed to compensate for the lack of fiber data. To achieve this, it is necessary to first study the effects of fibers on activation patterns.

Slab Tissue Experiments

Experiments were conducted on slab tissue. The fiber angle between endocardium and epicardium was varied, and the activation wave direction was also altered, creating a total of fifty scenarios. The results show that fibers have a stronger effect on conduction velocity when endocardium and epicardium fiber orientations are the same.

It was also found that the thickness ratio between endocardium and epicardium can affect conduction velocity. As shown in these two sets of experiments, an imbalanced thickness ratio results in a difference in the activation wavefront locations.

The observations from the slab tissue experiments indicate that fiber orientations can have a significant effect on activation patterns. However, the left atrium differs from slab tissue. Fiber organization in the left atrium is much more complex. The question that arises is whether the observations from the slab experiments still hold true in the left atrium.

Left Atrium Fiber Simulations

To investigate fiber effects on activation patterns in the left atrium, an ex-vivo fiber database is utilized. Fiber data from seven different patients are registered onto the same left atrium, and the same set of arrhythmias is simulated to observe differences in activation patterns.

To register fiber data from one mesh to another, a non-rigid iterative closest point transformation is first applied to the mesh. As illustrated here, the magenta mesh is morphed into the shape of the blue mesh. A reference frame transformation is then applied to the fiber vectors.

The results of the arrhythmia simulations with different fibers are shown below. Figures in the same row represent the same arrhythmia, with differences only in the fiber data. The results indicate that fibers do not produce large differences in activation patterns. The average activation time difference is 7.8 ms, or 4.3 percent of the activation time range.

The Cancellation Effect

A closer look reveals that fibers do affect local activation patterns. When activation direction is perpendicular to fiber orientation, conduction is slower. When activation direction is parallel to fiber orientation, conduction is faster.

However, the local effects of fibers do not accumulate. Fiber organization in the left atrium varies greatly. As shown in figure (c) below, for activation traveling along the red arrow, speed can increase and decrease, but the overall speed remains relatively unchanged. This phenomenon is called the cancellation effect.

Figure (a) below shows a path from point A to point B. The conduction speed along this path increases and decreases due to different fiber organizations, but the average conduction speeds remain similar regardless of fiber organization differences.