Dogbone Pattern Generation under Unequal Anisotropy

Introduction

Unequal anisotropy ratios, as seen in cardiac tissue, can be responsible for the formation of unexpectedly complex polarization patterns when applying stimuli through point sources. While an elliptical polarization pattern is normally expected when applying a strong source potential, a dogbone pattern can be achieved under unequal conductive anisotropy. The dogbone pattern is termed for the occurrence of two circular regions of opposite polarity (virtual electrodes) that arise. The pattern can be achieved by applying anodal (-) or cathodal(+) simulation.

Experimental Setup

The geometry and the electrodes are defined as follows:

Model: A hexahedral FE model with dimensions 20.0mm x 20.0mm x 10.0mm is submersed in a bath on both the upper and lower ends with thickness 5.0 mm. Fibers are assigned in the model ranging from -60 to 60 degrees. The model is automatically generated in mesher during the simulation.

Stimulus: A strong anodal or cathodal extracellular point stimulus (pt_stim) is applied just above the top surface of the mesh in the bath. An extracellular ground electrode (GND) is assigned to the bottom of the bath.

Assigning unequal anisotropy: Conductivities are applied either anisotropically using the default carp values to generate a dogbone pattern:

gregion[0].g_il          = 0.174
gregion[0].g_it          = 0.019
gregion[0].g_in          = 0.019
gregion[0].g_el          = 0.625
gregion[0].g_et          = 0.236
gregion[0].g_en          = 0.236

or to a single conductivity values of 0.625 to generate an elliptic pattern.

Usage

The following optional arguments are available (default values are indicated):

./run.py --visualize         #Visualize results with meshalyzer
         --pt_stim           = anode (default), cathode #type of applied point source stimulus
         --no_anisotropy     #Turn off conductive anisotropy
         --res               # changes the resolution of the mesh [0.25,0.75] with default 0.5.

If the program is run with the --visualize option, meshalyzer will automatically load the model with applied activation data showing the occurrence of the dog bone (8 seconds) in the presence of anisotropy or an elliptical fir pattern without.

Expected Results

The expected outcomes of the four cases of applying a cathode or anode point stimulus with or without anisotropy are shown: