Computer simulation
A simple, anatomically fixed reentry circuit was constructed in a custom software model of a cardiac tissue plane using the Matlab programming environment (Matlab R2020a, The Mathworks, Inc., Natick, MA). A propagation model was created consisting of an 1281 x 1281 matrix of excitable elements. The nominal dimensions of the simulation matrix are 40 mm x 40 mm with an imbedded 20 mm diameter circular scar. Within the non-conducting scar is an asymmetric gap of conducting elements. The matrix is isotropic except for a difference in refractory period within the scar gap. The excitable elements are 31.25 µm in diameter, though this is an arbitrary size chosen to provide as dense a matrix as possible within the constraints of the available computational resources. To allow more physiologic circular wavefronts within the rectangular simulation matrix, each activated element activates a roughly circular cluster of adjacent elements. The 16-element-radius size of this cluster was chosen to allow a wavefront as smooth as possible within the available computational constraints. Each active element activates its non-refractory cluster of neighboring elements immediately at each iteration interval, thus the conduction velocity through the matrix is determined by the cluster size and the iteration interval. That interval was chosen arbitrarily to be 4 ms to provide a reentry cycle length through the scar gap of ~300 ms. The conduction velocity within the scar gap is identical to that of the surrounding. After being excited, each element remains refractory to subsequent activation for an interval of 100 ms, though this interval is arbitrary and was chosen to facilitate visualization of a sequence of activation wavefronts. The refractory time within the scar conduction gap is 260 ms.
Programmed stimulation of the cardiac tissue plane is simulated by exciting specified locations within the tissue plane with a typical pacing protocol. In order to assess the range of VT inducibility, stimulation was tested at sequential points around the entire circumference of the scar while maintaining a constant distance from the outer edge of the scar. In these simulations, the last impulse of a 300 ms drive train is followed by a 220 ms extrastimulus. The simulation continues until no tissue is in the excited state or until a stable re-entrant circuit is demonstrated.