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.