  This paper investigates the properties of unstable single-spiral wave
solutions arising in the Karma model of two-dimensional cardiac tissue. In
particular, we discuss how such solutions can be computed numerically on
domains of arbitrary shape and study how their stability, rotational frequency,
and spatial drift depend on the size of the domain as well as the position of
the spiral core with respect to the boundaries. We also discuss how the
breaking of local Euclidean symmetry due to finite size effects as well as the
spatial discretization of the model is reflected in the structure and dynamics
of spiral waves. This analysis allows identification of a self-sustaining
process responsible for maintaining the state of spiral chaos featuring
multiple interacting spirals.
