  Cardiac arrhythmias such as ventricular tachycardia (VT) or ventricular
fibrillation (VF) are the leading cause of death in the industrialised world.
There is a growing consensus that these arrhythmias arise because of the
formation of spiral waves of electrical activation in cardiac tissue; unbroken
spiral waves are associated with VT and broken ones with VF. Several
experimental studies have been carried out to determine the effects of
inhomogeneities in cardiac tissue on such arrhythmias. We give a brief overview
of such experiments, and then an introduction to partial-differential-equation
models for ventricular tissue. We show how different types of inhomogeneities
can be included in such models, and then discuss various numerical studies,
including our own, of the effects of these inhomogeneities on spiral-wave
dynamics. The most remarkable qualitative conclusion of our studies is that the
spiral-wave dynamics in such systems depends very sensitively on the positions
of these inhomogeneities.
