Cardiac muscle is modelled by a simple, three-dimensional, coupled map lattice with component maps that show appropriate rate-dependent changes in action-potential duration. Anisotropy of conduction velocity and heterogeneity of action-potential duration is produced by a uniform spatial gradient along one axis of one parameter of the model, for parameters that control the coupling between maps and the action-potential duration. Non-uniformity of the coupling parameter in a direction orthogonal to the organising filament and of the parameter controlling the action-potential duration in a direction parallel to the organising filament gives rise to distortion and then breakdown of the scroll wave, producing new, interacting vortices.