An ectopic pacemaker is included in a heterogeneous oscillator model of cardiac conduction system.

It produces a model capable to generate electrocardiogram signals corresponding to healthy hearths as well as with arrhythmias.

The transition from normal to ventricular fibrillation is controlled by a single parameter of the model.

It is numerically shown that the transition from normal to ventricular fibrillation follows the torus or quasi-periodic route to chaos, as it has been suggested to occur in some experimental results.