Autointerference of demand pulse generators.

We describe in this report two cases of autointerference of demand pacemakers by signals originating within a pulse generator itself rather than the electrode or connection. Restoration of regular fixed rate pacing upon application of the magnet cannot differentiate between an external or internal source of oversensing as exemplified in our two cases. The absence of any visible interference on the surface electrocardiogram and the relatively late appearance of autointerference in the life of a pulse generator may mimic an intermittent fracture (or insulation defect) with over sensing of false signals. Autointerference of demand pacemakers, although rare, should be added to the list of causes producing oversensing and prolongation of the spike-to-spike interval.