[Ventricular fibrillation during testing of a demand pacemaker (author's transl)].

Ventricular fibrillation was precipitated in a patient with an implanted unipolar demand pacemaker while its function was being tested by switching over to a fixed-rate output by means of a magnet. There was no evidence of either myocardial infarction, abnormal electrolyte state, or digitalis intoxication. This, probably unique, case indicates that non-synchronized stimulation, even during brief switching over of a demand pacemaker to a fixed-rate one, may endanger the patient.