Shocking truths about implantable cardioverter defibrillator monitoring zones.

A 36 year-old man with hypertrophic cardiomyopathy and an ATLAS + DR implantable cardioverter defibrillator (ICD) (St. Jude Medical, Inc., St. Paul, MN, USA) for primary prevention received a shock while cycling. The ventricular fibrillation detection threshold was 182 beats/min. An additional monitoring zone was programmed to 156 beats/min with all discriminators "on" except morphology. On interrogation, the ICD shock followed sinus tachycardia. In the absence of a monitoring zone, device therapy would not have been expected. We explore the mechanisms by which monitoring zones could potentially contribute to inappropriate ICD therapy and offer trouble-shooting tips.