Case report: inappropriate discharge of an implantable cardioverter-defibrillator caused by the combined count criterion.

We describe a rare case of an inappropriate defibrillator discharge during a sinus tachycardia with single fast sensed beats fulfilling the ventricular fibrillation rate criterion, activating the "combined count criterion," although the prevalent sinus tachycardia was inside a ventricular tachycardia rate zone, programmed to monitor only mode. The "combined count criterion" is a safety feature that ensures defibrillation therapy in case of a tachyarrhythmia inside the ventricular tachycardia detection zone, that is temporarily crossing the ventricular fibrillation detection rate. This case reports shows that there is a low but unpredictable risk of device therapy even in a zone programmed to monitor only mode. This phenomenon should be considered when a monitor only zone is installed for revelation of slower tachyarrhythmias.