A medical device for detecting and treating tachyarrhythmia, comprising: a plurality of electrodes (24, 26, 28) for acquiring a cardiac signal; a sensing module coupled to the plurality of electrodes for sensing cardiac events in response to the cardiac signal; and a processing module configured to: perform an initial detection process comprising detecting a shockable cardiac rhythm by comparing the cardiac signal to a first set of detection criteria; characterized in that the processing module is further configured to: establish, by analyzing the cardiac signal, at least one patient-specific detection threshold during the initial detection process; and upon establishing the at least one patient-specific detection threshold, stop the initial detection process and start a next detection process comprising comparing the cardiac signal to a second set of detection criteria comprising the at least one patient-specific detection threshold, the first set of detection criteria relating to a first set of cardiac signal features, the second set of detection criteria relating to a second set of cardiac signal features different than the first set of cardiac signal features. a plurality of electrodes (24, 26, 28) for acquiring a cardiac signal; a sensing module coupled to the plurality of electrodes for sensing cardiac events in response to the cardiac signal; and a processing module configured to: perform an initial detection process comprising detecting a shockable cardiac rhythm by comparing the cardiac signal to a first set of detection criteria; characterized in that the processing module is further configured to: establish, by analyzing the cardiac signal, at least one patient-specific detection threshold during the initial detection process; and upon establishing the at least one patient-specific detection threshold, stop the initial detection process and start a next detection process comprising comparing the cardiac signal to a second set of detection criteria comprising the at least one patient-specific detection threshold, the first set of detection criteria relating to a first set of cardiac signal features, the second set of detection criteria relating to a second set of cardiac signal features different than the first set of cardiac signal features. perform an initial detection process comprising detecting a shockable cardiac rhythm by comparing the cardiac signal to a first set of detection criteria; characterized in that the processing module is further configured to: establish, by analyzing the cardiac signal, at least one patient-specific detection threshold during the initial detection process; and upon establishing the at least one patient-specific detection threshold, stop the initial detection process and start a next detection process comprising comparing the cardiac signal to a second set of detection criteria comprising the at least one patient-specific detection threshold, the first set of detection criteria relating to a first set of cardiac signal features, the second set of detection criteria relating to a second set of cardiac signal features different than the first set of cardiac signal features. establish, by analyzing the cardiac signal, at least one patient-specific detection threshold during the initial detection process; and upon establishing the at least one patient-specific detection threshold, stop the initial detection process and start a next detection process comprising comparing the cardiac signal to a second set of detection criteria comprising the at least one patient-specific detection threshold, the first set of detection criteria relating to a first set of cardiac signal features, the second set of detection criteria relating to a second set of cardiac signal features different than the first set of cardiac signal features.