A medical device and associated method for classifying an unknown cardiac signal sensing a cardiac signal over a plurality of cardiac cycles using a plurality of electrodes coupled to a sensing module, determining a template of a known cardiac signal in response to the cardiac signal sensed over the plurality of cardiac cycles, sensing an unknown cardiac signal over an unknown cardiac cycle, determining a fourth order difference signal corresponding to the template and a fourth order difference signal of the unknown cardiac signal, determining a first morphology match metric between the template fourth order difference signal and the fourth order difference signal of the unknown cardiac signal, and classifying the unknown cardiac signal in responsed to the determined first morphology match score.
