1. A method for classifying cardiac beats for use in detecting cardiac rhythms, comprising: sensing a cardiac signal over a plurality of cardiac cycles; 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; determining an R-wave onset and an R-wave offset in response to the fourth order difference signal of the unknown cardiac cycle signal; determining an R-wave width as the difference between the R-wave onset and the R-wave offset; determining a morphology analysis window in response to the R-wave width; and determining a first morphology match metric across the morphology analysis window.