Methods and systems are provided for discriminating heart arrhythmias. The methods and systems include identifying an arrhythmia, recording a predetermined number of beats during the arrhythmia as a base arrhythmia (BA) beats; delivering anti-tachy pacing (ATP) therapy to at least one chamber of the heart. After delivering the ATP therapy, the methods and system record at least one return beat representing cardiac activity following the ATP therapy, determines whether the return beat originated in a reference chamber of the heart, compares a morphology of the return beat to a morphology of the BA beat; and declares a VT or SVT based on the comparing operation.