Validated atrial and/or ventricular interval decreases are used to discriminate between VT and SVT. Atrial and/or ventricular intervals are monitored in order to detect decreases in such intervals (which are indicative in increases in rate). The atrial intervals can be, e.g., AA intervals, and the ventricular intervals can be, e.g., VV intervals. A detected atrial and/or ventricular interval decrease can be a decrease that is greater than an interval decrease threshold. Detected atrial and/or ventricular interval decreases can be validated by examining atrial and/or ventricular intervals before and after a detected atrial and/or ventricular interval decrease. The use of the validated atrial and/or ventricular interval decreases to classify an arrhythmia as SVT or VT can be called arrhythmia initiation analysis, since it is believed to determine whether the initiation of the arrhythmia is in an atrium or a ventricle.
