A method and apparatus for delivering therapy to treat ventricular tachyarrhythmias is described. A ventricular tachyarrhythmia can be classified into tachycardia or fibrillation zones using a rate criterion so that the tachyarrhythmia can be treated with either anti-tachycardia pacing or shock therapy, respectively. Rate stability and morphology criteria are employed to determine if a tachyarrhythmia in the fibrillation zone is actually monomorphic ventricular tachycardia which can be treated with ATP.
