A system comprising an implantable medical device includes a ventricular heart signal sensing circuit, a pacing therapy circuit, and a controller circuit coupled to the signal sensing circuit and the therapy circuit. The controller circuit is operable to detect a ventricular tachycardia heart rhythm from the cardiac signal, measure an amount of time the heart signal is one of inside or outside of an effective amplitude band, and initiate ATP therapy if the measured amount of time is less than a threshold amount of time.