In an example, an apparatus is described that includes an implantable housing, a heart signal sensing circuit configured to sense intrinsic electrical heart signals, a ventricular tachyarrhythmia (VT) detector circuit, operatively coupled to the heart signal sensing circuit, the detector circuit operable to detect a VT based on the sensed heart signals, a processor configured to control delivery of an anti-tachyarrhythmia pacing (ATP) therapy based on the detected VT, and an energy delivery circuit configured to deliver the ATP therapy in response to the detected VT, wherein the apparatus does not include a shock circuit capable of delivering a therapeutically-effective cardioverting or defibrillating shock.