Heart rate based control of cardiac resynchronization therapy

In some examples, controlling delivery of cardiac resynchronization therapy (CRT) includes storing, in a memory of an implantable medical device system and in association with each of a plurality of heart rates, at least one respective value for an interval between an atrial event and a ventricular event. Processing circuitry of the implantable medical device system may determine a heart rate of a patient and select one of the stored values for the interval between the atrial event and the ventricular event associated with the determined heart rate. The processing circuitry may further determine whether to control therapy delivery circuitry of the implantable medical device system to deliver fusion pacing or biventricular pacing, based on the selected one of the stored values for the interval between the atrial event and the ventricular event.