An implantable medical device includes a lead, a pulse generator, a cardiac signal module, a fusion detection module and a rate modification module. The lead includes electrodes that are configured to be positioned within a heart to sense cardiac signals of the heart. The pulse generator delivers stimulus pulses to the heart through at least one of the electrodes. The cardiac signal module monitors the cardiac signals and directs the pulse generator to deliver one or more of the stimulus pulses to the heart at a stimulation rate based on the cardiac signals. The fusion detection module identifies a presence of fusion-based behavior of the heart that is associated with delivery of the one or more of the stimulus pulses. The rate modification module then adjusts the stimulation rate based on the presence of the fusion-based behavior.