1. An implantable medical device comprising: a lead including electrodes configured to be positioned within a heart to sense cardiac signals of the heart; a pulse generator electrically coupled to the lead and adapted to deliver stimulus pulses to the heart through at least one of the electrodes; a cardiac signal module to monitor the cardiac signals and direct the pulse generator to deliver one or more of the stimulus pulses to the heart at a stimulation rate based on the cardiac signals; a fusion detection module to identify a presence of fusion-based behavior of the heart that is associated with delivery of the one or more of the stimulus pulses; and a rate modification module to adjust the stimulation rate based on the presence of the fusion-based behavior.