The atrio-ventricular conduction time (AVCT), a time interval which can be obtained from an electrocardiogram, was used for rate-responsive pacing in chronotropically incompetent patients. A control loop is always established as AVCT is sensitive to both the level of exertion and the pacing frequency. Based on experiments a set of plant models was derived for each patient. The controller design was governed by the necessity to attenuate the disturbances acting on AVCT which are correlated with the respiratory cycle. In an ongoing experimental study individual controllers are realized for each patient, but with the same closed-loop bandwidth.