RESUMO
Chronotropic incompetent patients are unable to adapt their heart rate adequately to the level of exertion. One treatment for this disease is a rate-responsive pacemaker that stimulates the heart at a rate proportional to an exertion-related variable derived from an implemented sensor (e.g., ventilation). The atrio-ventricular conduction time (AVCT), which can act as such a sensor, corresponds to a well-defined interval in the intracardial electrogram. The AVCT is coupled to the sympathetic/parasympathetic activity of the autonomous nervous system (dromotropic effect), leading to a shortening of AVCT during exercise. We found that AVCT is also sensitive to the pacing frequency (pf). Hence, an AVCT-based pacemaker always constitutes a closed-loop system. General design rules for an AVCT pacemaker have been developed from our experimental results and a system-theoretical treatment with reasonable assumptions. Problems addressed were uncertainties concerning the AVCT dynamics and disturbances in the signal. The following rules for the controller design have been derived: 1) The controller gain strongly depends on the therapeutic range of the pf, which is set by the medical doctor; 2) the closed-loop bandwidth of the system should be smaller than the respiratory frequency; and 3) a robust control strategy, capable of guaranteeing stability for a certain set of plant models, should be applied.
