The dromotropic pacemaker: System analysis and design considerations
Department of Biomedical Engineering, Medical Faculty, University Center of Medical Engineering (UZMT), Ruhr-University Bochum, Germany.Biomedizinische Technik (Impact Factor: 1.46). 12/2004; 49(11):300-5. DOI: 10.1515/BMT.2004.056
Patients suffering from chronotropic incompetence are generally treated with a rate-responsive pacemaker that stimulates the heart at a rate derived from a strain related sensor signal. The pacemaker concept described here uses a well-defined time interval in the electrogram as sensor parameter (AVCT: atrio-ventricular conduction time). AVCT is directly controlled by the autonomic nervous system. The design of the new algorithm was based on a thorough experimental analysis of AVCT subject to variations of the exercise rate and the pacing frequency. There it was demonstrated that AVCT is disturbed by the respiratory activity. The new rate-responsive algorithm which uses the internal model control principle explicitly takes into account the closed-loop nature of the underlying system. The major design objectives were: a) extended range of the individual heart rate, b) effective attenuation of the respiratory related disturbance and c) dynamic stability. Seven patients undergoing an incremental exercise test were paced with the new AVCT-based algorithm. When paced with this algorithm the paced heart rate was 126 +/- 12 min(-1) whereas the peak intrinsic heart rate was only 100 +/- 20 min(-1). The increase which was significant (26 +/- 13 min(-1); 15.53 min(-1)) clearly demonstrated the potential of this concept to restore chronotropic competence. A reanalysis of the experiments was undertaken in order to facilitate the individual parameterization in clinical practice. It could be demonstrated that a rather simple screening test is sufficient to gain the knowledge necessary for the individual parameterisation.
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ABSTRACT: Rate-responsive pacing based on the atrio-ventricular conduction time is a promising therapy for restoring physiological heart rate control in chronotropic incompetent patients. This paper compares four different algorithms. Three of them had been formulated as patents, but no real test or application has been reported up to now. The fourth was recently published by the authors of this paper. There the steps involved in the development process were stationary and dynamic system identification, control system design and a pilot study with patients. The data obtained were used to formulate a simulation model of the cardiac system by means of which the other algorithms were tested. Test criteria were stability, the attenuation of disturbances and the response time to changes of the exercise rate. None of the three patents worked when being strictly implemented as described. The problems encountered were instability, unusable parameterisations and some questionable adaptation mechanisms. In a redesign we tried to improve the patents, but only in one case would the results obtained justify real use. In the other cases the variability of the pacing frequency was intolerably high.