Restoration of cardio-circulatory regulation by rate-adaptive pacemaker systems: the bioengineering view of a clinical problem.

In the past, the development of rate-adaptive (sensor-controlled) pacemaker systems seems to have been determined primarily by the availability, compatibility and other properties of the technical sensor. This paper, however, focuses on the system-physiological aspect in an attempt to answer the question to what extent physiological cardiovascular control is restored by the pacemaker system. This is a question which should be asked before attempting to design a sensor-controlled system and especially before designing multisensor systems with infinite combinations. Four categories are defined: direct bridging ("shunting"), open loop systems, closed systems using cardiorespiratory or metabolic coupling and those using cardiac signals. Further subdivisions are shown. From the bioengineering as well as from the physiological viewpoint a system should preferably not combine sensors from one and the same of these categories. At present direct bridging is available only for the atrioventricular (AV)-block, so that for sick-sinus-syndrome (SSS) patients feedback control via cardiac signals ("inotropic" pacemaker) comes nearest the goal without, however, ideally bridging the gap. Open-loop systems should no longer be developed as single-sensor systems. A well developed activity sensor, however, which quickly pinpoints the most prominent stressor of cardiovascular control is best suited to complement another sensory system achieving closed-loop control. New and promising concepts orientated toward direct bridging are the analysis of monophasic action potentials and the "dromotropic" concept, both of which seek direct correlation with the "chronotropic" information not available in SSS patients.