A physiological approach to different concepts of rate adaptive cardiac pacing.

Physiological aspects of rate adaptive pacing are dealt with in the context of the cardiovascular control loop with the cardiac output as the controlling quantity and mean arterial blood pressure as the controlled quantity. Different methods proposed or clinically investigated are analyzed with respect to cardiovascular control mechanisms. To qualify as physiologic, the sensor-driven pacing rate must respond to disturbance variables similarly as the sinus node. To achieve this, the feedback signals, which control the cardiac output and are summarized as the sympathetic and parasympathetic innervation of the heart, are prime candidates as input signals for rate control in the presence of chronotropic insufficiency. Since only sympathetic tone can serve to increase contractility and thereby stroke volume and cardiac output in the absence of rate support, its meaningful measurement is the key for physiological sensor-driven rate control. PEP clinically has proved to be a measure of contractility, which represents sympathetic tone as it affects central hemodynamics. PEP-controlled pacing rate adaptation therefore enables restoration of a closed-loop pacing system utilizing those components of the physiological system that are still functioning.