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Various sensor systems that indicate the hypermetabolic state have been used to provide input signals for controlling rate responsive pacing systems. The sensitivity of an indicator may be related directly or indirectly to metabolic requirements. Metabolic, respiratory, and cardiac dynamics during exercise have been described. Direct measurement of oxygen consumption provides the best index of work rate and exercise capacity. In pacemaker-dependent patients, heart rate is not an independent variable. During exercise, changes in pacing rate and hemodynamics do not show the specificity of a rate-responsive system, but are the result of a combination of sensor response and electronic or signal processing. Following the general patterns of change in various measurable parameters during exercise with progressively increasing work rates, the hypermetabolic indicators have been categorized into five groups according to the accuracy of their relationship to oxygen consumption. The indicators that are used or proposed as sensors for the regulation of rate-responsive systems are described. Sensor sensitivity, specificity, reliability, and physiologic sensitivity are defined and discussed with the single rate-responsive system in clinical or experimental use.
Paolo Rossi (1987) studied this question.