Hysteresis in a rate-responsive pacemaker
A rate-responsive pacemaker includes programmable hysteresis means for automatically extending an escape interval, EI.sub.0, in the presence of sensed intrinsic cardiac activity, and returning the escape interval to its initial value in the presence of pacemaker-stimulated (paced) cardiac activity. The escape interval sets the rate at which stimulation pulses are generated on demand in the absence of sensed intrinsic cardiac activity. The initial value of the escape interval is selected to be the lessor of: (a) a programmed escape interval (determined from a minimum programmed rate), or (b) a sensor-indicated escape interval (determined from a physiological or metabolic sensor used as part of the rate-responsive pacemaker). In addition to the hysteresis mode, a scan mode is optionally provided wherein the escape interval, EI.sub.0, is gradually extended (lengthened) in small incremental steps if no intrinsic activity is sensed during the prior escape interval. In this manner, the pacemaker incrementally "scans" through an increasingly longer window of time looking for the occurrence of intrinsic cardiac activity before generating a stimulation pulse. A hysteresis rate adjust (HRA) mode is also optionally provided wherein the escape interval is gradually returned to EI.sub.0 over several pacing cycles, rather than abruptly returning to EI.sub.0 in a single step after the hysteresis mode has been ended due to no sensed intrinsic activity during the hysteresis escape interval EI.sub.H.
Siemens Pacesetter, Inc.
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Raymond A. James L.
Kristall Duncan