Clinical applications of the threshold tracking pacemaker.
The concept of automatic threshold tracking is the most recent adaptation of artificial cardiac pacemakers to physiologic needs. Earlier pacemaker innovations were primarily in the area of timing of impulse delivery, whereas more recent changes have focused on alteration of the amplitude and duration of the pacemaker stimulus. Matching of this stimulus to the cardiac excitation threshold is important because a stimulus greater than necessary is wasteful of battery energy and may produce arrhythmias, whereas a stimulus of less than threshold intensity fails to pace the heart. Because of the wide range of clinically encountered threshold levels it is impossible to design a fixed output pacer that is efficient and safe for all patients at all times. The threshold tracking pacemaker searches for and finds the threshold level, and adjusts its stimulus to just above this level. The device ensures an adequate safety margin of stimulation, but reduces the stimulus level to the lowest safe level possible. It is fully automatic, adjusts immediately to any threshold changes and is not dependent on manual alteration.