Automatic refractory period.

Pacemakers have become more and more complex over the years, whereby technological advances were used to create more and new possibilities in existing pacemaker applications. As a result a demand has grown for more automatic functions to reduce the time to program the pacemakers and eleviate follow-up procedures. One of the parameters which can he adjusted automatically is the refractory period. We measured the pacing threshold as a function of coupling interval between the stimulus and the previous paced complex and related this to the morphology of the paced evoked T-wave. When the stimulus fails far away from the T-wave the pacing threshold is constant. When it approaches the T-wave the threshold reduces slightly (superconductivity) and at the peak of the T-wave there is a sharp increase of the pacing threshold. The refractory period coincides with the period from the stimulus until the top of the T-wave (stim-T interval). This result is used in the Rhythmyx pacemaker where the stim-T interval is used to automatically adjust the pace refractory period. Automatically adjusting the refractory period has the extra advantage of higher allowable pacing rates as the refractory period shortens with increasing heart rates. Future applications will include use antitachycardia pacing where the stim-T interval can be used to prevent pacing in the refractory period or in the vulnerable zone.