What is pacemaker hysteresis?

What is pacemaker hysteresis?

It is generally desirable to preserve intrinsic cardiac rhythm if the rate and rhythm are adequate to maintain an acceptable cardiac output. If the heart rate transiently decreases below that to which the pacemaker is set, it may not be necessary for the pacemaker to discharge. This is especially important for patients whose intrinsic cardiac rhythm is normal sinus and who have ventricular pace-makers. Considering the contribution of the atrial “kick” to cardiac output, the patient may have a better cardiac output in a slow sinus rhythm than would be created by single-chamber pacing. The additional delay that the pace-maker allows before initiating discharge after an intrinsic beat is known as hysteresis.

For example, if the pacemaker is set to pace at a rate of 60 beats per minute, then the pacing interval is 1,000 msec (1 second) between each paced beat. If we wish to program the pacemaker to wait until the intrinsic heart rate is less than 50 per minute (1,200 msec between beats) before firing, then the hysteresis is considered to be 200 msec. Once 1,200 msec have elapsed since the last intrinsic beat, the pacemaker will begin pacing every 1,000 msec. For the intrinsic rhythm to inhibit the pacemaker, an intrinsic beat will have to occur within the 1,000 msec interval following the last paced beat.