AV Delay Hysteresis AR Interval PR Interval

Figure 6.19. Schematic diagram of one manufacturer's differential AVI, designated "AV delay hysteresis." (Modified from Chorus II Model 6234, 6244 Dual Chamber Pulse Generator Physician's Manual. Minnetonka, Minnesota: ELA Medical, 1994.)

One DDD pacemaker automatically calculates the AVI differential between paced and sensed atrial events.23 When an atrial paced event occurs, the AR interval is measured. When an atrial sensed event follows an atrial paced event, a new PR interval is measured. The AV delay hysteresis is set equal to the maximum value (AR or PR) minus the PR interval (Fig. 6.19).

Most dual-chamber pacemakers allow the paced and sensed AV delays to be programmed independently. Although the two may be nominally different, wider differences up to 100 milliseconds are programmable.

Rate-Variable or Rate-Adaptive Atrioventricular Interval: Most DDD and DDDR pacemakers can shorten the AVI as the atrial rate increases, either by an increase in sinus rate or by a sensor-driven increase in paced rate (Fig. 6.20). Rate-adaptive or rate-variable AVI is intended to optimize cardiac output by mimicking the normal physiologic decrease in the PR interval that occurs in the normal heart as the atrial rate increases.24-26 The rate-related shortening of the AVI may also improve atrial sensing by shortening the TARP and thereby extending the time for the ASW.

Rate-adaptive AVI may be designed in several ways. The more common method is to allow linear shortening of the AVI from a programmed baseline AVI to a programmed minimum AVI. Another method allows a limited number of stepwise shortenings of the AVI. These steps may or may not be programmable.

Atrioventricular Interval Hysteresis: The term AVI hysteresis has been used variously but most commonly describes alterations in the paced AVI relative to the patient's intrinsic AV conduction. For example, a longer paced AVI is permitted, to allow maintenance of intrinsic AV conduction. However, once the intrinsic PR or AR interval triggers the programmed AVI hysteresis, consistent AV pacing at the programmed AVI occurs. Commonly, AVI hysteresis is programmed by selecting the desired AV delay during pacing with an additional programmable delta. If there is AV pacing, the system periodically extends the AV delay by the programmed delta. If a native R wave is detected within this extended interval, the longer interval remains in place and results in functional single-chamber atrial pacing. However, with the first cycle of AV pacing, which may occur with a transient increase in vagal tone or even intermittent pathologic AV block, the AV delay returns to the programmed value.

Figure 6.20. As heart rate increases, AV delay dynamically adapts to the change in cycle length. (From Hayes DL, Ketelson A, Levine PA, et al. Understanding timing systems of current DDDR pacemakers. Eur JCPE 1993;3:70-86. By permission of Mayo Foundation.)

This programming accomplishes two goals. In a patient with a normal ventricle, normal ventricular activation sequence (narrow QRS), and a normal PR interval, single-chamber atrial pacing provides hemodynamics superior to those of dual-chamber pacing. Ventricular stimulation causes a disordered ventricular activation sequence. However, an AV delay that is too long, as in first-degree AV block, may be hemodynamically deleterious. In this situation, hemodynam-ics may be superior with a shorter AV delay despite the disordered ventricular activation sequence. AVI hysteresis allows for both a longer AV delay when AV nodal conduction is intact and a shorter AVI when conduction is compromised.

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