Page 706 - Cardiac Nursing

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682 PA R T I V / Pathophysiology and Management Disease
Search Begins Return to Programmed
AV Delay
31 32 1 2 2
200 ms 400 ms 350 ms 400 ms 200 ms
AV Delay AV Delay PR Interval AV Delay AV Delay
■ Figure 28-32 Diagram of AV Search Hysteresis function. After 32 beats the AV delay extends to 400 mil-
liseconds to encourage intrinsic conduction. On beat 2 intrinsic conduction occurs with a PR interval of 350
milliseconds; this will continue until intrinsic conduction fails. At the end of the strip, on beat 1 intrinsic con-
duction failed and a ventricular paced beat occurred at an AV delay of 400 milliseconds, returning the device
to the programmed AV delay of 200 milliseconds. After another 32 beats the search will begin again. (From
Boston Scientific brochure for Altrura Pacemaker, with permission.)
imal effect on HF and was associated with an increased risk of death to occur rather than forcing RV pacing. 45,46 The device can be
and HF when compared to VVI pacing as a backup pacing mode in programmed to extend the AV delay every x cycles (x is a pro-
ICD patients. 40,44 A high percentage of ventricular pacing from the grammable number from 32 to 1024) to look for intrinsic AV
RV apex is thought to play a large role in the progression of HF by conduction in the next eight cycles. If intrinsic conduction occurs
creating ventricular dysynchrony, and has been associated with in- during the search, the AV delay remains at the extended value and
creased risk of HF hospitalization and atrial fibrillation. 38,42,43 allows intrinsic conduction to continue until intrinsic conduction
Although dual-chamber pacing maintains AV synchrony, which fails; then the device returns to its programmed AV delay until the
should improve ventricular function, it can also result in a high per- next search begins. Figure 28-32 illustrates AVSH.
centage of ventricular pacing because it paces the ventricle at the end
of the AV delay in the absence of intrinsic AV conduction. Managed Ventricular Pacing. MVP is a pacing mode in
In the last few years, the recognition of the deleterious effects some dual-chamber pacemakers that paces the atrium only (AAI/R
47
of RV pacing has led to efforts to reduce unnecessary RV pacing, mode) while monitoring the ventricle for intrinsic conduction. If
including new pacing modes and functions to prevent RV pacing intrinsic conduction fails (i.e. an atrial paced or sensed event that
whenever possible. Some of these features to reduce ventricular is not followed by an intrinsic QRS), a ventricular back-up pace
pacing include AV search hysteresis (AVSH [Boston Scientific]), occurs and AAI/R pacing continues. If loss of intrinsic conduction
managed ventricular pacing (MVP [Medtronic]), and ventricular persists, the device switches from AAI/R to DDD/R for 1 minute.
intrinsic preference (VIP [St. Jude]). To test for return of intrinsic AV conduction, a ventricular pace is
inhibited for one cycle; the conduction test is repeated at progres-
AV Search Hysteresis. AVSH is a feature of some dual- sive time intervals (e.g. 1, 2, 4, 8, . . . minutes). If intrinsic conduction
chamber pacemakers and ICDs that searches for the presence of returns, the device switches back to AAI/R pacing with ventricular
intrinsic AV conduction by extending the AV delay by a pro- monitoring. Figure 28-33 is an example of MVP mode operation.
grammable percentage in an effort to allow intrinsic conduction VIP is a similar function in St. Jude devices.
AAI pacing with long AV delay Intrinsic conduction failure Back-up V pace
Back-up V pace Switch to DDD pacing

■ Figure 28-33 MVP function. In the top strip the pacemaker is functioning in the AAI/R mode with a very
long AV delay to allow intrinsic conduction with an A–R interval of 400 milliseconds. The sixth atrial pacing
spike fails to conduct, triggering a backup ventricular pace at the end of the top strip. In the bottom strip, an-
other backup ventricular pace occurs and the device switches to DDD pacing with a programmed AV delay of
160 milliseconds. DDD pacing will continue for a programmable number of minutes and then the AV delay
will extend again to encourage intrinsic conduction. As long as intrinsic conduction is present the device will
operate in AAI/R mode. The star beat appears to be a PVC that occurred right after the atrial pacing spike.