The Atrioventricular Interval

Recent data35 suggesting that right ventricular pacing may worsen symptoms of congestive heart failure in those with underlying cardiomyopathy has re-focused attention on appropriate programming of the AV interval. Certainly, in those with diminished LV function, one should make attempts to prolong the AV delay sufficiently to minimize RV pacing. This can be accomplished with programming a long fixed AV delay or programming "on" an AV search hysteresis feature which is being increasingly incorporated into new devices. With this feature, a backup AV interval is set as well as a further delay which is added to the programmed value allowing additional time to encourage native conduction. If no intrinsic R wave has been sensed by the end of the summed interval, the programmed "physiologic" AV delay will become active but with periodic searches for native conduction. Programming the device to DDI(R) mode is sometimes worthwhile as well. In contrast, patients with normal LV function are rarely able to distinguish between native conduction and P wave synchronous pacing. Nevertheless, the above techniques to minimize ventricular pacing may still be worthwhile if only to maximize battery duration. However, there is clearly an AV delay sufficiently long that the benefits of

Table 11.4. General Guidelines for Programming Common Pacemaker Parameters

Parameter Situation Chronic Setting Comments

Lower rate limit

Upper rate limit

General

Minimal pacing desired Heart failure

General

Children/athletes

Coronary artery disease/angina

Pacing output Fixed voltage

Fixed pulse width Atrium

Ventricle

Sensitivity

AV delay

AV block

Intrinsic AV

conduction (no CHF)

Intrinsic AV conduction (CHF) Hypertrophic cardiomyopathy

50-70 bpm 40-60 bpm

70-90 bpm

85% maximal predicted heart rate; [(220~ age) X 0.85] bpm (220~ age) bpm

110-120 bpm

3-4X pulse width threshold

2—3X voltage threshold

25-50% of threshold value

25-50% of threshold value

150-180 msec paced AV delay, sensed AV delay 25-50 msec < paced AV delay

Up to 220 msec

Often set even longer AV delays

Use rate hysteresis

Benefit of high rate pacing not proven

Based on average levels of activity

May require programming short refractory periods

Approximates peak heart rates on maximal beta blockade

Minimizing voltage output is most efficient

Use autothreshold functions

Need <1mV setting for mode switching

Evaluate oversensing in unipolar systems

Turn on rate adaptive AV delay in active patients

Longer AV delays may compromise hemodynamics, use AV interval hysteresis to promote intrinsic conduction Pacing induced desynchrony of very long AV delay

Approximately 100msec Optimize by Doppler

Use negative hysteresis to "force" ventricular pacing inhibiting ventricular pacing are outweighed by the adverse hemodynamic effects which can result with marked first degree block. Much of this detrimental effect may be linked to atrial contraction occurring during or immediately after ventricular systole creating a "pseudo-pacemaker syndrome." Certain studies have demonstrated that this detrimental effect is seen with PR intervals of greater than 220 milliseconds.36 In occasional patients who continue to complain of breathlessness after a pacemaker is implanted, an attempt at AV optimization using Doppler echocardiography or impedance cardiography may be warranted. Both AV as well as LV/RV optimization is a subject of particular interest in those with bi-ventricular pacemakers. Traditional mitral inflow and aortic outflow Doppler measurements as well as newer tissue Doppler techniques are being investigated.37,38

In contrast to the previous situations, there are also situations in which ventricular pacing is desired. This is particularly true when dual-chamber pacemakers are placed for symptom relief in those with hypertrophic obstructive cardiomyopathy. Programming a short fixed AV delay is one option although more sophisticated algorithms such as negative AV interval hysteresis can be quite effective in allowing for the longest AV interval that still provides 100% ventricular pacing. Patients with biventricular pacemakers represent another group where continuous ventricular pacing is desired. This is often difficult when these patients develop rapid atrial fibrillation. New algorithms are being developed to assist in this situation. One such algorithm automatically increases the ventricular pacing rate after every sensed ventricular event. Another provides a "triggered" mode where each sensed ventricular beat immediately results in LV pacing resulting in a fused QRS complex which is more synchronized than the intrinsic beat would be alone. Finally, in patients with a "traditional" dual-chamber pacemaker, underlying cardiomyopathy and a baseline right bundle branch block, biventricular pacing can often be simulated. To accomplish this, one carefully programs the AV delay so as to create fusion between the RV paced complex and the native QRS (which, due to the RBBB, is activating the left ventricle first) and thus, create some measure of re-synchrony.

A differential AV delay is now incorporated into virtually all modern pacemakers. This allows for programming of a shorter AV delay for the sensed AV interval (SAV) compared with the paced AV interval (PAV). This accounts for the fact that the pacemaker does not recognize a sensed atrial event (and start the SAV timer) until atrial depolarization is well under way. The differential AV delay thus keeps the time between atrial and ventricular contraction constant regardless of the presence of atrial pacing. A differential value of 25 to 40 milliseconds is considered appropriate. Most pacemakers also now have a rate responsive AV delay feature. This provides for a shortening of the AV interval during exercise mimicking the normal positive dromotropic effect seen in those with normal AV conduction. This feature provides a hemodynamic benefit as well as allow for the programming of a higher maximum tracking rate. This feature should ordinarily be programmed "on" in those with AV conduction disturbances.

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