Color Atlas and Synopsis of Electrophysiology, 1st Ed.


Malini Madhavan, MBBS, Paul A. Friedman, MD, FACC, FHRS


A 68-year-old man with ischemic cardiomyopathy, ejection fraction of 35%, and intermittent AV block underwent implantation of a Medtronic dual chamber ICD. The device was programmed DDDR 60-120 bpm, sensed AV delay of 150 ms, and paced AV delay of 120 ms. The patient presented after 1 year with worsening heart failure symptoms. Device interrogation revealed that 80% of ventricular events were paced. The patient’s intrinsic rhythm was sinus rhythm with first degree AV block. Figure 69-1 was obtained during device interrogation.


FIGURE 69-1 Thoracic impedance monitoring for heart failure monitoring.


Several manufacturer-specific algorithms exist to monitor for impending heart failure. Figure 69-1 shows the OptiVol fluid status monitoring algorithm available in Medtronic devices. The daily thoracic impedance trend is monitored and plotted at the bottom. The presence of pulmonary edema will decrease the daily thoracic impedance. The OptiVol fluid index is the difference between the daily impedance and the patient’s reference impedance, which is an internal reference that is updated on a continuous basis. This index will increase as risk of incipient overt heart failure increases. Figure 69-1 shows an upward trend in the OptiVol index for a few weeks prior to presentation consistent with the clinical impression of worsening heart failure. Thoracic impedance drop can also be due to fluid in the pocket or lead revision and pleural effusion. Hence, clinical correlation is essential in interpreting heart failure monitoring tools. In the absence of other precipitants of heart failure, clinical worsening in our patient was suspected to be due to high percentage of RV apical pacing. RV pacing avoidance algorithm, managed ventricular pacing, was enabled. Subsequently, the rate of RV pacing declined to 10%, and his ejection fraction improved to preimplantation levels.


The premise behind programming to minimize ventricular pacing is the finding that frequent right ventricular pacing may induce ventricular dyssynchrony and left ventricular dysfunction. The Dual Chamber and VVI Implantable Defibrillator (DAVID) trial randomized patients with LV dysfunction undergoing implantation of an implantable cardioverter defibrillator (ICD) to either DDD pacing at 70 bpm or backup VVI pacing at 40 bpm.1 The trial noted a significant increase in death or hospitalization for heart failure in patients with DDD-70 compared to VVI-40. This was attributed to a significant reduction in the percentage of ventricular pacing in patients programmed to VVI-40 (4% versus 78%). The risk for heart failure increases with the percentage of RV pacing, with significant risk associated with >40% pacing. Post-hoc analysis of the Multicenter Automatic Defibrillator II (MADIT II) trial showed increased incidence of ventricular tachycardia with greater percentage of RV pacing.2 Hence several algorithms and programming strategies have been developed to promote intrinsic ventricular rhythm and minimize RV pacing.


Several programming strategies can be employed to avoid unnecessary RV pacing. Atrial only pacemaker (AAI) can be considered in patients with sinus node dysfunction and normal AV node. This is generally not preferred in patients with a history or risk of dysfunction of AV conduction.

Programming a low ventricular “back-up” pacing rate (such as 40 bpm) is recommended in patients with single chamber ICD who do not require antibradycardia therapy. This is supported by the DAVID trial described previously. Moreover, trials such as INTRINSIC RV and MVP trial did not establish superiority of advanced ventricular avoidance algorithms such as AV search hysteresis and MVP respectively over VVI back-up pacing in this population.3,4

Patients with ICDs and a traditional indication for pacing often receive a dual chamber device. Programming a long “fixed” AV delay can encourage intrinsic conduction in these patients. However, this can result in unfavorable hemodynamic effects and “upper rate behavior” and is generally not preferred. This has led to the introduction of several algorithms to minimize RV pacing, many of which are manufacturer specific. We provide a brief general description of these algorithms.

AV Search Hysteresis

AV hysteresis avoids the disadvantages of a long fixed AV delay by allowing the AV delay to vary. If a ventricular-sensed event occurs, the device extends the AV delay to a programmable long interval to promote intrinsic conduction. If there is failure to conduct with a ventricular-paced event for a programmed number of cycles, the device switches back to the shorter AV interval. The device then performs periodic search for intrinsic conduction by prolonging the AV delay for a programmed number of intervals. Figure 69-2 shows operation of the AV search+ algorithm (Boston Scientific).


FIGURE 69-2 AV search hysteresis (AV Search+, Boston Scientific) in a 65-year-old with a dual chamber ICD and intermittent AV block. (A) Dual chamber pacing with a short AV interval of 120 ms is noted. The algorithm then extends the AV interval to “search” for intrinsic AV conduction (*). Since a sensed ventricular event occurs, the longer AV interval persists allowing intrinsic conduction until in (B) two consecutive P waves fail to conduct within a programmed interval of 400 ms (arrows). Following this, the short programmed AV interval takes effect. (C) The algorithm extends the AV interval again for a programmed number of eight beats. Since no intrinsic conduction is seen during this search the device reverts to a short AV delay. (Used with permission of Boston Scientific.)

AAI ←→ DDD Mode Switch

The dual chamber device operates in the functional AAI mode with active ventricular surveillance when there is intrinsic conduction. If AV block develops for a programmed number of atrial events, the device switches to DDD mode with the programmed AV delay. The device then periodically searches for intrinsic conduction using either the AV search hysteresis (RHYTHMIQ, Boston Scientific) or by mode switching to AAI (MVP, Medtronic). Figure 69-3 shows operation of the managed ventricular pacing (MVP) algorithm (Medtronic).


FIGURE 69-3 Operation of managed ventricular pacing (Medtronic) in a 68-year-old man with a dual chamber pacemaker implanted for intermittent AV block. The rhythm was observed on telemetry 1 day following the implantation. The device is programmed with lower rate limit of 70 bpm, upper rate limit of 110 bpm, and AV interval of 180 ms. There are two atrial paced beats (AP) that are not followed by a ventricular complex. This demonstrates functioning of managed ventricular pacing (MVP), an algorithm used by Medtronic devices to minimize ventricular pacing and does not represent device malfunction. The device is functioning in the AAI mode at the beginning of the tracing with “ventricular surveillance” (section 1). This allows intrinsic conduction of the first two atrial events with a long AV interval of 260 ms. The third atrial paced beat fails to conduct to the ventricle which triggers ventricular pacing with the next cycle at a short nonphysiologic AV delay of 80 ms. If a programmed number of atrial events fail to conduct to the ventricle as in section 2, the mode switches to DDD (section 3) resulting in ventricular pacing with the programmed AV delay. The device then checks periodically for intrinsic conduction. (Reproduced with permission from Hayes DL, Asirvatham SJ, Friedman PA. Cardiac Pacing, Defibrillation and Resynchronization: A Clinical Approach. 3rd ed. Wiley- Blackwell; 2012.)

Proarrhythmia Due to Ventricular Pacing Avoidance Algorithms

While ventricular pacing avoidance algorithms are generally effective and safe in the majority of patients, proarrhythmia has been occasionally reported. We present a few examples.

Pacemaker-Mediated Tachycardia Due to AV Hysteresis

Figure 69-4 shows the AV hysteresis algorithm, ventricular intrinsic preference (VIP) operational in a St. Jude dual chamber pacemaker. The patient is initially paced in the atrium and ventricle sequentially with a short AV interval of 170 ms. The AV hysteresis algorithm then extends the AV interval to 290 ms to search for intrinsic conduction. Since a sensed ventricular event does not occur, the ventricle is paced when the long AV interval expires. The lack of antegrade AV node conduction and the long AV delay promote retrograde VA conduction. If the retrograde atrial event is sensed outside the postventricular atrial refractory period, an episode of pacemaker mediated tachycardia is initiated.


FIGURE 69-4 Initiation of pacemaker-mediated tachycardia due to AV hysteresis in dual chamber pacemaker.

Polymorphic Ventricular Tachycardia Due to AAI ←→ DDD Mode Switch

The AAI ←→ DDD mode switch algorithms permit single nonconducted P waves in contrast to standard dual chamber operation. A second nonconducted P wave is not allowed. Hence, following a nonconducted P wave, the device will pace the ventricle for one cycle in the DDD mode. As demonstrated in Figure 69-5, this results in a short-long-short coupling sequence, which in the vulnerable patient can lead to polymorphic VT.5 This is a rare phenomenon.


FIGURE 69-5 Polymorphic ventricular tachycardia initiated by short-long-short sequence due to operation of the managed ventricular pacing (MVP) algorithm in a dual chamber ICD. (Reproduced with permission from Hayes DL, Asirvatham SJ, Friedman PA. Cardiac Pacing, Defibrillation And Resynchronization: A Clinical Approach. 3rd ed. Wiley- Blackwell; 2012.)


High percentage of RV apical pacing can lead to ventricular dyssynchrony and increased incidence of heart failure. Several strategies are available to minimize RV pacing. The selection of device and AP programming should be tailored to the individual patient.


1. Wilkoff BL, Cook JR, Epstein AE, et al. Dual-chamber pacing or ventricular backup pacing in patients with an implantable defibrillator: the Dual Chamber and VVI Implantable Defibrillator (david) trial.JAMA. 2002;288(24):3115-3123.

2. Steinberg JS, Fischer A, Wang P, et al. The clinical implications of cumulative right ventricular pacing in the multicenter automatic defibrillator trial II. J Cardiovasc Electrophysiol. 2005;16(4):359-365.

3. Olshansky B, Day JD, Moore S, et al. Is dual-chamber programming inferior to single-chamber programming in an implantable cardioverter-defibrillator? Results of the INTRINSIC RV (Inhibition of Unnecessary RV Pacing With AVSH in ICDs) study. Circulation. 2007;115(1):9-16.

4. Sweeney MO, Ellenbogen KA, Tang AS, et al. Atrial pacing or ventricular backup-only pacing in implantable cardioverter-defibrillator patients. Heart Rhythm. 2010;7(11):1552-1560.

5. Vavasis C, Slotwiner DJ, Goldner BG, et al. Frequent recurrent polymorphic ventricular tachycardia during sleep due to managed ventricular pacing. Pacing Clin Electrophysiol. 2010;33(5):641-644.