Device therapy for atrial fibrillation

Cardiol Clin 22 (2004) 71–86

Device therapy for atrial fibrillation Lai Chow Kok, MB, BSa,b,*, Kenneth A. Ellenbogen, MDc a

Division of Cardiology, Department of Internal Medicine, Medical College of Virginia/Virginia Commonwealth University, 1101 East Marshal Street, Richmond, VA 23298, USA b Cardiology Section, Hunter Holmes McGuire Veterans Affairs Medical Center, 111J-3 McGuire VA Medical Center, 1201 Broad Rock Boulevard, Richmond, VA 23249, USA c Cardiac Electrophysiology, Division of Cardiology, Department of Internal Medicine, Medical College of Virginia/Virginia Commonwealth University, 1101 East Marshal Street, Richmond, VA 23298, USA

The pharmacological treatment of atrial fibrillation (AF) directed toward restoration and maintenance of sinus rhythm has been limited by the low efficacy and adverse effect profiles of antiarrhythmic drugs [1,2]. New insights into the underlying pathophysiology of AF have fueled enthusiasm for developing potentially curative nonpharmacological therapies. Cardiac pacing is one such therapy. Pacemakers traditionally have been used to treat or prevent symptomatic bradyarrhythmias, but rapid advances in the understanding of AF and new developments in computer technology over the last decade have provided an opportunity to use pacemakers as therapeutic tools for the treatment of AF in patients. The role of pacemakers in AF therapy has evolved from conventional bradycardia pacing in sick sinus syndrome (SSS) patients or following atrioventricular (AV) junction ablation to atrial pacing for AF prevention and antitachycardia pacing for AF termination. The ultimate goals of AF therapy are to eliminate the consequences of AF, which include prevention of thromboembolic events, prevention of tachycardia-induced cardiomyopathy, and relief of symptoms. In light of this, this article reviews the role of pacing on these end points whenever

* Corresponding author. Division of Cardiology, Department of Internal Medicine, Medical College of Virginia/Virginia Commonwealth University, 1101 East Marshal Street, Richmond, VA 23298. E-mail address: [email protected] (L.C. Kok). 0733-8651/04/$ - see front matter. Published by Elsevier Inc. doi:10.1016/S0733-8651(03)00113-9

possible. The role of the stand-alone atrial defibrillator in AF therapy is still in a state of infancy and will not be discussed. Atrial pacing to prevent atrial fibrillation Various experimental studies have shown that AF is caused by the presence of either multiple reentrant wavelets or a dominant single re-entrant wavelet [3]. For AF to be established, it requires the appropriate triggers (premature atrial complex [PAC]) for initiation and appropriate atrial substrate (nonhomogeneous refractoriness) for maintenance of re-entrant wavelets. Preventive pacing algorithms have been designed to suppress AF triggers, and new pacing lead designs may compensate for underlying atrial substrate by allowing pacing from multiple sites or alternative sites in the atria. Suppression of triggers The exact role of PACs in AF initiation is a matter of debate, although several investigators have observed that PACs triggering AF tend to have shorter coupling intervals [4,5]. PAC suppression can be achieved by permanently elevating the lower pacing rate or by dynamic overdrive pacing. The results of fixed rate atrial pacing on AF prevention have been mixed. In a small trial, AF was eliminated in 14 of 22 patients over a 30-day period when the atrial pacing rate was programmed to 10 beats per minute (bpm) faster than the mean heart rate [6]. In another study, overdrive pacing

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did not offer any advantage for AF suppression over rate responsive pacing, with a base rate of 60 bpm in SSS patients, even though a significant increase in total duration of atrial pacing was achieved with the overdrive pacing algorithm [7]. The Atrial Pacing Peri-Ablation for Paroxysmal Atrial Fibrillation (PA3) investigators reported similar outcome in a series of 97 drugrefractory AF patients who were randomized to DDI with a lower rate of 30 bpm (DDI 30) or DDIR with lower rate of 70 bpm (DDIR 70) before planned AV junction ablation [8]. Patients in this study had at least three episodes of symptomatic AF in a year, were refractory or intolerant to medical therapy, and were being considered for AV junction ablation. Diagnostic data were collected after a 2-week stabilization period to allow for lead maturation and antiarrhythmic therapy stabilization. After 3 months of atrial pacing, the atrium was found to be paced 67%  31% of time in patients randomized to DDIR 70 compared with no atrial pacing in patients with DDI 30. Although atrial pacing significantly reduced the burden of PACs from baseline (3.8/hour to 0.5/hour, P<0.01), it had no effect on AF prevention. The time to first episode of AF recurrence was identical between atrial pacing and no pacing (1.9 days, 95% confidence interval [CI], 0.8 to 4.6 versus 4.2 days; 95% CI, 1.8 to 9.5, P = NS [not significant]). In fact, pacing may be deleterious, as a lower AF burden was observed in patients randomized to no pacing (0.24 hours/day versus 0.67 hours/day, P = 0.08). Eleven patients in this study who crossed over to the pacing arm were found to have earlier AF recurrence after pacing, compared with no pacing, and the total AF burden was significantly greater during atrial pacing. Sixty-seven patients who subsequently underwent AV junction ablation participated in the second phase of the PA3 trial [9]. Following ablation, they were randomized to DDDR pacing or VDD pacing in a crossover study design with 6 months of pacing in each arm. The trial was designed to study the potential benefits of atrial pacing over AV synchronization in AF prevention. The outcome of the second phase was identical to the first phase. There was no difference in the time to first AF episode between DDDR pacing and VDD pacing (0.37 days versus 0.5 days, P = NS). The AF burden increased over time in both groups, with 6.93 hours/day of AF reported in the DDDR group and 6.30 hours/day reported in the VDD group at the 6-month follow-up. At the end of 1 year, 43% of patients

developed permanent AF. Atrial based pacing did not prevent progression to chronic AF in this group of symptomatic paroxysmal AF patients. Dynamic atrial overdrive pacing achieves a similar effect of ensuring constant atrial capture without having to program an elevated lower base rate. It allows for preservation of a normal circadian heart rate pattern. The pacing rate is increased incrementally upon detection of intrinsic atrial events until no further atrial sensed events occur. Pacing is maintained for a programmable duration before it is decreased again in steps until the intrinsic rate appears or lower pacing rate or sensor rate is reached. Again, sensing of atrial events will result in an increase of atrial pacing rate. There are variations with each device manufacturer regarding the specific details of dynamic overdrive pacing algorithms (Fig. 1). The Atrial Preference Pacing algorithm (Medtronic, Minneapolis, Minnesota) monitors the P–P interval and paces 30 milliseconds shorter, while the Dynamic Atrial Overdrive algorithm (St. Jude Medical, Sylmar, California) increases atrial paced rate for a programmable duration of time based on detection of intrinsic atrial activity. The overall mean heart rate was not increased significantly while using these algorithms, and more importantly, the algorithms were tolerated well by patients [10,11]. In a short-term follow-up, dynamic overdrive pacing did not have any impact on the overall incidence of AF in 31 patients with paroxysmal AF [12]. In a small series of 15 patients with SSS, this algorithm (DDDR plus consistent atrial pacing) was shown to reduce the frequency of PACs significantly, but it did not alter the number of mode-switching episodes (47  90 versus 42  87, DDDR versus DDDR plus consistent atrial pacing, P > 0.05) [11]. Similar reduction of PACs was demonstrated in a larger series of 61 patients, but this had no effect on reduction of symptomatic paroxysmal AF episodes [13]. The dynamic atrial overdrive algorithm was tested further in the ADOPT-A trial, which enrolled 399 patients with conventional bradycardia pacing indications and implanted DDDR pacemakers, with the dynamic atrial overdrive algorithm programmed on or off. Based on an intention-to-treat analysis, the AF burden was reduced by 25% in patients with the algorithm enabled. There was also a remodeling effect of pacing seen in both groups, with a progressive reduction in episodes of AF at 1, 3 and 6 months of follow-up. These studies present

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Fig. 1. Description of different dynamic atrial pacing algorithms of major US device manufacturers.

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a mixed picture. Most studies show that suppression of PACs is possible with the newer pacing algorithms, but only few studies have shown a clinically important or significant benefit in terms of reduction in overall AF incidence or AF burden. Effect on atrial substrate suppression of compensatory pause Premature atrial complexes also may initiate AF by altering the underlying atrial substrate to support re-entry. The short–long cycles caused by postextrasystolic pauses following PACs may increase dispersion of atrial refractoriness making it conducive to the maintenance of re-entrant wavelets [14]. Pacing algorithms, which respond to spontaneous PACs, have been designed to maintain homogeneous refractoriness. The pacing response to a PAC can be a single paced beat, a series of paced beats coupled with autoadaptive decay, or pacing rate acceleration for a programmable duration. A pacing algorithm using a series of paced beats in response to PACs was studied in 70 patients with a history of frequent atrial ectopics [15]. The algorithm was tested in a cross-over study design involving alternating 2 hours of programming the algorithm on and off for a 24hour period. This pacing algorithm had no effect on the overall PAC burden (by pacemaker counters or Holter analysis) or overall AF episodes. Only patients with frequent AF (>5 episodes in 24 hours) experienced a lower AF burden with this pacing algorithm. Furthermore, this algorithm was inactivated frequently by salvos of PACs in over 75% of episodes. This would suggest that while the algorithm may be effective for suppression of PACs, it is not able to prevent short bursts of PACs that are more likely to induce AF. Pacemakers that incorporate separate algorithms for PAC suppression and for maintaining homogenous atrial refractoriness have been designed. The Atrial Therapy Efficacy and Safety Trial (ATTEST) evaluated the role of AT500 (Medtronic, Minneapolis, Minnesota) pacemakers that incorporated three different pacing algorithms (Atrial Preference Pacing, Atrial Rate Stabilization and Post Mode Switching Overdrive) in AF prevention [16]. The study enrolled 368 patients with standard bradycardia indications and history of AF to receive the AT500 pacemakers, and patients were randomized to algorithms programmed on or off. A history of

paroxysmal AF was present in 78% of the study population. At the end of 3 months, there was no difference observed between the two groups with regards to median atrial tachyarrhythmias (AT/ AF) burden (4.2 hours/month on versus 1.1 hour/ month off, P = 0.20) and frequency of AT/AF (1.3 episodes/month on versus 1.2 episodes/month off, P = 0.65). This study suggests that preventive pacing has minimal short-term impact on the natural history of AF. Reducing dispersion of refractoriness—multi-site pacing Patients with paroxysmal AF can have profound intra- and interatrial conduction delay as evidenced by broader P waves on surface ECG [17,18]. This conduction delay increases dispersion of atrial refractoriness, favoring induction of AF [19,20]. Pacing may correct the conduction delay, and this can be achieved by simultaneous pacing from more than one site (multi-site or dual-site) to allow synchronization of left and right atrial activation. Dual-site pacing has been shown to be effective in preventing AF by reducing the conduction delay [17]. D’Allones et al demonstrated significant reduction of the P-wave duration (from 187  29 milliseconds to 106  14 milliseconds, P = 0.0001) by pacing simultaneously from high right atrial and distal coronary sinus in patients with P-wave duration >120 milliseconds [21]. They reported a 64% success rate of maintenance of sinus rhythm in 86 patients with intra-atrial delay after a mean follow-up of 33 months. An initial concern of dual-site pacing was raised with reports of a high dislodgement rate of the coronary sinus lead, but this issue has been resolved with the development of a specifically designed lead for the coronary sinus (Medtronic SP 2188, Medtronic, Minneapolis, Minnesota) [22]. Saksena et al proposed a different method of reducing total atrial activation time by placement of a screw-in pacing lead near the coronary sinus os instead of in the distal coronary sinus. An advantage of this method is ease of extraction should this become necessary later. The initial experience reported by these investigators appeared promising [23]. They showed that dual-site pacing significantly increased the duration of the AF-free interval in a series of 15 patients with AF and bradycardia, although no difference in AF-free interval was observed between single-site compared with dualsite pacing. Pacing therapy, regardless of the

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number of sites, also reduced the number of reported symptomatic episodes and the number of antiarrhythmic drugs compared with before pacemaker implantation. A subsequent report on 30 patients with longer follow-up demonstrated better AF control with pacing regardless of pacing site (high right atrium versus coronary sinus os) [24]. To provide a more definitive answer, this same group of investigators initiated a multi-center study to examine the effectiveness of dual-site pacing therapy in AF prevention [25]. The study randomized 118 patients with symptomatic AF and bradycardia to one of three pacing modes (overdrive high right atrial pacing, dual-site atrial pacing, or support pacing at lower rate of 50 bpm or in VDI pacing mode) using a cross-over trial design. The study found no significant difference in the time between dual-site and single-site pacing (relative risk [RR] 0.835, P = 0.175) for freedom from symptomatic AF during a mean follow-up of 12.1 months ( 6.9 months). In patients who developed recurrent AF, the median time to AF recurrence was 1.77 months with dual-site pacing, 0.62 months with high right atrial pacing, and 0.44 months with support pacing (P<0.09, dual versus high right atrial; P<0.05, dual versus support; P > 0.7 high right atrial versus support). Patients reported better quality-of-life scores when randomized to overdrive high right atrial pacing or dualsite pacing compared with the support mode, but no difference was observed between the first two pacing modalities. The study did not show any superiority of dual-site pacing over high right atrial pacing with regard to the use of antiarrhythmic drugs or frequency of AF episodes. Dual-site pacing was found to be more effective for suppressing AF when compared with high right atrial pacing in the presence of concomitant class I or III antiarrhythmic agents, but the number of patients in this analysis was too small to arrive at any firm conclusions. Although this study had demonstrated the safety and feasibility of dual-site atrial pacing, it did not provide convincing evidence for the role of multi-site pacing in AF prevention, particularly in AF patients with bradycardia. Paroxysmal AF patients with prolonged P-wave of at least 120 milliseconds may benefit from dualsite pacing, as suggested by a nonrandomized study [26], but this will need further confirmation from larger prospective randomized trials. The role of dual-site pacing in patients with paroxysmal AF without a bradycardia indication for pacemaker also was examined [27]. Lau et al

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randomized 22 patients with recurrent AF on sotalol in a cross-over study design to 12 weeks of either pacing with sotalol or sotalol alone. All patients received pacemakers with the Continuous Atrial Pacing algorithm to ensure a higher percent of atrial pacing. The study showed that dual-site pacing reduced the frequency of PACs and prolonged the time to first symptomatic AF episode but had no effect on frequency of AF. Both arms had the same number of patients with AF recurrence. This result has been confirmed by two other studies of similar size involving the same patient population [28,29]. These studies seem to suggest that dual-site pacing had at best modest efficacy in reducing the overall AF burden, and the complexity of the additional pacing leads may relegate this pacing mode to patients with frequent symptomatic AF who have failed a combination of antiarrhythmic and ablation therapies (Table 1). Atrial pacing following cardiac surgery Although the results of permanent biatrial pacing for AF prevention have been mixed, the results of temporary biatrial pacing for postoperative AF prevention following coronary artery bypass surgery have been more consistent. AF is a common arrhythmia following bypass surgery often associated with prolonged hospitalization and increased hospital cost [30,31]. The strategy of preoperative use of antiarrhythmic drugs for AF prevention is limited to patients undergoing elective cardiac surgery [32,33]. Routine epicardial pacing lead placement following cardiac surgery provides an opportunity to study the role of pacing in prevention of postoperative AF. Two recently published studies have shown that biatrial pacing lowered the incidence of postoperative AF [34,35]. A metaanalysis also showed that biatrial pacing reduced incidence of postoperative AF and shortened hospital stay [36]. Fan et al found that pacing decreased the mean P-wave duration irrespective of the pacing sites, although biatrial pacing showed the greatest reduction in P-wave dispersion compared with left or right atrial pacing (42%  8% versus 13%  6%, biatrial versus left atrial, P<0.05; 42%  8% versus 10%  9%, biatrial versus right atrial, P<0.05) [35]. In their study, there was no difference in the baseline mean P-wave duration or P-wave dispersion between patients who developed postoperative AF and those who did not, but patients who maintained

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Table 1 Clinical trials of dual-site pacing and alternative site pacing for atrial fibrillation suppression Study

N

Study design

End point

Follow-up

Results

Multicenter prospective randomized crossover trial of DAP versus sotalol with 12 weeks for each phase Single center prospective randomized DAP versus RAA pacing crossover study 1 month for each phase

Time to first symptomatic AF

40 weeks

1. Number of PAF episodes 2. Duration of PAF (days) 3. AF burden (total % of time in 1 month) Time to PAF or permanent AF (AF lasting > 3 months)

1 month

DAP versus sotalol 50  30 days vs 15  17 days (p<0.01). No. of patients with AF recurrence with DAP versus sotalol 16 versus 16 (p = ns) RAA versus DAP pacing 1. 77  98 vs 52  78 (p = ns) 2. 4.8  5.4 days vs 6.3  9.8 days (p = ns) 3. 14  16 % vs 19  30 % (p = ns)

NIPP-AF [27]

22

No bradycardia + PAF ($ 2 episodes in 3 months) All patients on sotalol

Levy et al [29]

20

No bradycardia + drug refractory PAF ($ 3 episodes in 1 week)

Leclercq et al [26]

83

Drug refractory PAF ($ 2 episodes in 6 months)

Single-center prospective nonrandomized. DAP (30 patients all with P wave $ 120 months). RAA (53 patients; 21 patients with P-wave$120 months)

D’Allones et al [21]

86

Drug refractory PAF P-wave $ 120 months

Single-center prospective nonrandomized

Sinus rhythm

Mean 33 months (6-109)

Bradycardia + PAF ($ 2 episodes in 3 months) 46 patients on Class I or III

Multi-center prospective randomized crossover DAP versus RAA versus support pacing. 6 months in each phase

Time to first symptomatic AF

12.1  6.9 months

DAPPAF [25]

118

18  15 months

Patients with P-wave $ 120 months: PAF incidence DAP versus RAA, 30% versus 71% (p<0.01) Permanent AF DAP vs. RAA, 3% vs. 38% (p<0.01) Patients with P-wave<120 months; PAF incidence DAP versus RAA 30% versus 28% (p = ns) Permanent AF incidence DAP versus RAA, 3% versus 12% (p = ns) At follow-up, 64% (55 patients) in sinus rhythm, of which 33% (27 patients) with $ 1 AF recurrence. Drug therapy in the 55 patients reduced compared to baseline from 1.7  0.5 drugs/pt to 1.4  0.6 drugs/pt, (p = 0.011) Freedom from symptomatic AF DAP versus RAA, RR = 0.835 (p = 0.175) DAP versus support, RR = 0.715 (p = 0.073) RAA versus support, RR = 0.709 (p = 0.188)

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Patient population

Mirza et al [28]

19

Bailin et al [37]

120

46

Bradycardia + PAF

Bradycardia + PAF ($ 2 episodes of AF in 3 months)

Single-center prospective randomized crossover RAA versus CS versus DAP (70months inter atrial delay) versus DAP (16 months inter atrial), 3 months in each phase Multi-center prospective randomized trial of RAA versus BB pacing Multi-center prospective randomized RAA versus pacing for 6 months, each arm randomized to CAP-off and CAP-on programming

Number of AF episodes

15 months

Time to chronic AF (2 consecutive ECG with AF 2 months apart) 1. Time to first AF (days) 2. Symptomatic AF episodes/ month 3. PAF burden (mid/day)

1 year

6 months

Mean number of AF episodes/ month compared to baseline of 16/month RAA: 10.5 (p = 0.003) CS: 9.3 (p = 0.015) DAP 70: 7.3 (p = 0.0007) DAP 16: 7.2 (p = 0.0007) Freedom from chronic AF at 1 year BB versus RAA pacing, 75% vs 47% (p<0.05) CS versus RAA pacing 1. CAP-on 6.7 vs 6.7, p = ns; CAP=off 9.6 versus 6.8, p = ns 2. CAP-on 0.2  0.5 versus 1.9  3.8, p<0.05; CAP-off 0.2  0.5 versus 2.1  4.2, p<0.05 3. CAP-on 41  72 versus 193  266, p<0.05; CAP-off 47  84 versus 140  217, p<0.05

Abbreviations: BB, Bachmann’s bundle; CAP, consistent atrial pacing; CS, coronary sinus; DAP, dual-site atrial pacing; PAF, paroxysmal atrial fibrillation; Pt, patient; RAA, right atrial appendage; RR, relative risk.

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Padeletti et al [41]

No bradycardia + drug refractory PAF

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sinus rhythm postoperatively were found to have significant reduction in P-wave duration and Pwave dispersion with biatrial pacing compared with their baseline. This observation suggests that failure to reduce dispersion of atrial refractoriness with pacing led to development of postoperative AF. This observation may also explain the less than satisfactory outcome of dual-site atrial pacing studies discussed earlier in non-postcardiac surgery patients. Reducing dispersion of refractoriness—alternative site atrial pacing Another method of shortening the total atrial activation time is by pacing the atrium from selected sites. Generally, it is accepted that routes of preferential interatrial conduction exist, and pacing at these sites will pre-excite the left atrium. Bachmann’s bundle is the largest and most important anatomical interatrial communication and probably accounts for most interatrial conduction. The structure is a band of tissue that extends from the right of the superior vena cava transversally to the anterior wall of the left atrium up to the left atrial appendage. This alternative pacing site had been shown to be effective in AF prevention [37]. All 120 patients in this study had paroxysmal AF, and about a third had undergone AV nodal ablation previously. The incidence of chronic AF at 1 year was lower with Bachmann’s bundle pacing compared with atrial appendage pacing (47% versus 75%, P<0.05), and most patients who progressed to chronic AF did so within the first 4 months following device implantation. Bachmann’s bundle pacing resulted in shorter P-wave duration compared with sinus rhythm or right atrial appendage pacing. Pacing from this site has been shown to reduce P-wave duration to the same extent as dual-site (high right atria and coronary sinus os) pacing [38]. Bachmann’s bundle pacing also has been shown to reduce the window of AF inducibility in an animal model, and this may be another mechanism whereby AF incidence can be reduced [39]. Low interatrial septum, near the triangle of Koch, is another alternative pacing site that has been studied. A recent necropsy study on the human heart has demonstrated consistent myo-

cardial connections between the right and left atrium along the coronary sinus, especially proximally [40]. In a series of 46 patients, low interatrial septum pacing was found to be more effective than atrial appendage pacing in reducing AF burden, although either site reduced the AF frequency when compared with preimplantation frequency [41]. Again, P-wave duration during low interatrial septum pacing was reduced significantly compared with sinus rhythm. These studies suggest that over an intermediate period of time, pacing at these selected sites may prevent progression of AF, and leads positioned at these sites appear to be as stable and safe as the conventional atrial appendage position (see Table 1). Atrial pacing for atrial fibrillation termination Antitachycardia pacing (ATP) is a reliable mode of therapy in arrhythmias with a large excitable gap such as atrial flutter. On the other hand, AF with shorter excitable gap does not lend itself easily to pace termination. Recent animal studies have shown that by timing the delivery of pacing, the success of AF termination can be improved [42]. There are several potential advantages to incorporating ATP into devices for management of AF. Atrial flutter, atrial tachycardia (AT) and AF frequently can coexist in the same patient, and not uncommonly, atrial flutter can degenerate into AF. Delivery of ATP while the patient is in atrial flutter may prevent AF initiation. Alternatively, AF may be converted to atrial flutter with antiarrhythmic agents, and the availability of ATP would allow for termination of atrial flutter (Fig. 2). Atrial ATP is available in some implantable cardioverter defibrillators (ICDs) and pacemakers. The effectiveness of ATP in reducing atrial tachyarrhythmias (AT/AF) was examined in patients who received Medtronic 7250 Jewel AF dual-chamber ICDs. This study enrolled 269 patients receiving ICDs with at least two prior episodes of AF or AT in the preceding year before implant [43]. Patients were randomized to 3-month periods of ATP on or ATP off with cross-over. The ATP algorithms for AT/AF termination in this study were atrial burst + (atrial burst followed by two extra stimu-

c Fig. 2. Demonstration of termination of atrial arrhythmia in a patient with a Guidant Vitality AVT model A135 ICD. The upper tracing is the atrial electrogram; the middle tracing is the ventricular electrogram, and the lower tracing is an electrogram recorded from the tip to the distal coil of the ICD lead. As shown, AF organized to atrial flutter (278 millisecond cycle length) before delivery of atrial therapy and sinus rhythm was restored following burst pacing. Paper speed 25 mm/sec.

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li), atrial ramp (atrial autodecremental ramp) and atrial 50 Hz burst pacing up to 3 seconds. Atrial burst 50 Hz pacing successfully converted 24% of the 121 AF episodes identified in 52 patients. The burden of AT/AF was reduced during periods with therapies on, but the actual reduction of AF burden with ATP could not be determined, as no distinction was made between AF and other atrial tachycardias in the analysis. A larger study by Adler et al involving 537 patients implanted with the same ICDs showed that ATP (50 Hz burst) was successful in converting 30% of 880 AF episodes in 101 patients [44]. The relatively lower success rate of pace termination is attributable to the shorter excitable gap present in AF as evidenced by the indirect relationship between successful outcome and underlying cycle length of the AT/AF. The success of ATP for termination of AT/AF was 29% if the cycle length 190 milliseconds but 65% when cycle length was >320 milliseconds [44]. They also observed that ATP had higher success rate (34% versus 10%, P<0.01) if it was delivered within the first 10 minutes of AF onset. Prompt AF detection and delivery of ATP may prevent electrical remodeling after AF onset and promote sinus rhythm. Similar ATP efficacy of 54% for AT/AF termination was reported in the ATTEST study that uses the same atrial termination algorithms in patients who received pacemakers [16]. No further analysis in outcome of ATP in AF alone was provided in this study, however. An added benefit to incorporating atrial ATP therapy in implanted devices is that AF may increase the risks of spontaneous ventricular tachycardia (VT) or ventricular fibrillation (VF) in ICD patients. Retrospective analysis has shown that 8.6% of all VT/VF episodes were initiated or preceded by an episode of AT or AF (dual tachycardia), and this was present in 20.3% of 537 ICD patients [45]. AT/AF was terminated successfully during ventricular therapy (ATP or shock) for VT/VF in 40% of the dual tachycardia episodes. The median interdetection interval for the subsequent VT/VF was 108 hours when AT/ AF was terminated successfully, compared with 15 minutes if AT/AF persisted after ventricular therapy (P<0.001). This finding suggests that successful AF termination with ATP may delay the onset of spontaneous VT or VF. Conventional pacing in atrial fibrillation patients AF is a common clinical manifestation of SSS, and patients with SSS often require pacing for

conventional bradycardia indications. Earlier uncontrolled studies found that VVI pacing was deleterious to ventricular function and was associated with higher incidence of AF, stroke, congestive heart failure, and thromboembolism [46–49]. It has been suggested that ventricular pacing may promote AF by atrial stretching from transient high atrial pressures as a result of asynchronous atrial contraction from retrograde VA conduction during VVI pacing. The choice of pacing modality in patients with SSS would be important if the incidence of AF is to be minimized, and this clinical issue has been the subject of several recently published prospective randomized clinical trials. Andersen et al was the first to establish the effectiveness of atrial pacing over ventricular pacing in reducing AF incidence in SSS patients. They randomized 225 patients with SSS to either single-chamber atrial or single-chamber ventricular pacing [50]. Significant reduction in thromboembolic events was reported in patients randomized to atrial pacing (6 patients in atrial group versus 20 patients in ventricular group, P = 0.008) during the initial mean follow-up of 3.3 years. Patients randomized to atrial pacing had lower incidence of AF compared with ventricular pacing (13.6% versus 23.5%, P = 0.12). The beneficial effect of AF reduction was seen only after 2 years of randomization and persisted with longer follow-up (23.6% versus 34.8%, P = 0.012). Furthermore, improved survival was demonstrated in the atrial pacing group when follow-up was extended to 8 years (RR 0.66, P = 0.045) [51]. This study, however, includes only small numbers of patients with long-term follow-up. A more modest effect on AF incidence was reported in a larger study involving 407 patients, the Pacemaker Selection in the Elderly (PASE) trial [52]. All patients received DDD pacemakers and then were programmed randomly to VVIR or DDDR. There was no difference in the overall AF incidence between the VVIR and DDDR mode (19% versus 17%, P = 0.80) after mean follow-up of 1.5 years, although a higher AF incidence was observed in the subgroup of SSS patients randomized to single-chamber ventricular pacing (28% versus 19%, P = 0.06). No difference in AF incidence was found in the subgroup of patients with AV block. The lack of statistical difference in overall AF incidence may be attributed to high cross-over rates from VVIR to DDDR pacing (26%) because of pacemaker syndrome.

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To minimize the high cross-over rates observed in PASE, where all patients received dual chamber pacemakers and were randomized to either single chamber programming or dual chamber programming, the Canadian Trial of Physiologic Pacing (CTOPP) study randomized patients to implantation of physiologic (dual-chamber) pacemakers or ventricular-based (single-chamber) pacemakers [53]. This was the first large-scale multi-center prospective randomized trial that compared the effects of physiologic pacing and ventricular pacing mode on overall cardiovascular death and stroke. A total of 2569 patients were enrolled in the study, and after a mean follow-up of 3.5 years, patients randomized to physiologic pacing were found to have a lower incidence of AF (annual rate of 5.3% versus 6.6%, P = 0.05). There was no significant difference in the primary outcome of stroke or cardiovascular death (4.9% versus 5.5%, P = 0.33) between the two pacing modes. Failure to demonstrate a significantly lower AF incidence with physiologic pacing may be attributed to the fact that less than half of the patients in the study had sinus node dysfunction. In another report, the investigators reported relative reduction of 27% for progression to chronic AF with physiologic pacing from 3.8%/ year to 2.8%/year, P = 0.0016 [54]. To avoid the confounding variable of AV conduction disease on the outcome of AF and pacing, the MOST (Mode Selection Trial in Sinus Node Dysfunction) trial enrolled only SSS patients. A total of 2010 patients with SSS were randomized to ventricular pacing or dual-chamber pacing [55]. The effect of pacing mode on AF incidence was almost immediate and obvious within the first 12 months of follow-up. Overall AF incidence was lowered with dual-chamber pacing (hazard ratio 0.79, P = 0.008), and fewer patients progressed to chronic AF with dual-chamber pacing (15.2% versus 26.7%, P<0.001). Patients receiving dualchamber pacing with no prior history of AF had lower incidence of AF after randomization compared with ventricular pacing (hazard ratio 0.50, P = 0.001). A smaller reduction in AF incidence was observed in the dual-chamber group if patients had a prior history of AF (hazard 0.86, P = 0.12). Patients receiving dual-chamber pacing also reported small but measurable improvements in quality of life over the study period. This trial, however, showed no difference in outcome with respect to hospitalization for heart failure, death, or nonfatal stroke (primary end points) between dual-chamber and ventricular pacing.

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None of the larger pacemaker trials have shown the dramatic benefit of reduced AF incidence, lowered stroke rate, and mortality as initially reported by Andersen et al. A possible explanation is the high incidence of AAI pacing in the Andersen et al’s study, while patients in the larger multi-center clinical trials received dualchamber pacemakers. Continuous right ventricular pacing with resulting left bundle branch block morphology and asynchronous ventricular activation is increasingly being recognized as arrhythmogenic. This effect may have blunted the benefits of atrial pacing. A recent retrospective analysis has provided further evidence for this adverse effect. A linear relationship was found between cumulative percent of ventricular pacing (Cum%VP) and development of AF in over 1300 patients with normal QRS duration who were enrolled in the MOST study [56]. In this study, the risk of AF was increased by 1% for each 1% increase in Cum%VP up to 85% in the DDDR arm and 0.7% for each 1% increase in Cum%VP up to 80% in the VVIR arm despite adjusting for known baseline predictors of AF. Nonetheless, all these studies suggest that atrialbased pacing is the preferred mode in patients with SSS for prevention of AF (Fig. 3). By programming a longer AV delay to ensure intrinsic AV conduction, the incidence of AF may be even lowered further in this patient population. Atrioventricular junction ablation In some AF patients, achieving adequate rate control is difficult despite therapy with maximal doses of AV nodal blocking agents. The strategy of AV junction ablation and pacing (or ablate and pace) may be appropriate in this circumstance. This strategy has been shown to result in significant improvements in symptoms and quality of life compared with medical therapy, especially in patients with chronic AF and heart failure [57]. Furthermore, several studies have noted significant improvement in functional class, exercise tolerance, and ejection fraction following AV junction ablation and pacing [58]. In patients with paroxysmal AF, dual-chamber pacemakers with mode switching capability are implanted in an effort to maintain AV synchrony during sinus rhythm following ablation. Automatic mode switching prevents rapid ventricular tracking when patients develop AF and have underlying AV block. Dual-chamber devices can be reprogrammed to VVIR when AF becomes persistent or

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Fig. 3. Prospective randomized studies comparing the development of AF in pacemaker patients programmed to atrial or physiological-based pacing and ventricular-based pacing. The duration of follow-up for each study is as indicated. Abbreviation: SND, sinus node dysfunction.

chronic. There has been concern about an increased mortality from polymorphic ventricular tachycardia directly related to the ablation procedure. Programming a higher baseline pacing rate for the first 2 months after AV junction ablation can minimize this complication [59]. The ablate and pace strategy may gain wider acceptance now as results from the AFFIRM trial did not show any mortality benefit for rhythm control over rate control arm in patients with recent AF onset [60]. Furthermore, long-term follow-up has shown this strategy to be cost effective, particularly in symptomatic drug-refractory patients [61]. Newer pacing and therapeutic algorithms The irregularity of the ventricular depolarization during AF has been associated with poorer cardiac performance and increased sympathetic nervous system activation. Ventricular rate stabilization algorithms have been developed by various device manufacturers to regularize the R–R interval during AF in attempts to improve patients

symptoms. This can be achieved by increasing the ventricular pacing rate during AF, without causing a significant increase in mean heart rate and has been shown to suppress the R–R intervals at shorter cycle lengths and regularize the rate (Fig. 4). Although a stand-alone atrial defibrillator has been shown to be effective, its role in AF management remains limited. This is primarily because of patient discomfort despite shocking at low energy levels and development of immediate or early recurrence of AF following shock therapy. On the other hand, delivery of synchronized shock therapy for AF termination has been incorporated into the newer ICDs (see Fig. 4). The availability of ICD therapy for ventricular fibrillation obviates the safety concerns of stand-alone atrial defibrillators. This therapy feature can be programmed to deliver shocks controlled by the patient or after a predetermined duration of time. Additional studies would determine its unique role in management of AF and its acceptance among ICD patients with atrial arrhythmias.

c Fig. 4. Electrograms recorded from a patient with ischemic cardiomyopathy and history of atrial tachyarrhythmias implanted with Guidant Vitality AVT model A135 ICD. The upper tracing is the atrial electrogram; the middle tracing is the ventricular electrogram, and the lower tracing is an electrogram recorded from the tip to the distal coil of the ICD lead. Atrial fibrillation was converted successfully to an atrial-paced rhythm following a 10 J biphasic synchronized shock. Note the regular V–V cycle during atrial fibrillation caused by the Ventricular Rate Regulation feature of the device (in circle). Abbreviation: VP–VR, Ventricular Pace-Ventricular Rate Regulation.

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Summary Currently, there are more therapeutic options available for AF patients with recurrent symptomatic AF episodes. The studies on the role of device therapy for AF have been promising thus far, but they have not achieved any of the goals of AF management. Based on large randomized clinical trials, patients with SSS should receive atrial-based pacing devices. Moderate sized randomized studies have shown minimal benefit of multi-site pacing in AF prevention, even when combined with antiarrhythmic agents. Alternative site pacing such as septal pacing (high or low), however, may be more advantageous, as it achieves similar results in terms of AF reduction with less hardware. The role of ATP in AF prevention is still in its infancy and will need further studies to determine its role in conjunction with antiarrhythmic agents. Furthermore, the role of radiofrequency ablation of pulmonary veins and other sites of AF initiation has been evolving and may be offered to more patients in the future. This approach may be more acceptable to patients and may gain wider acceptance for some groups of AF patients rather than device therapy. In any event, there is still a large role for pacemaker therapy in the management of AF, especially in patients who cannot benefit from curative ablation or surgery procedures, or patients who have failed these procedures, and particularly elderly patients who typically do not undergo these procedures. Based on current understanding, careful selection of pacing sites and pacing algorithms may help in reducing AF episodes in patients receiving devices.

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