(Circulation. 1996;94:578-583.)
© 1996 American Heart Association, Inc.
Articles |
Dual-Chamber Versus Ventricular Pacing
Critical Appraisal of Current Data
the Departments of Medicine and of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ont (S.J.C., M.G., S.Y.) and the Department of Medicine, University of British Columbia, Vancouver, BC (C.K.), Canada.
Key Words: pacing • heart-assist device • electrical stimulation • pacemaker • stroke • survival
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Current Use of Dual-Chamber Pacing |
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 Top Current Use of Dual-Chamber... Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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Electrical stimulation of the heart to prevent bradycardia has been a practical treatment option for 40 years,1 and over this period pacemaker technology has advanced at a rapid rate. Many innovations in lead and generator technology have now become incorporated into standard practice and have made cardiac pacing very reliable and effective.2 Virtually all pacemakers implanted today use highly biocompatible materials and long-lasting lithium-based batteries; they are inhibited by spontaneously occurring cardiac activity and are multiprogrammable. Dual-chamber pacing more closely resembles the normal physiology of cardiac activation than does asynchronous ventricular stimulation because it maintains the usual synchrony of atrial and ventricular contraction and dominance of the sinus node. However, despite the theoretical advantages of dual-chamber pacing, this technology is not widely used in most countries. In a world survey of pacing done in 1989,3 dual-chamber pacemaker use varied between regions from 2% to 32% (median, 14%) of patients and was used in
30% of patients in 3 of 13 countries or regions surveyed. Dual-chamber pacemaker use remains below expectations despite a statement in favor of dual-chamber pacing in guidelines for pacemaker implantation published jointly by the American Heart Association and the American College of Cardiology, which said "Although this may be less important at rapid rates, at slow rates it is almost always desirable to maintain AV synchrony. Long-term absence of AV synchrony increases the incidence of atrial fibrillation and stroke and may reduce patient life expectancy, particularly in patients with impaired left ventricular function, idiopathic hypertrophic subaortic stenosis or aortic stenosis."5 The British Pacing and Electrophysiology Group (BPEG) (a specialist subgroup of the British Cardiac Society) recently published guidelines for pacemaker selection that include a recommendation that dual-chamber or atrial pacemakers are optimal except in chronic atrial fibrillation.6 In response to the BPEG recommendation, two British groups7 8 calculated that the increased cost of pacemaker hardware for their implementation would be 75% and 94%, respectively. In a subsequent editorial, Petch9 speculated that dual-chamber implant rates remain low because of cost and because of the advanced age of the majority of new pacemaker recipients, who (some clinicians believe) may often not be able to benefit from small improvements in functional capacity. He stated that the recommendation for broad use of dual-chamber pacing is based on observational data that may not be reliable and not on the results of randomized controlled trials. These articles point out two important reasons why the rate of dual-chamber device implantation remains lower than recommended: (1) the significant incremental cost of dual-chamber pacing (cost of leads and generators, more intensive follow-up, and shorter battery life) and (2) the lack of reliable comparative data from large randomized clinical trials regarding the effects on mortality and morbidity of dual-chamber compared with ventricular pacing. Many physicians appear unwilling to adopt a more expensive new technology without reliable evidence from randomized controlled trials that a clinically worthwhile benefit truly exists.
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Potential Benefits of Dual-Chamber Pacing |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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Intuitively, one expects a clinical benefit from the closer approximation of sinus rhythm provided by dual-chamber pacing. Maintaining the normal sequence of atrial and ventricular activation will tend to optimize ventricular filling and cardiac output. This, in turn, should improve exercise capacity and quality of life. The pacemaker syndrome, a constellation of symptoms associated with asynchronous or retrograde atrial activation from ventricular pacing, may also be prevented by dual-chamber pacing. Observational studies suggest that dual-chamber pacing reduces the risk of atrial fibrillation, stroke, and death compared with ventricular pacing.10 11 12 13 14 15 16 17 18 19 20 21 22 A reduction in stroke has been demonstrated in one small randomized study.23 If important reductions in mortality, stroke, and atrial fibrillation were reliably demonstrated, it would amply justify the increased cost of dual-chamber pacing. It is therefore timely to review critically the evidence supporting the proposed benefits of dual-chamber pacing and to discuss briefly the randomized controlled trials of pacemaker device and mode selection now in progress.
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Effect of Pacing Mode on Effort Tolerance and Quality of Life |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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The effect of pacing mode on effort tolerance has been assessed in many small crossover trials using a variety of treadmill and bicycle exercise protocols.24 25 26 27 28 29 30 31 32 33 34 While early studies compared AV sequential pacing modes to fixed-rate ventricular pacing, more recent studies have evaluated dual-chamber against rate-adaptive ventricular pacing (Table 1
). There is a clear contrast between the results of those studies in which ventricular pacing is at a fixed rate24 25 26 27 28 and of those in which ventricular pacing is rate adaptive.29 30 31 32 33 34 Dual-chamber pacing improves effort tolerance compared with fixed-rate ventricular pacing but not compared with rate-adaptive ventricular pacing. This can be explained by the observation that much of the physiological increase in cardiac output required for exercise comes from a twofold to threefold increase in heart rate. In most patients with AV block, ventricular rate does not increase adequately with exercise (chronotropic incompetence). In such patients, dual-chamber pacing allows an increase in ventricular rate, whereas this does not occur with fixed-rate ventricular pacing. However, the ability of ventricular pacemakers with rate-adaptive capability to increase heart rate in response to exertion appears to minimize the benefit of dual-chamber over ventricular pacing.
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Several crossover trials24 25 26 27 30 31 32 33 35 36 have compared quality of life between dual-chamber and ventricular pacing (Table 2). The majority of studies reported improved quality of life with dual-chamber pacing compared with ventricular pacing whether or not rate-adaptive features were used. It would be wise, however, to interpret these findings cautiously. Very few of the studies used standardized or validated quality-of-life instruments. In crossover trials, the potential for patient unblinding is high, which can result in biases in assessment due to higher expectations on the part of the patient. Nonetheless, the general consistency of these small studies suggests a benefit in quality of life for physiological pacing. In summary, the small crossover trials available suggest that, compared with rate-adaptive ventricular pacing, dual-chamber pacing does not improve effort tolerance. They indicate a benefit in quality of life, but this should be confirmed in larger randomized trials using standardized quality-of-life instruments.
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Pacemaker Syndrome |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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One of the potential benefits of dual-chamber pacing is the prevention of the pacemaker syndrome, a constellation of symptoms associated with ventricular pacing that is attributed primarily to asynchronous atrial and ventricular contraction. Ausubel and Seymour37 in their review identified 17 different potential symptoms as part of the pacemaker syndrome, including not only most major cardiovascular symptoms (dyspnea, presyncope, angina) but also lethargy, cough, chest colds, and palpitations. More recently, Travill and Sutton38 reviewed the definition of pacemaker syndrome and focused on syncope, presyncope, dyspnea, chest pain, lethargy, and palpitations as being the main symptoms. These symptoms are nonspecific and very common among cardiac patients with or without pacemakers. No standardized diagnosis of the pacemaker syndrome has been adopted, and for this reason, the incidence of the pacemaker syndrome in patients receiving pacemakers is uncertain. Cohen and Frank39 reported that 13 of 260 patients (5%) had symptoms during ventricular pacing that were alleviated by returning normal AV synchrony. Conversely, Escher et al40 identified 90 of 1400 ventricular pacing patients who were symptomatic without pacemaker malfunction, but of these, only 2 (0.1%) were thought to clearly have the pacemaker syndrome.
Given that the average age of patients receiving a pacemaker is >70 years, it is not surprising that some patients continue to complain of syncope or presyncope after pacemaker implantation. Several studies have analyzed the origin of this symptom in patients with ventricular pacemakers and report that persistent syncope and presyncope after pacemaker implantation are most often not due to pacemaker syndrome. Pavlovic et al41 studied 46 patients who complained of recurrent syncope after ventricular pacemaker implantation. Upright tilt tests were positive in 37%, and a variety of other pathophysiological mechanisms, other than the pacemaker syndrome, were identified in an additional 33%. Sgarbossa et al42 likewise found that persistent syncope in 44 ventricular pacemaker patients could be explained by a variety of mechanisms other than the pacemaker syndrome: vasovagal mechanisms (18%), orthostatic mechanisms (26%), tachyarrhythmia (16%), myocardial ischemia (3%), or pacemaker malfunction (7%). In only 30% was there no obvious explanation other than possible pacemaker syndrome. Many of these patients had dual-chamber pacemakers. Conversely, Nishimura et al43 evaluated 9 patients with ventricular pacemakers who had recurrent syncope or presyncope after implantation. They reported that after dual-chamber pacing was instituted, 6 had no further syncope and 3 had only presyncope. However, without a true concurrent control group, the above data are difficult to interpret.
By comparing the same patients who received both ventricular and dual-chamber pacing either after they had changed from ventricular to dual-chamber or after a period of successful dual-chamber pacing, some studies have suggested that a subclinical pacemaker syndrome may exist. Using quality-of-life questionnaires, Sulke et al44 compared ventricular and dual-chamber pacing in 16 patients who had been satisfied with ventricular pacing. Dual-chamber pacing was preferred by 75%, whereas no patient preferred ventricular pacing. In a similar study of 40 patients, Heldman et al45 found that 83% of patients had fewer symptoms with dual-chamber than with ventricular pacing.
Some patients undoubtedly experience symptoms from pacing that does not maintain AV synchrony that are relieved by restoration of AV synchrony. The extent of this problem is perhaps quite large but is currently unknown. No study has ever compared the severity of typical pacemaker syndrome symptoms in patients randomized to receive either dual-chamber or ventricular pacing. Such studies will clarify the importance of maintaining AV synchrony for preventing pacemaker syndrome and optimizing quality of life.
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Prevention of Atrial Fibrillation, Stroke, Heart Failure, and Death |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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Atrial fibrillation occurs commonly in patients receiving pacemaker therapy, related in part to underlying sick sinus syndrome. Several studies suggest that ventricular pacing may increase the risk of developing atrial fibrillation,10 11 12 13 14 15 16 17 18 19 20 21 22 23 and atrial fibrillation, in turn, is a major risk factor for stroke.46 Several randomized clinical trials, which enrolled several thousand patients with atrial fibrillation, have established that these patients face an annual risk of stroke or other systemic embolism of 5% and that this risk is lowered to 1.7% with anticoagulant therapy.47 Pacemaker patients may not receive anticoagulation promptly when they develop atrial fibrillation because their conduction abnormalities may prevent them from becoming aware of palpitations. Their physicians also may have difficulty recognizing atrial fibrillation because of the regular ventricular rate of the pacemaker. Therefore, preventing atrial fibrillation by dual-chamber pacing could have a significant impact on the risk of stroke and other systemic embolism in pacemaker patients.
There are theoretical reasons why dual-chamber pacing might reduce mortality from congestive heart failure (CHF). The initial compensating mechanism of the failing heart is to increase sarcomere length, which optimizes the Frank-Starling relationship between end-diastolic volume and contractile force.48 Dual-chamber pacing should help maintain cardiac output by augmenting ventricular filling, and it may protect the patient who is prone to the development of heart failure. There is limited evidence of improved cardiac performance with dual-chamber pacing in heart failure. Nitsch et al49 used radionuclide angiography in 16 patients with coronary disease or heart failure to compare resting cardiac output with AV synchrony against single-chamber ventricular pacing at similar rates. Cardiac output increased slightly from 4.86 L/min with ventricular pacing to 5.12 L/min with AV pacing (P<.01). Mukharji et al50 studied the effects of atrial and ventricular pacing on cardiac index in 10 normal patients and in 10 patients with severe CHF. They reported significant and comparable increases in cardiac output at rest with atrial compared with ventricular pacing in both groups. Conversely, Greenberg et al51 reported that the atrial contribution to cardiac output was inversely related to left ventricular filling pressure as measured by the pulmonary capillary wedge pressure (PCWP). The atrial contribution to cardiac output was 9.3±1.3 mL/m2 per beat in those with PCWP below 20 mm Hg, whereas it was 2.4±1.2 mL/m2 per beat in those with a higher wedge pressure. This suggests that the atrial contribution to ventricular filling may be less in heart failure, implying that contrary to intuition, the mode of pacing may be less important in patients with CHF.
In summary, it is reasonable to hypothesize lower rates of atrial fibrillation, stroke, and CHF with dual-chamber pacing compared with ventricular pacing. Whether these theoretical expectations and physiological observations are indeed associated with a reduction in major clinical outcomes requires careful and prospective evaluation.
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Long-term Observational Studies |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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A variety of observational studies indicate that atrial or dual-chamber pacing is associated with an overall two-thirds lower risk of atrial fibrillation and a one-third lower risk of death compared with ventricular pacing (Table 3
).10 11 12 13 14 15 16 17 18 19 20 21 22 These studies were all retrospective, nonrandomized comparisons of patients receiving atrial or dual-chamber versus ventricular pacing for sinus node disease and/or AV block. In most patients, the decision about which pacemaker to use was based largely on physician preference and is likely to have been influenced by individual patient characteristics. For example, patients expected to have a shorter survival may have been more likely to receive the less expensive ventricular devices. Therefore, the potential for bias in these studies is large. It is useful to examine two of the larger studies in detail. Rosenqvist et al18 retrospectively compared 79 patients who received ventricular pacing for sinus node disease with 89 patients who received atrial pacing. Exclusion of AV nodal disease in the atrial pacing group undoubtedly excluded some sicker patients from this group. The atrial pacing group was also slightly younger and was less likely to have heart enlargement. No adjustment was made for imbalances in baseline prognostic factors. During 4 years of follow-up, mortality was lower in those receiving atrial pacing (2.2%/y) compared with those receiving ventricular pacing (5.5%/y). Atrial fibrillation occurred less frequently (12.1%/y versus 1.8%/y) with atrial pacing.
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In one large study,22 950 pacemaker patients were followed for up to 7 years. Dual-chamber pacing was used in 665 patients and ventricular pacing in 285 patients. Most ventricular devices were implanted before dual-chamber devices became generally available, but the specific determinants of pacing mode selection are unspecified. At the time of implantation, all patients were in sinus rhythm, and 59% of patients had sinus node disease. The mean age was 70 years and was similar between groups. No other prognostic indexes were reported. The mortality rates were significantly higher for ventricular compared with dual-chamber pacing at 13.6%/y and 8.9%/y, respectively. The rates of atrial fibrillation were also significantly higher for ventricular compared with dual-chamber pacing at 5.3%/y and 1.6%/y, respectively. As with other nonrandomized studies, because imbalances in baseline prognostic factors can have a major influence on outcomes, one should interpret the observed difference very cautiously.
Two other observational studies have examined the effect of pacing mode selection in CHF. Linde-Edelstam et al52 evaluated the effect of pacing mode on survival in 144 patients with either ventricular or AV synchronized pacing for high-degree AV block. Total mortality did not differ between the two groups, but in the subgroup with heart failure (18 AV synchronous, 24 ventricular pacemakers), mortality rate was four times higher with ventricular pacing (P=.04) than with synchronous pacing. Alpert et al,53 in a similar nonrandomized study of 148 pacemaker patients, found no overall difference in mortality except in the subgroup with heart failure (23 ventricular, 16 dual-chamber), in which mortality was significantly lower with dual-chamber pacing. The results of these studies are promising, but they should be interpreted cautiously, because both studies were small and nonrandomized, with high potential for selection bias. Thus, there is only weak evidence that dual-chamber pacing improves prognosis in heart failure compared with ventricular pacing.
A single randomized trial comparing two modes of pacing has recently been published.23 In this study, 225 patients with sinus node disease and intact AV nodal conduction referred for a first pacemaker were randomly allocated to receive either atrial or ventricular pacing and followed for a mean of 40 months. There was no difference in either overall mortality or cardiovascular mortality, nor was there a significant difference in atrial fibrillation, although a lower rate of atrial fibrillation was observed with atrial (14%) compared with ventricular (23%) pacing (P=.12). There was a highly significant reduction in the risk of systemic thromboembolism (stroke or peripheral arterial) with atrial pacing (5%) versus ventricular pacing (17%) (P=.0083). However, the 5.5% annual rate of stroke in the ventricular pacemaker group was unusually high. In those with atrial fibrillation, it was 14%/y, which is much higher than reported in the meta-analysis of the five recent large randomized trials of antithrombotic therapy in atrial fibrillation,47 in which the annual risk of stroke or systemic embolism was 5%/y without anticoagulation and 1.7%/y on anticoagulation. Thus, the rate of thromboembolism in the ventricular pacemaker group with atrial fibrillation was eight times higher than expected for atrial fibrillation patients treated with warfarin. The rate of use of anticoagulant therapy after randomization in this study is not reported. This study is important because it is the first randomized trial of pacemaker device selection that assesses clinically relevant outcomes. Because of its small size, a clinically important reduction in the risk of atrial fibrillation and death could easily have been missed. The reduction in thromboembolism observed with atrial pacing is encouraging, but the unusually high risk of stroke with ventricular pacing in this study raises concern about the generalizability of the results to the average pacemaker patient.
In summary, many observational studies and one small randomized study indicate that atrial or dual-chamber pacing reduces important clinical outcomes. Although these reports are encouraging, they require confirmation by large randomized trials.
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Ongoing Large Randomized Trials of Pacemaker Selection |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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Three large multicenter randomized trials of pacemaker selection are either in the final planning stage or already under way. The Canadian Trial of Physiological Pacing (CTOPP) is a multicenter randomized trial examining the hypothesis that physiological pacing (either atrial or dual-chamber) reduces the risk of the combined outcome of cardiovascular death or stroke. Patients scheduled for an initial pacemaker implant who are not in chronic atrial fibrillation are potentially eligible for enrollment. Patients are randomized to receive either a physiological (atrial or dual-chamber) or a ventricular pacemaker. Rate-adaptive pacing is required for all patients with chronotropic incompetence or complete AV block. Patients randomized to physiological pacing may receive an atrial pacemaker if AV nodal Wenkebach phenomenon does not occur with atrial pacing at
130 bpm. However, the vast majority of patients randomized to physiological pacing actually receive dual-chamber devices. In addition to the primary outcome measure of cardiovascular death or stroke, the rates of occurrence of atrial fibrillation, other systemic embolism, and admission to hospital with CHF are specified secondary outcomes. Quality-of-life and economic evaluations are also being done. Thirty-five centers will contribute 2450 patients (expected completion of recruitment is by February 1996) who will be followed for a minimum of 2 years. The study is anticipated to have 90% power to detect a 28% reduction in the primary outcome. Final results are expected by late 1998.
The UK-PACE trial is a multicenter trial in the United Kingdom that has recently begun enrollment. Patients 70 years old with AV conduction abnormality requiring pacing will be randomized to receive either a dual-chamber or ventricular pacemaker. The primary outcome measure will be all-cause mortality. It is anticipated that 2000 patients will be enrolled by 40 centers and followed for a mean of 3 years.
The Pacemaker Selection in the Elderly (PASE) study in the United States has recently completed enrollment for a pilot study of 400 patients. Patients 65 years old were potentially eligible for the pilot study if they required a pacemaker. All enrolled patients received a rate-adaptive dual-chamber pacemaker and were randomized to be initially programmed to either dual-chamber or ventricular pacing. The primary outcome measure in the pilot study is quality of life (according to the Short Form 36). Results of the pilot study are anticipated by spring of 1996. The Mode Selection Trial (MOST) is now under way. Patients with sinus node disease will receive a dual-chamber pacemaker and will have pacing mode randomized between dual-chamber and ventricular pacing. Two thousand patients will be enrolled, and the primary outcome measure will be death or stroke.
These three trials are expected to be completed within 5 years, and the question of whether dual-chamber pacing prevents death, stroke, and heart failure compared with ventricular pacing will probably be resolved. It is estimated that between 250 and 300 deaths, between 70 and 90 strokes, and between 150 and 200 hospitalizations for heart failure will occur in each of these studies. Thus, each study will have at least 80% statistical power to detect mortality reductions of 30%. A meta-analysis of these studies will allow detection of mortality reductions of as low as 20%, stroke reductions of 30%, and reductions in heart failure hospitalization of 25%. The patient populations of the three studies are complementary: elderly patients with AV block in UK-PACE, patients with sinoatrial node disease in MOST, and all patients requiring a first pacemaker without chronic atrial fibrillation in CTOPP. A likely limitation of each study will be insufficient numbers of patients in specific subsets, such as those with heart failure. However, combining data in an eventual meta-analysis should also assist in clarifying the role of dual-chamber pacing in these selected populations. The effects of different modes of pacing on quality of life, functional capacity, and cost will undoubtedly be much better understood as a result of these trials.
In summary, although there are reasons to believe that dual-chamber pacing improves patient tolerance of pacing and reduces morbidity and mortality, careful analysis of existing reports reveals surprisingly few studies that are free of confounding influences. Only one small randomized trial of pacemaker selection has been reported, and its results are encouraging but not definitive. The evidence for improved patient well-being with dual-chamber pacing compared with rate-adaptive ventricular pacing requires confirmation. If dual-chamber pacing can be demonstrated by carefully designed large randomized trials to prevent stroke, heart failure, or death, these results will have a major impact on clinical practice.
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Footnotes |
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Reprint requests to Stuart J. Connolly, MD, McMaster Clinic Room 501, Hamilton General Hospital, 237 Barton St E, Hamilton, Ont L8L 2X2, Canada.(Circulation. 1996;94:578-583.)
Received July 24, 1995; revision received November 7, 1995; accepted November 19, 1995.
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References |
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TopCurrent Use of Dual-Chamber...Potential Benefits of Dual...Effect of Pacing Mode...Pacemaker SyndromePrevention of Atrial...Long-term Observational StudiesOngoing Large Randomized Trials...References |
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