Atrial pacing for sick sinus syndrome

Heart rate during exercise: mechanisms, behavior, and therapeutic and prognostic implications in heart failure patients with reduced ejection fraction

Exercise intolerance is a typical manifestation of patients affected by heart failure with reduced ejection fraction (HFrEF); however, the relationship among functional capacity, mortality, and exercise-induced heart rate response during exercise remains unclear in either sinus rhythm or atrial fibrillation subjects. Heart rate increase during incremental load exercise has a typical pattern in normal subjects, whereas it is commonly compromised in HFrEF patients, mainly due to the imbalance of the autonomic nervous system. In the present review, we aim to describe the behavior of heart rate during exercise in normal subjects and in HFrEF patients in sinus rhythm and atrial fibrillation, understanding and explaining the mechanism leading to a different exercise performance and functional limitation. Moreover, the role of chronotropic incompetence and the need of standardizing the cutoff criteria are also discussed in order to clarify the clinical importance, the prognostic relevance, and the potential therapeutic implications of this condition. Looking into the relative contribution and interaction of heart rate response during exercise might represent an important issue to guide individualized therapeutic interventions and prognostic assessment in HFrEF patients.

Deactivation vs. asynchronous pacing - prospective evaluation of a protocol for rhythm management in patients with magnetic resonance conditional pacemakers undergoing adenosine stress cardiovascular magnetic resonance imaging

Background

Cardiovascular Magnetic Resonance (CMR) imaging with adenosine stress is an important diagnostic tool in patients with known or suspected coronary artery disease (CAD). However, the method is not yet established for CAD patients with pacemakers (PM) in clinical practice. A possible reason is that no recommendations exist for PM setting (paused pacing or asynchronous mode) during adenosine stress. We elaborated a protocol for rhythm management in clinical routine for PM patients that considers heart rate changes under adenosine using a test infusion of adenosine in selected patients.

Methods

47 consecutive patients (mean age 72.3 ± 10,0 years) with MR conditional PM and known or suspected CAD who underwent CMR in clinical routine were studied in this prospective observational study. PM indications were sinus node dysfunction (SND, n = 19; 40,4%), atrioventricular (AV) block (n = 26; 55.3%) and bradyarrhythmia in permanent atrial fibrillation (AF, n = 2; 4.3%). In patients with SND, normal AV-conduction and resting HR >45 bpm at the time of CMR and in AF the PM was deactivated for the scan. In intermittent AV-block a test infusion of adenosine was given prior to the scan. All patients with permanent higher degree sinuatrial or AV-block or deterioration of AV-conduction in the adenosine test were paced asynchronously during CMR, in patients with preserved AV-conduction under adenosine the pacemaker was deactivated. CMR protocol included cine imaging, adenosine stress perfusion and late gadolinium enhancement.

Results

The adenosine test was able to differentiate between mandatory PM stimulation during CMR and safe deactivation of the device. In patients with permanent sinuatrial or AV-block (n = 11; 23.4%) or deterioration of AV conduction in the adenosine test (n = 5, 10.6%) asynchronous pacing above resting heart rate did not interfere with intrinsic rhythm, no competitive stimulation was seen during the scan. 10 of 15 (66,7%) patients with intermittent AV-block showed preserved AV-conduction under adenosine. As in SND and AF deactivation of the PM showed to be safe during CMR, no bradycardia was observed.

Conclusion

Our protocol for rhythm management during adenosine stress CMR showed to be feasible and safe and may be recommended for pacemaker patients undergoing routine CMR.

Dual-chamber pacemakers for treating symptomatic bradycardia due to sick sinus syndrome without atrioventricular block: a systematic review and economic evaluation

BACKGROUND

Bradycardia [resting heart rate below 60 beats per minute (b.p.m.)] can be caused by conditions affecting the natural pacemakers of the heart, such as sick sinus syndrome (SSS) and atrioventricular (AV) blocks. People suffering from bradycardia may present with palpitations, exercise intolerance and fainting. The only effective treatment for patients suffering from symptomatic bradycardia is implantation of a permanent pacemaker.

OBJECTIVE

To appraise the clinical effectiveness and cost-effectiveness of dual-chamber pacemakers compared with single-chamber atrial pacemakers for treating symptomatic bradycardia in people with SSS and no evidence of AV block.

DATA SOURCES

All databases (MEDLINE, EMBASE, Cochrane Central Register of Controlled Trials, Health Technology Assessment database, NHS Economic Evaluations Database) were searched from inception to June 2014.

METHODS

A systematic review of the clinical and economic literature was carried out in accordance with the general principles published by the Centre for Reviews and Dissemination. Randomised controlled trials (RCTs) evaluating dual-chamber and single-chamber atrial pacemakers and economic evaluations were included. Pairwise meta-analysis was carried out. A de novo economic model was developed.

RESULTS

Of 493 references, six RCTs were included in the review. The results were predominantly influenced by the largest trial DANPACE. Dual-chamber pacing was associated with a statistically significant reduction in reoperation [odds ratio (OR) 0.48, 95% confidence interval (CI) 0.36 to 0.63] compared with single-chamber atrial pacing. The difference is primarily because of the development of AV block requiring upgrade to a dual-chamber device. The risk of paroxysmal atrial fibrillation was also reduced with dual-chamber pacing compared with single-chamber atrial pacing (OR 0.75, 95% CI 0.59 to 0.96). No statistically significant difference was found between the pacing modes for mortality, heart failure, stroke, chronic atrial fibrillation or quality of life. However, the risk of developing heart failure may vary with age and device. The de novo economic model shows that dual-chamber pacemakers are more expensive and more effective than single-chamber atrial devices, resulting in a base-case incremental cost-effectiveness ratio (ICER) of £6506. The ICER remains below £20,000 in probabilistic sensitivity analysis, structural sensitivity analysis and most scenario analyses and one-way sensitivity analyses. The risk of heart failure may have an impact on the decision to use dual-chamber or single-chamber atrial pacemakers. Results from an analysis based on age (> 75 years or ≤ 75 years) and risk of heart failure indicate that dual-chamber pacemakers dominate single-chamber atrial pacemakers (i.e. are less expensive and more effective) in older patients, whereas dual-chamber pacemakers are dominated by (i.e. more expensive and less effective) single-chamber atrial pacemakers in younger patients. However, these results are based on a subgroup analysis and should be treated with caution.

CONCLUSIONS

In patients with SSS without evidence of impaired AV conduction, dual-chamber pacemakers appear to be cost-effective compared with single-chamber atrial pacemakers. The risk of developing a complete AV block and the lack of tools to identify patients at high risk of developing the condition argue for the implantation of a dual-chamber pacemaker programmed to minimise unnecessary ventricular pacing. However, considerations have to be made around the risk of developing heart failure, which may depend on age and device.

STUDY REGISTRATION

This study is registered as PROSPERO CRD42013006708.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

Group Purchasing Organizations: Optimizing Cardiac Device Selection, Therapy Delivery, and Fiscal Responsibility

Group purchasing organizations (GPOs) have played a major role in supporting health care delivery in recent years as the healthcare industry has faced stronger economic pressures. Consequently, a position statement was drafted to act as a guideline for a GPO in creating a fiscally responsible, yet unrestricted environment for physicians to select the most appropriate cardiac device for their patients. This cardiac device selection guideline is to be implemented in hundreds of member hospitals but may be of use in non-member hospitals as well. The guideline will only be effective when the physicians or cardiac device caregivers have the knowledge and skills to optimally program and match device therapies and algorithms to individual patient needs.

Predictors of chronotropic incompetence in the pacemaker patient population

AIMS

We prospectively evaluated results from cardiopulmonary exercise testing for chronotropic incompetence (CI) in a cohort of 292 pacemaker patients. In addition, we evaluated comorbidity and antiarrhythmic patient data as indicators of CI.

METHODS AND RESULTS

On the basis of exercise stress testing and application of the definition of CI by Wilkoff, 51% of our cohort was categorized as having CI. Indications for pacemaker implant for this patient group were 42% atrioventricular block, 56% sinus node disease, and 59% atrial fibrillation. Maximum oxygen uptake (VO(2) max) and exercise duration were significantly reduced among CI pacemaker patients, whereas oxygen uptake at the anaerobic threshold remained unchanged. The following clinical characteristics were significant predictors of CI: existence of coronary artery disease (P = 0.038), presence of an acquired valvular heart disease (P = 0.037), and former cardiac surgery (P = 0.041). Age, gender, arterial hypertension, cardiomyopathy, congenital heart disease, left ventricular ejection fraction, and time period between stress-exercise examination and pacemaker implantation were not significant predictors of CI. Chronic antiarrhythmic therapy with digitalis (P = 0.013), beta blockers (P = 0.036), and amiodarone (P = 0.045) were significant predictors of CI. In contrast, medication with class I and IV antiarrhythmics had no significant correlation with CI.

CONCLUSION

We found the following characteristics predictive of CI in this pacemaker patient population: VO(2) max, existence of coronary artery disease or acquired valvular heart disease, previous cardiac surgery, as well as medication with digitalis, beta blockers, and amiodarone.

The reactive adventitia: fibroblast oxidase in vascular function

The vascular adventitia is activated in a variety of cardiovascular disease states and has recently been shown to be a barrier to nitric oxide bioactivity. Vascular fibroblasts produce substantial amounts of NAD(P)H oxidase-derived reactive oxygen species (ROS) that appear to be involved in fibroblast proliferation, connective tissue deposition, and perhaps vascular tone. However, the physiological and pathophysiological roles of the adventitia have not been extensively studied, possibly because of its location in large blood vessels remote from the vascular endothelium. In recent years, substantial information has been gathered on pathways leading to oxidase activation in smooth muscle cells and fibroblasts and the downstream signaling pathways leading to hypertrophy and proliferation. A clearer understanding of the molecular mechanisms involved will likely lead to therapeutic strategies aimed at preventing vascular dysfunction in diseases such as atherosclerosis, in which these pathways are activated.

Clinical trials of pacing mode selection

Current recommendations in favor of dual-chamber over single-chamber ventricular pacing for patients with sinus node dysfunction or AV conduction disorders were made largely based on observational data and expert opinions. The first randomized pacing mode selection study was relatively small and suggested survival advantage with physiologic pacing only after an extended follow-up duration of 5.5 years. Preliminary results of the first large-scale multicenter randomized pacing mode selection trial revealed only modest reduction in atrial fibrillation without survival advantage after 3 years of physiologic pacing. Two other large-scale multicenter randomized trials comparing physiologic versus ventricular pacing are currently ongoing. They may provide further scientific evidence based on which more objective recommendations can be made with respect to pacing mode selection.

Incidence and significance of chronotropic incompetence in patients with indications for primary pacemaker implantation or pacemaker replacement

This prospective study was undertaken to evaluate the incidence and significance of chronotropic incompetence in 211 patients [age 71.1 6 10.6 years (mean 6 SD)] by means of maximum exercise test in order to determine the indication for rate-responsive pacing before primary pacemaker implantation (147 patients) or pacemaker replacement (64 patients). There were 112 (53%) patients with second- or third-degree AV block, 63 (30%) with sick sinus syndrome, and 36 (17%) with chronic atrial fibrillation. Chronotropic incompetence was defined as maximum heart rate lower than age-adjusted norm calculated by the formula: 0.7x(220 - age) and its significance as the difference between the two rates. The overall incidence of chronotropic incompetence was 42%. The incidence was significantly higher in patients with atrial fibrillation (67%, P<0.0005) and sick sinus syndrome (49%, P<0.012) than in those with AV block (30%). The mean difference between maximum heart rate and the age-adjusted norm was 18% (range 2%-63%). The mean difference was significantly higher in patients with atrial fibrillation (27%, range 8-63%) than in those with sick sinus syndrome (19%, range 2%-45%, P<0.01), or with AV block (12%, range 6%-26%, P<0.000001). The rate-responsive pacemakers were implanted in 44% of 211 patients studied and in 43% of 196 patients excluded from the study due to the apparent (contra)indication of rate-responsive pacing (NS). Thus, chronotropic incompetence seems to be common in the pacemaker patient population. The highest incidence and significance was found in patients with chronic atrial fibrillation. Systematic evaluation of chronotropic competence can double the rate of implantation of rate-responsive pacemakers; however, further studies are needed to clarify relation between the significance of chronotropic incompetence and functional benefit of rate-responsive pacing.

Exercise response of the recipient atrial remnant after orthotopic cardiac transplantation: implications for recipient atrial triggered pacing

The assumption that the recipient atrial remnant in the cardiac transplant recipient is normal has led to the suggestion that it is an appropriate trigger for permanent pacing in transplant recipients who need pacing or to restore chronotropic competence and/or mechanical synchrony of the composite atrium. We examined the chronotropic response to exercise in 12 orthotopic cardiac transplant recipients (mean age 49 years) at a mean time of 17 months posttransplantation. Recipient and donor atrial rates were noted and compared and chronotropic competence determined. Two of 12 recipient atrial remnants were in atrial fibrillation. Only six of the remaining 10 recipient atria exhibited chronotropic competence. Seven of 10 recipient atria had rates higher than that of the donor. Only four of ten recipient atria in sinus rhythm satisfied both criteria. Two of these had abnormally high atrial responses early into exercise. Of the remaining two, only one recipient atrial remnant demonstrated a > or = 20% increase in heart rate above that of the donor at peak exercise. Hence only 1 of 12 (8.3%) transplant recipients potentially could benefit from recipient atrial triggered pacing. While recipient atrial triggered pacing is an attractive theoretical concept for restoring chronotropic competence following orthotopic cardiac transplantation, it may rarely be practical because the recipient atrial remnant displays rhythm abnormalities, chronotropic incompetence, and abnormalities in its exercise response.

Induction of chymase that forms angiotensin II in the monkey atherosclerotic aorta

Chymase shows a catalytic efficiency in the formation of angiotensin (Ang) II. In the present study, the characterization and primary structure of monkey chymase were determined, and the pathophysiological role of chymase was investigated on the atherosclerotic monkey aorta. Monkey chymase was purified from cheek pouch vascular tissue using heparin affinity and gel filtration columns. The enzyme rapidly converted Ang I to Ang II (Km = 98 microM, k(cat) = 6203/min) but did not degrade several peptide hormones such as Ang II, substance P, vasoactive intestinal peptide and bradykinin. The primary structure, which was deduced from monkey chymase cDNA, showed a high homology to that of human chymase (98%). The mRNA levels of the aorta chymase were significantly increased in the atherosclerotic aorta of monkeys fed a high-cholesterol diet. These results indicate that monkey chymase has a highly specific Ang II-forming activity and may be related to the pathogenesis of atherosclerosis.

Cardioprotective effect of angiotensin-converting enzyme inhibitors in patients with coronary artery disease

Clinical and experiments study with angiotensin-converting enzyme (ACE) inhibitors suggest that these agents may improve coronary artery disease by acting at multiple sites in the series of events leading to end-stage heart disease. These agents reduce blood pressure, improve prognosis and symptoms in patients with severe heart failure and in patients after acute myocardial infarction with left ventricular dysfunction. They are useful in the early, acute phase of myocardial infarction. More recently, ACE inhibitors have been shown to reduce in vitro vascular hypertrophy, to attenuate arteriosclerosis, and to maintain endothelium function. Whether these effects occur at clinical levels is still uncertain. The exciting clinical data have led to the proposal that alteration of ACE activity, particularly in tissue, is an important factor in development and progression of CAD. The ACE system is complex, with endocrine, paracrine, and autocrine effects. ACE is present in cardiac and vascular tissue. Therefore, the beneficial effects of ACE inhibitors can be classified as "cardio" and "vasculo" protective. This article summarizes a number of independent and complementary mechanisms pointing to a role of ACE and ACE inhibition in coronary artery disease.

Chronotropic incompetence--Part II: Clinical implications

The definition of chronotropic incompetence as inadequate chronotropic response to metabolic demand is, in theory, quite satisfactory. However, the method used in clinical practice for determination of chronotropic incompetence is far from established. The determination of chronotropic incompetence has important diagnostic, therapeutic, and prognostic implications although the exact mechanism underlying chronotropic incompetence is at present unclear. From a pacing viewpoint, chronotropic incompetence is clinically relevant only when there is a functional improvement associated with rate-responsive pacing. Rate-responsive pacing has improved the physiologic approach to artificial pacing.

Evolution of the chronotropic response to exercise after cardiac transplantation

The chronotropic response to exercise is abnormal in cardiac transplant recipients as a result of autonomic denervation. Differences in the response between recent transplant recipients and longer-term survivors have been described in previous cross-sectional studies. These changes have not been assessed directly using serial studies. The effect of sinus node dysfunction on the chronotropic response has not previously been determined. Thirty-one transplant recipients underwent serial treadmill exercise tests using the chronotropic exercise assessment protocol 3 and 6 weeks and 3 and 6 months after transplantation. Sinus node function was assessed using standard electrophysiologic techniques. The chronotropic response increased between 3 and 6 weeks after transplantation in all subjects. Six months after transplantation, there was a further marked increase in the response in a subgroup of 5 subjects. These subjects also had a dramatic decrease in heart rate on cessation of exercise. Three subjects had abnormal sinus node function. Although heart rates and chronotropic response were below average in these subjects, 2 other subjects with normal sinus node function on electrophysiologic testing had lower heart rates and worse chronotropic responses. Thus, the chronotropic response to exercise evolves over the first 6 weeks after cardiac transplantation in all subjects. In a number of recipients (16%), there is a marked increase in chronotropic response between 3 and 6 months, which suggests efferent sympathetic reinnervation. There was no clear difference in chronotropic response between subjects with and without evidence of sinus node dysfunction.

Comparison of ventricular function in atrial rate adaptive versus dual chamber rate adaptive pacing during exercise

The hemodynamic effects of two different pacing modes--rate adaptive atrial (AAIR) versus dual chamber (DDDR) pacing--were assessed in 12 patients with DDDR pacemakers during upright bicycle exercise first-pass radionuclide angiography using a multiwire gamma camera with tantalum-178 as a tracer. All patients had sinus node disease with intact AV conduction. Patients exercised to the same heart rate in random order in these two different pacing modes, AAIR and DDDR with AV delay (of 100 msec) selected to maintain 100% ventricular capture. Cardiac output increased significantly above baseline values during exercise in both pacing modes: 154 +/- 41% (mean +/- SEM, P = 0.002) with AAIR, versus 95 +/- 24% (P = 0.004) with DDDR (P = NS between the two modes). The peak filling rate, likewise, increased in both pacing modes (2.3 +/- 0.21 end-diastolic volumes/sec to 3.8 +/- 0.31 end-diastolic volumes/sec in AAIR [P = 0.0004] and 2.2 +/- 0.18 end-diastolic volumes/sec to 3.4 +/- 0.27 end-diastolic volumes/sec in DDDR [P = 0.0008]). LV ejection fraction was normal at rest (60 +/- 4%, SEM) and did not significantly change with submaximal exercise in either pacing mode (both 56%, P = NS). No significant changes in end-diastolic volume or stroke volume indexes occurred with exercise in either pacing mode. Our study demonstrates that in patients with normal resting LV function, AAIR and DDDR pacing are equally effective in attaining appropriate increases in cardiac output and LV filling during exercise.

Atrial adaptive rate pacing in sick sinus syndrome: effects on exercise capacity and arrhythmias

OBJECTIVE

To test the hypotheses that adaptive rate atrial (AAIR) pacing: significantly increases maximal exercise capacity, and results in significant suppression of supraventricular and ventricular arrhythmia compared with fixed rate atrial (AAI) pacing.

DESIGN

Prospective, randomised, single blind, crossover study with maximal treadmill exercise testing and 24 hour ambulatory electrocardiographic monitoring in AAIR and AAI modes.

SETTING

Regional pacing centre.

PATIENTS

30 consecutive patients (mean SD age 65 (12) years) with sick sinus syndrome who required permanent pacing, without evidence of conduction disturbance on 12 lead electrocardiograms or 24 hour ambulatory electrocardiographic monitoring and without other cardiovascular or systemic disease.

INTERVENTIONS

Activity sensing or minute ventilation driven systems (AAI/AAIR) were implanted alternately.

RESULTS

The mean (SD) peak heart rate in AAI mode was 122(28)v 130(22) in AAIR mode (p < 0.02) for the whole group and 104(17) v 120(5) (p < 0.003) for the patients with chronotropic incompetence. Exercise time was 12.3 (4.1) minutes in AAI and 12.3 (3.8) minutes in AAIR mode (NS) in the chronotropically incompetent patients. There were no significant differences in the Borg scores at peak exercise in AAI v AAIR mode in either group. The frequency per hour of atrial and ventricular arrhythmias showed no significant differences between the two modes in either the group as a whole or in the subgroups with chronotropic incompetence.

CONCLUSION

AAIR pacing confers little benefit in sick sinus syndrome compared with AAI pacing.

Chronotropic incompetence: a common and progressive finding in pacemaker patients

Thirty-eight patients (ages 40 to 77 years, mean 63) followed in a pacemaker clinic underwent exercise treadmill tests to determine chronotropic incompetence. There were 28 men and 10 women. Twenty-seven patients had atrioventricular (AV) block and 11 patients had sick sinus syndrome. All patients were exercised to fatigue. None of the patients were receiving beta-blockers or other drugs that could reduce heart rate. Maximum heart rate (MHR) and percent predicted maximum heart rate (% PMHR) were used as an index of chronotropic incompetence. Chronotropic incompetence was defined as inability to achieve a % PMHR of greater than 80%. The overall incidence of chronotropic incompetence was 58% (22 of 38 patients). We examined the relationship between chronotropic incompetence and the time to pacemaker implantation. We found that in patients who had pacemakers for less than 2 years, the mean MHR was 125 +/- 21.6 beats/min compared with 111.9 +/- 23.6 beats/min for patients who had pacemakers implanted for longer than 4 years. Similarly, the mean % PMHR decreased from 76.5 +/- 12.5% to 68.7 +/- 15.4% in patients with pacemakers less than 2 years versus those with pacemakers for more than 4 years. Fifty-three percent of the patients with a pacemaker less than 2 years old were chronotropic incompetent versus 70% of the patients with a pacemaker more than 4 years old. These data suggest that chronotropic incompetence worsens with time after pacemaker implant. To further support this, eight patients with AV block underwent a second stress test an average of 2 years following the first.(ABSTRACT TRUNCATED AT 250 WORDS)

"Subclinical" pacemaker syndrome: a randomised study of symptom free patients with ventricular demand (VVI) pacemakers upgraded to dual chamber devices

OBJECTIVE

To determine whether symptom free patients with single chamber pacemakers benefit from dual chamber pacing.

DESIGN

A randomised double blind crossover comparison of ventricular demand (VVI), dual chamber demand (DDI), and dual chamber universal (DDD) modes after upgrading from a VVI device.

SETTING

Cardiology outpatient department.

PATIENTS

Sixteen patients aged 41-84 years who were symptom free during VVI mode pacing for three or more years.

INTERVENTION

Pacemaker upgrade during routine generator change.

MAIN OUTCOME MEASURES

Change in subjective (general health perception, symptoms) and objective (clinical assessment, treadmill exercise, and radiological and echocardiographic indices) results between pacing modes before and after upgrading.

RESULTS

75% preferred DDD, 68% found VVI least acceptable with 12% expressing no preference. Perceived general well-being and exercise capacity (p less than 0.01) and treadmill times (p less than 0.05) were improved in DDD mode but VVI and DDI modes were similar. Clinical, echocardiographic, radiological, and electrophysiological indices confirmed the absence of overt pacemaker syndrome, although mitral and tricuspid regurgitation was greatest in VVI mode (p less than 0.01).

CONCLUSIONS

Most patients who were satisfied with long term pacing in VVI mode benefited from upgrading to DDD mode pacing suggesting the existence of "subclinical" pacemaker syndrome in up to 75% of such patients. The DDI mode offered little subjective or objective benefit over VVI mode in this population and should be reserved for patients with paroxysmal atrial arrhythmias. VVI mode pacing should be used only for patients with very intermittent symptomatic bradycardia or atrial fibrillation with a good chronotropic response during exercise.

Long-term functional integrity of atrial leads

The effectiveness and reliability of atrial leads has been questioned. We studied retrospectively, all atrial leads implanted at our center (n = 494; 438 Medtronic Model 6957J, 56 Medtronic Model 4512) over a 5-year period ending December 31, 1987, to determine the frequency of atrial lead failure (pacing, sensing, or both) and the median duration of proper pacing and sensing function for each lead model studied. Eighty-eight percent of the polyurethane atrial leads continued to function satisfactorily at 5 years, results somewhat better than those reported heretofore in the literature, as well as our own past results with a variety of different lead types. There were 29 failures of pacing, sensing, or both (6% of implants). The cumulative survival of the atrial leads at 5 years was 88%. Pacing and sensing survival were 91% +/- 2.4% and 88% +/- 2.9%, respectively. We conclude that the choice of pacing mode for a new pacemaker should be based solely on the clinical indication and not on the concern that atrial pacing and sensing will be unreliable.