Determination of the optimal atrioventricular delay in DDD pacing. Comparison between echo and peak endocardial acceleration measurements.

The goal of this study was to compare two methods determining the optimal atrioventicular delay (AVD) in 19 patients implanted with the BEST-Living system for complete heart block. The definition of the optimal AVD was: the AVD with the echo method that provided the longest diastolic filling time without interruption of the A wave, and the AVD with the peak endocardial acceleration (PEA) method, corresponding to the knee of the PEA curve vs AV delay. The amplitude of the PEA was measured for every AVD programmed via an automatic scanner in steps of 60 to 300 ms (40 ms steps): in the VDD pacing mode with a low base rate, to obtain 100% sensed P waves; in DDD with a base rate = sinus rate + 20%, to obtain 100% paced P waves. Echocardiographic (Echo) measurement of the left ventricular filling time were performed in the same AV delay settings in VDD and DDD as the ones tested in the PEA method, which were manually programmed. The optimal AVDs obtained in DDD and those obtained in VDD were compared in the echo and the PEA tests by a paired Student's t-test. The optimal AVDs obtained by both Echo and by PEA were also compared by a paired Student's t-test in VDD and DDD. The r value of the correlation between the optimal AVDs obtained by Echo and those obtained by PEA was calculated. Similar values of optimal AVD were obtained with both methods. The optimal AVDs given by the Echo technique (179 +/- 25 ms in DDD and 124 +/- 18 ms in VDD) were slightly, but significantly shorter than the ones obtained with the PEA method (202 +/- 21 ms in DDD and 145 +/- 18 ms in VDD, P < 0.05). A highly significant difference between AVD VDD and AVD DDD was found with both methods (P < 0.001). The correlation between the AVDs obtained with the echo and the PEA methods was highly significant (r = 0.78, P < 0.01). Pacemaker software could be modified to determine automatically the optimal AVDs to be applied throughout the heart rate range.     

Possible acute and chronic synergistic effect of dual chamber pacing and disopyramide in obstructive hypertrophic cardiomyopathy: a case report

It remains unclear whether the combination of dual‐chamber (DDD) pacing and disopyramide can achieve prolonged left ventricular outflow tract (LVOT) gradient reduction and symptom relief in patients with obstructive hypertrophic cardiomyopathy (HCM). In an HCM patient with a severe LVOT gradient, the combination of DDD pacing and disopyramide achieved marked improvement of gradient in the catheter laboratory and also after medium‐term follow‐up. The patient's severe dyspnoea was alleviated during the follow‐up period. This combination might enable physicians to treat and manage elderly symptomatic obstructive HCM patients with a severe LVOT gradient more effectively and less invasively.     

Upgrade to dual chamber pacing after long-term ventricular stimulation: feasibility and intermediate term follow-up.

AIMS: To evaluate the feasibility and follow-up results of atrial lead implantation and a change to dual chamber pacing following long-term treatment with single chamber ventricular stimulation. METHODS AND RESULTS: During a 30-month period, 70 consecutive patients with ventricular pacemakers were referred for pulse generator exchange or lead reoperation. Using defined criteria, an upgrade procedure was considered indicated in 34 of the cases (49%); these patients had a mean age of 74.8+/-8.8 years, and had been treated with VVI or VVIR pacing for a mean time of 7.8+/-3.8 years (range 1.8-17). An atrial lead was successfully implanted via ipsilateral subclavian venipuncture through the existing pectoral pacemaker pocket in 33 of the 34 cases (97% of the attempts). Postoperatively, one atrial lead dislodgement was seen, and another patient required atrial lead adjustment due to P wave undersensing. The mean follow-up period was 14+/-10 months. During this time, four patients developed permanent atrial fibrillation (annual incidence 11%. In 82% of the patients in whom an upgrade procedure was attempted, dual chamber pacing was maintained at the end of follow-up. CONCLUSION: Restoration of AV synchrony is possible in a substantial proportion of patients treated with long-term ventricular stimulation. Atrial lead placement through ipsilateral subclavian venipuncture is generally feasible, and the vast majority of cases remain in dual chamber pacing with normal function during intermediate term follow-up.     

Esophageal ECG recording--a valuable diagnostic tool in dual chamber pacing

The interpretation of the ECG of a dual chamber pacemaker necessitatesthe identification of atrial activity. This is a prerequisitefor the evaluation of pacemaker function and for the correctadjustment of programmable pulse generators. The assessmentof atrial capture in standard 12-lead ECGs is, however, sometimesrather difficult. Esophageal ECG recording by means of a reusableunipolar electrode, inserted transnasally, and connected toa standard ECG recorder, is a simple, rapid and inexpensivemethod for the reliable identification of P-waves. Clinical examples are presented to illustrate the value of thistechnique in determining atrial capture and as a tool for thedifferential diagnosis of pacemaker-involved tachycardias. Theuse ofesophageal ECG recording in the clinical follow-up ofpatients with dual chamber pacemakers isrecommended.     

Left univentricular pacing for cardiac resynchronization therapy using rate-adaptive atrioventricular delay

Objective To evaluate left univentricular (LUV) pacing for cardiac resynchronization therapy (CRT) using a rate-adaptive atrioventricular delay (RAAVD) algorithm to track physiological atrioventricular delay (AVD). Methods A total of 72 patients with congestive heart failure (CHF) were randomized to RAAVD LUV pacing versus standard biventricular (BiV) pacing in a 1: 1 ratio. Echocardiography was used to optimize AVD for both groups. The effects of sequential BiV pacing and LUV pacing with optimized A-V (right atrio-LV) delay using an RAAVD algorithm were compared. The standard deviation (SD) of the S/R ratio in lead V1 at five heart rate (HR) segments (R_S/R-SD5), defined as the “tracking index,” was used to evaluate the accuracy of the RAAVD algorithm for tracking physiological AVD. Results The QRS complex duration (132 ± 9.8 vs. 138 ± 10 ms, P < 0.05), the time required for optimization (21 ± 5 vs. 50 ± 8 min, P < 0.001), the mitral regurgitant area (1.9 ± 1.1 vs. 2.5 ± 1.3 cm2, P < 0.05), the interventricular mechanical delay time (60.7 ± 13.3 ms vs. 68.3 ± 14.2 ms, P < 0.05), and the average annual cost (13,200 ± 1000 vs. 21,600 ± 2000 RMB, P < 0.001) in the RAAVD LUV pacing group were significantly less than those in the standard BiV pacing group. The aortic valve velocity-time integral in the RAAVD LUV pacing group was greater than that in the standard BiV pacing group (22.7 ± 2.2 vs. 21.4 ± 2.1 cm, P < 0.05). The R_S/R-SD5 was 4.08 ± 1.91 in the RAAVD LUV pacing group, and was significantly negatively correlated with improved left ventricular ejection fraction (LVEF) (?LVEF, Pearson’s r = ?0.427, P = 0.009), and positively correlated with New York Heart Association class (Spearman’s r= 0.348, P = 0.037). Conclusions RAAVD LUV pacing is as effective as standard BiV pacing, can be more physiological than standard BiV pacing, and can decrease the average annual cost of CRT.     

Dual chamber pacing with optimal AV delay in congestive heart failure: a randomized study.

AIM: A prospective randomized trial was set up to evaluate contractile parameters and quality of life in patients with congestive heart failure. METHODS AND RESULTS: We describe the results from 38 patients in sinus rhythm and with chronic heart failure due to congestive cardiomyopathy, prospectively randomized to optimal medical therapy (Group 1, 19 patients) or optimal medical therapy plus dual chamber pacemaker programmed to optimal AV delay (Group 2, 19 patients). At a 6 month follow-up, 7/19 patients in Group 1 had died compared with 5/19 patients in Group 2. During follow-up, there were few significant changes in evaluated parameters except for mitral regurgitation time, which was prolonged in Group 1 and shortened in Group 2. The systolic left ventricular diameter shortened significantly only in Group 2. An energy and activity questionnaire showed that the effect of DDD pacing in the latter patient population was beneficial. CONCLUSIONS: From these results we may conclude that at the 6 month follow-up DDD pacing with echo-optimized AV interval programming can improve quality of life without affecting survival.     

房室间期自动搜索功能对心室起搏及心功能的影响

目的 观察自动AV间期搜索功能双腔起搏器减少心室起搏和高频心房事件的效果及对心功能的影响.方法 60例置入DDD/R起搏器的患者（有AV搜索功能30例,无AV搜索功能30例）,术后1年内程控获取右室起搏百分比、高频心房事件,检查超声心动图,测试血浆利钠肽（BNP）值.结果 有AV搜索功能组术后6个月和12个月右室起搏百分比明显小于无AV搜索功能组[（21.2±6.0）％比（78.3±7.5）％,（19.1±6.5）％比（73.4±7.9）％,P均＜0.05）.AV搜索功能组左室射血分数、左室Tei指数均明显改善（0.57±0.03比0.53±0.05,0.48±0.15比0.68±0.20,P均＜0.05）;BNP水平明显降低[（75.2±34.5）pg/ml）比（37.0±16.4）pg/ml,P＜0.05];高频心房事件也显著减少[（42±10）次比（19±11）次,P＜0.05].结论 AV自动搜索功能起搏器可有效减少不必要的右室起搏及高频心房事件,改善血流动力学效应.     

Functional assessment of patients treated with permanent dual chamber pacing as a primary treatment for hypertrophic cardiomyopathy

Patients with severe hypertrophic cardiomyopathy pase a difficultmanagement problem. Between 1984 and 1986, 11 such patientshave been treated by dual chamber pacing. (DDD). Subjectivelyall patients improved and objectively there was an increasein exercise tolerance during paced rhythm.     

Usefulness of bifocal pacing in patients with heart failure and intraventricular conduction delay

BACKGROUND: Bifocal pacing (BFP) has been proposed as a feasible alternative to cardiac resynchronization therapy. AIM: To evaluate BFP in patients with severe heart failure and significant intraventricular conduction delay and to compare it with biventricular pacing (BVP). METHODS: Echocardiographic examination including TDI and invasive measurement of haemodynamics was performed under basal conditions, during BFP and during BVP. RESULTS: Fifty patients were included: 29 with ischaemic heart disease (IHD), 21 with idiopathic dilated cardiomyopathy (IDCM). LV dp/dt(max) increased during BFP compared to the basal state (13.4%, 95% CI 9.2-17.6%, p<0.0001) and a further increase was achieved during BVP (29.5%, 95% CI 23.7-35.4%, p<0.0001). A significant correlation was found between the distance of the right ventricular apical and outflow tract leads and percentage of dp/dt(max) increase in IDCM patients (r=0.72), but not in IHD patients. Interventricular mechanical delay (IVMD) decreased in BFP (43+/-22 ms vs. 53+/-31 ms, p=0.006), but BVP produced even shorter IVMD (22+/-19 ms, p<0.0001). In all patients, regional systolic contraction times were significantly shortened, corresponding with prolongation of the respective regional diastolic filling times during both BFP (p<0.05 for all segments) and BVP (p<0.001 for all segments). CONCLUSIONS: BFP improves LV haemodynamics by decreasing the inter- and intraventricular conduction delays. The leads in the right ventricle should be placed at the longest achievable distance. BVP is superior to BFP.     

Use of a new cardiac pacing mode designed to eliminate unnecessary ventricular pacing.

AIMS: To examine the performance of AAIsafeR2, a new pacing mode to minimize the cumulative proportion of ventricular pacing in patients who do not need regular ventricular support. METHODS AND RESULTS: The safety of AAIsafeR2 was examined in 123 recipients (73 +/- 12 years old, 51% men) of dual chamber pacemakers implanted for sinus node dysfunction, paroxysmal AV block or the bradycardia-tachycardia syndrome. Data were collected from pacemaker diagnostics, and the first 43 patients underwent 24-h Holter recordings before being discharged from the hospital with AAIsafeR2 activated. No adverse event related to AAIsafeR2 was observed. All ventricular pauses detected on Holter tapes triggered immediate back-up ventricular pacing. Appropriate switches to DDD occurred in 97 of 123 patients. In 69 of 123 devices (56%) switches to DDD were non-sustained, and the average % ventricular pacing in this group was 0.2+/-0.5%. CONCLUSION: AAIsafeR2 mode seems to be safe and reliable in patients with infrequent slowing or pauses in ventricular activity, while maintaining ventricular pacing below 1%.     

Non-invasive study of dual chamber pacing by pulsed Doppler. Prediction of the haemodynamic response by echocardiographic measurements

The increase in stroke volume with DDD compared with VVI pacingwas measured at rest using pulsed Doppler echocardiography in23 patients at a pacing rate of 70 beats min^–1. Strokevolume was assessed by measuring the velocity integral of theflow at the level of the aortic annulus using the apical five-chamberwindow. Pulsed Doppler echocardiography allowed determinationof the least and most favourable A V delay haemodynamically.TVI was also measured at each nominal value of AV delay. The percentage increase in stroke volume was determined in everypatient changing from VVI to optimum DDD pacing and was usedas a measurement of the ‘sensitivity’ to optimumDDD pacing; the mean increase was 27 ±19%. The increasein stroke volume accompanying the change from DDD pacing withthe least favourable to the optimum A V delay was also measured,and used as a measurement of ‘sensitivity’ to changesin A V delay; the mean increase was 23.7 ± 16.3%. Clinicaland standard echocardiographic parameters were studied in orderto determine which variable might best identify the patientsmore likely to benefit from DDD pacing, and to identify thosemore sensitive to the A V delay setting. With respect to sensitivityto DDD pacing, three echocardiographic variables were selectedby linear discriminant analysis from 11 clinical and echocardiographicvariables. These were, in order of importance, left ventricularsystolic diameter (LVSD), left ventricular wall thickness (LVWT)and left atrial size (LAS) which allowed the prediction of agood or a bad response to optimal DDD pacing with an accuracyof 91–3%. The discriminant values between responders (morethan 25% increase in TVI) and non–responders (less than25% increase) were 36.4mm (LVSD), 10.1 mm (LVWT) and 35.8 mm(LAS) respectively. With respect to sensitivity to changes inA V delay, the best predictive parameter was left ventriculardiastolic diameter (LVDD), which gave a predictive accuracyof 87.5%.     

Mode switching in dual chamber pacemakers: effect of onset criteria on arrhythmia-related symptoms.

AIMS: Various mode-switching algorithms are available with different tachyarrhythmia detection criteria to be satisfied to initiate mode-switching. This study evaluated three different mode-switching algorithms in patients with paroxysmal atrial fibrillation. METHODS AND RESULTS: Seventeen patients completed the study. Three mode-switching algorithms were downloaded as software into the pacemaker, each for 1 month in a single-blind, randomized sequence. The criteria to initiate mode-switching were: mean atrial rate ('standard'), '4-of-7' or '1-of-1' atrial intervals to exceed the atrial detection rate. Symptoms for each were measured using the Symptom Checklist Frequency and Severity index. The median number of mode-switch episodes increased from 20 for 'standard' to 39 for '4-of-7' (P=0.029 vs 'standard') and 103 for '1-of-1' (P=0.0012 vs 'standard') onset criteria. Median duration of episodes decreased from 2.5 min with 'standard' to 1.4 min with '4-of-7' and 0.4 min with '1-of-1' onset criteria. Frequency of symptoms was lower using '4-of-7' (18.2 +/- 12.0 vs 23 +/- 12.0, P=0.08) or '1-of-1' (20.4 +/- 12.4 vs 23 +/- 12.0, P=0.07) than 'standard' onset criteria. Severity of arrhythmia tended to be less with either '4-of-7' (16 +/- 10.4 vs 19.1 +/- 19.4, P=0.12) or '1-of-1' (17.5 +/- 10.3 vs 19.1 +/- 9.4, P=0.18) than with 'standard' onset criteria. CONCLUSIONS: The more sensitive onset criteria for detection of atrial tachyarrhythmias were associated with lower frequency and severity of symptoms.     

不同起搏方式对心房电机械延迟影响的对比研究

了解双腔起搏 (DDD)患者右心耳起搏 (RAA)方式对心房电机械延迟 (AEMD)的影响。对 2 1例置入DDD的患者 ,用M型超声心动图结合同步心电图分别测量DDD方式及心房感知心室起搏 (VDD)方式下的AEMD。结果 :RAA起搏与窦性节律比较 ,AEMD明显增加 ,其中P波起始至中央纤维体 (CFB)运动发生的时间增加 2 8± 4ms、至CFB最大收缩振幅出现的时间增加 42± 3ms、至左房侧壁 (LLA)运动发生的时间增加 35± 5ms、至LLA最大收缩振幅出现的时间增加 34± 4ms (所有P <0 .0 0 1)。结论 :DDD患者右心耳起搏能明显增加AEMD     

Interatrial septum pacing avoids the adverse effect of interatrial delay in biventricular pacing: an echo-Doppler evaluation.

AIMS: Biventricular pacing (BVP) can improve haemodynamics in patients with dilated cardiomyopathy (DCM) and left bundle branch block by reducing interventricular delay (IVD). Since in DCM interatrial delay (IAD) and IVD frequently coexist, the aim of this study was to test the hypothesis that IVD reduction associated with IAD produces an imbalance between the programmed right atrioventricular (AV) delay and the effective AV delay on the left, and that interatrial septum pacing (IASP) combined with BVP overcomes this adverse effect. METHODS AND RESULTS: IAD, IVD, left and right mechanical atrioventricular delay (L-RMAVD) were measured by echo-Doppler in 29 patients with BVP: 17 patients (Group A) had the atrial lead in the right atrial appendage, 12 patients (Group B) who experienced paroxysmal atrial fibrillation had the atrial lead on the interatrial septum. In Group A, LMAVD was significantly shorter than RMAVD (172 +/- 24 vs 207 +/- 24 ms, P<0.002), IAD was significantly longer than IVD (52 +/- 24 vs 21 +/- 18 ms, P<0.0001). In Group B, no differences were observed between LMAVD and RMAVD (187 +/- 32 vs 185 +/- 28 ms, NS), and between IAD and IVD (11 +/- 12 vs 13 +/- 16 ms, NS). CONCLUSIONS: IAD produces different left and right atrioventricular sequences in BVP. IASP combined with BVP, by resynchronizing both atria and ventricles, is able to avoid this adverse effect.     

有自动夺获功能的双腔起搏器的临床随访

目的观察双腔心室自动阈值夺获起搏器长期的参数变化及安全性.方法我院1999年10月至2000年6月住院植入具有心室自动阈值夺获功能的双腔起搏器20例,观察术中、术后1周、2周、1个月、3个月及6个月后的心室起搏阈值、输出电压、ER振幅、阻抗、极化电位及自动工作方式转换功能.结果术后早期心室起搏阈值、极化电位轻度升高,2周后趋于稳定,其余参数无明显变化,长期的心室起搏电压平均为(1.26±0.38)V.15例术后即刻可以启动自动夺获功能,4例术后1周可启动自动夺获功能,随访期间功能正常,无起搏脱漏现象,且5例伴有阵发性心房颤动的患者,心房颤动时均可发生工作方式转换,未出现起搏器介入性心动过速.1例肥厚梗阻性心肌病患者因术中及术后极化电位较高致自动夺获功能不能启动.结论有自动夺获功能的双腔起搏系统耗能低,安全可靠,满足了患者的生理需要.但对有心内膜病变患者有待进一步观察.     

Cost benefit analysis of single and dual chamber pacing for sick sinus syndrome and atrioventricular block: An economic sensitivity analysis of the literature

The benefits of dual (DDD) over single chamber pacing (VVI)have been demonstrated in haemodynamics, exercise capacity,quality of life and reduced complications in atrioventncularblock and sick sinus syndrome. The literature was reviewed to provide complication rates fordual and VV1 pacing. Cost calculations were based on UnitedKingdom 1991 prices. Over a 10-year period, a computer modelcalculated the incidence and prevalence of atrial fibrillation,stroke, permanent disability, heart failure and mortality insix patient categories: sick sinus syndrome paced VVI, sicksinus syndrome upgraded to DDD, sick sinus syndrome paced DDDfrom outset, atrioventricular block paced VYI and those upgradedto DDD and atrio ventricular block paced initially DDD. Calculationswere based on intention to treat. The 10 year survival with DDD vs YVI pacing was 71% vs 57% insick sinus syndrome and 61% vs 51%, respectively, in atrioventricularblock. In both indications the prevalence of heart failure inthe 10 year survivors was 600 lower with DDD pacing. In sicksinus syndrome patients paced VVI, 36% had severe disabilitywhile only 8% experienced this with DDD pacing. For atrioventricularblock the figures were, respectively, 22% vs 3%. The differencein 10 year cumulative cost between VVI and DDD is 13 times thepurchase price of a VVI pulse generator for sick sinus syndromeand 7 times for atrioventricular block. In the third year afterimplantation the cumulative costs of DDD were lower than forVVI for both indications. Dual chamber pacing for both indications, sick sinus syn dromeand atrioventricular block, is both clinically and cost effective.     

Repetitive reentrant and non-reentrant ventriculoatrial synchrony in dual chamber pacing.

Repetitive retrograde ventriculoatrial (VA) conduction in patients with dual chamber pacemakers may cause two forms of VA synchrony. (1) Endless loop tachycardia (pacemaker-mediated tachycardia) or repetitive reentrant VA synchrony occurs when the pacemaker senses retrograde P waves. Appropriate programming can prevent pacemaker reentrant tachycardia in almost all cases. However, the measures used to control tachycardia may themselves create new problems. (2) AV desynchronization arrhythmia or repetitive non-reentrant AV synchrony occurs when the pacemaker does not sense retrograde P waves. In this form of VA synchrony, the atrial stimulus is ineffectual because it falls in the atrial myocardial refractory period generated by the preceding unsensed retrograde P wave. A long atrioventricular interval and a relatively fast lower rate (or sensor-driven rate with DDDR pacing) favor the development of AV desynchronization arrhythmia and its unfavorable hemodynamic consequences.     

双心腔起搏治疗肥厚梗阻性心肌病患者的长期随访

目的　评价双心腔起搏对肥厚梗阻性心肌病患者的远期疗效。方法　 15例确诊为肥厚梗阻性心肌病并植入双心腔起搏器的患者 ,男性 11例 ,女性 4例 ,平均年龄 (5 0 2± 15 5 )岁 ,程控为DDD起搏方式。对患者术前、术后第 1次随访平均 (5 1± 1 6 )个月及最近 1次随访平均(2 7 3± 3 7)个月的临床症状、心功能、和超声心动图下的血流动力学指标进行分析。结果　 9例晕厥患者在第 1次评价时即未再发作 ,2例先兆晕厥的患者 ,在第 1次评价时仍有 1例有偶发 ,但在第2次评价时症状也消失。 15例气促、 13例胸痛、 12例心悸患者 ,在第 1次评价时分别有 6例、 7例、6例症状完全消失 ,总消除率约 4 0 %。在第 2次评价时又分别有 5例、 2例、 3例患者症状完全消失 ,进一步症状消除率约 2 0 %。患者左心室流出道跨瓣压差由术前平均 (79 9± 32 6 )mmHg (1mmHg=0 133kPa) ,经半年起搏治疗后降为 (43 2± 18 9)mmHg ,3年后进一步降为 (40 4± 2 4 1)mmHg ,两次结果与术前比较均有统计学意义 (P <0 0 1) ;心功能由术前的 (2 6± 0 4 )级 (NYHA分级 ) ,提高到第 1次随访时的 (1 7± 0 3)级 (P <0 0 1) ,第 2次随访时的 (1 1± 0 2 )级 (P <0 0 1) ;E/A峰值 ,在术前、第 1次随访、第 2次随访时分别为 0 8±     

Antiarrhythmic benefits of dual chamber stimulation with rate-response in patients with paroxysmal atrial fibrillation and chronotropic incompetence: a prospective, multicentre study.

BACKGROUND: While the haemodynamic benefits of DDDR pacing compared with DDD pacing in patients with brady-tachy syndrome and chronotropic incompetence (CI) are well demonstrated, the antiarrhythmic advantage is controversial and so far not clearly demonstrated. AIM: We have performed a prospective, randomized, multicentre study to evaluate the efficacy of DDDR and DDD pacing modes in preventing paroxysmal atrial fibrillation (PAF) episodes in patients with brady-tachy syndrome and CI. METHODS AND RESULTS: Seventy-eight patients were included in the study. All patients had a dual chamber pacemaker implanted and were randomly programmed to DDD or DDDR with a cross over (DDD --> DDDR or vice versa) at 3 months. The final evaluation was performed at 6 months by means of two self-administered symptom questionnaires to evaluate activity. Symptoms of palpitations were analysed and scored. The patients were less symptomatic with the DDDR mode. The number of mode-switch activations compared with symptomatic episodes of PAF confirmed the high rate of asymptomatic PAF episodes in patients with brady-tachy syndrome. We conclude that in a small but well defined population of patients affected by sick sinus syndrome with CI and severely symptomatic PAF, DDDR pacing compared with DDD pacing may offer an additional antiarrhythmic benefit and should be considered the primary mode of pacing.