***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: BACKGROUND In a previous study of 225 patients with sick-sinus syndrome r and omised to either single-chamber atrial pacing ( n=110 ) or single-chamber ventricular pacing ( n=115 ) , we found that after a mean follow-up of 3.3 years , atrial pacing was associated with significantly less atrial fibrillation and thromboembolism whereas there was no significant difference in mortality and heart failure between the two groups . We aim ed to find out whether this beneficial effect of atrial pacing is maintained during extended follow-up of up to 8 years . METHODS Follow-up visits for all patients were at 3 months , 12 months , then once a year at which patients had a physical examination , ECG recording , and pacemaker check-up . Endpoints were mortality , cardiovascular death , atrial fibrillation , thromboembolic events , heart failure , and atrioventricular block . Data was analysed on Dec 31 , 1996 . FINDINGS At long-term follow-up , 39 patients from the atrial group had died versus 57 from the ventricular group ( relative risk 0.66 [ 95 % CI 0.44 - 0.99 ] ; p=0.045 ) . 19 patients from the atrial group and 39 patients from the ventricular group died from a cardiovascular cause ( 0.47 [ 0.27 - 0.82 ] ; p=0.0065 ) . The cumulative incidences of atrial fibrillation and chronic atrial fibrillation were also significantly lower in the atrial group than in the ventricular group ( 0.54 [ 0.33 - 0.89 ] , p=0.012 and 0.35 [ 0.16 - 0.76 ] , p=0.004 , respectively ) . Thromboembolic events occurred in 13 patients in the atrial group and 26 in the ventricular group ( 0.47 [ 0.24 - 0.92 ] , p=0.023 ) . Heart failure was less severe in the atrial group than in the ventricular group ( p<0.05 ) . In multivariate analysis , atrial pacing was significantly associated with freedom from thromboembolic events ( 0.47 [ 0.24 - 0.92 ] , p=0.028 ) and survival from cardiovascular death ( 0.52 [ 0.30 - 0.91 ] , p=0.022 ) , but no longer with overall survival ( 0.71 [ 0.46 - 1.08 ] , p=0.11 ) or chronic atrial fibrillation ( 0.45 [ 0.20 - 1.05 ] , p=0.063 ) . Atrioventricular block occurred in four patients in the atrial group ( 0.6 % annual risk ) . INTERPRETATION The beneficial effect of atrial pacing found in our previous study is enhanced substantially over time . Patients with sick-sinus syndrome should be treated with an atrial rather than ventricular-pacing system because after long-term follow-up , atrial pacing is associated with a significantly higher survival , less atrial fibrillation , fewer thromboembolic complications , less heart failure , and a low-risk of atrioventricular block OBJECTIVES To evaluate the long-term functional and hemodynamic effects of right ventricular outflow tract ( RVOT ) pacing by comparison with right ventricular apical ( RVA ) pacing . BACKGROUND Acute studies have suggested that RVOT pacing could significantly improve cardiac performance in comparison with RVA pacing but no data are available in chronically implanted patients . METHODS Sixteen patients with chronic atrial tachyarrhythmia and complete AV block were included . Left ventricular ejection fraction ( LVEF ) was > or = 40 % in ten and < 40 % in six . Patients were implanted with a st and ard DDDR pacemaker connected to two ventricular leads . A screw-in lead was placed at the RVOT and connected to the atrial port . A second lead was positioned at the RVA and connected to the ventricular port . Right ventricular outflow tract and RVA pacing was achieved by programming either the AAIR or the VVIR mode respectively . Four months later patients were r and omized so as to undergo either RVOT or RVA pacing for three months according to a blind crossover protocol . Apart from the pacing mode , programming remained unchanged throughout the study . At the end of each period , NYHA class , LVEF , exercise time and maximal oxygen uptake were assessed . RESULTS No significant difference was observed between the two modes for all the parameters analyzed . These identical results were observed in all patients globally , in patients with LVEF > or = 40 % as in those with LVEF < 40 % . CONCLUSIONS Within the limits of this study , no symptomatic improvement or hemodynamic benefit was noted after three months of RVOT pacing , by comparison with RVA pacing Background Dual‐chamber ( DDDR ) pacing preserves AV synchrony and may reduce heart failure ( HF ) and atrial fibrillation ( AF ) compared with ventricular ( VVIR ) pacing in sinus node dysfunction ( SND ) . However , DDDR pacing often results in prolonged QRS duration s ( QRSd ) as the result of right ventricular stimulation , and ventricular desynchronization may result . The effect of pacing‐induced ventricular desynchronization in patients with normal baseline QRSd is unknown . Methods and Results Baseline QRSd was obtained from 12‐lead ECGs before pacemaker implantation in MOST , a 2010‐patient , 6‐year , r and omized trial of DDDR versus VVIR pacing in SND .  Cumulative percent ventricular paced ( Cum%VP ) was determined from stored pacemaker data . Baseline QRSd < 120 ms was observed in 1339 patients ( 707 DDDR , 632 VVIR ) . Cum%VP was greater in DDDR versus VVIR ( 90 % versus 58 % , P=0.001 ) . Cox models demonstrated that the time‐dependent covariate Cum%VP was a strong predictor of HF hospitalization in DDDR ( hazard ratio [ HR ] , 2.99 [ 95 % CI , 1.15 to 7.75 ] for Cum%VP > 40 % ) and VVIR ( HR 2.56 [ 95 % CI , 1.48 to 4.43 ] for Cum%VP > 80 % ) . The risk of AF increased linearly with Cum%VP from 0 % to 85 % in both groups ( DDDR , HR 1.36 [ 95 % CI , 1.09 , 1.69 ] ; VVIR , HR 1.21 [ 95 % CI 1.02 , 1.43 ] , for each 25 % increase in Cum%VP ) . Model results were unaffected by adjustment for known baseline predictors of HF hospitalization and AF . Conclusions Ventricular desynchronization imposed by ventricular pacing even when AV synchrony is preserved increases the risk of HF hospitalization and AF in SND with normal baseline QRSd . ( Circulation . 2003;107:2932‐2937 . OBJECTIVES We sought to evaluate the long-term effects of alternative right ventricular pacing sites on myocardial function and perfusion . BACKGROUND Previous studies have demonstrated that asynchronous ventricular activation due to right ventricular apical ( RVA ) pacing alters regional myocardial perfusion and functions . METHODS We r and omized 24 patients with complete atrioventricular block to undergo permanent ventricular stimulation either at the RVA ( n = 12 ) or right ventricular outflow ( RVOT ) ( n = 12 ) . All patients underwent dipyridamole thallium myocardial scintigraphy and radionuclide ventriculography at 6 and 18 months after pacemaker implantation . RESULTS After pacing , the mean QRS duration was significantly longer during RVA pacing than during RVOT pacing ( 151 + /- 6 vs. 134 + /- 4 ms , p = 0.03 ) . At six months , the incidence of myocardial perfusion defects ( 50 % vs. 25 % ) and regional wall motion abnormalities ( 42 % vs. 25 % ) and the left ventricular ejection fraction ( LVEF ) ( 55 + /- 3 % vs. 55 + /- 1 % ) were similar during RVA pacing and RVOT pacing ( p > 0.05 ) . However , at 18 months , the incidence of myocardial perfusion defects ( 83 % vs. 33 % ) and regional wall motion abnormalities ( 75 % vs. 33 % ) were higher and LVEF ( 47 + /- 3 vs. 56 + /- 1 % ) was lower during RVA pacing than during RVOT pacing ( all p < 0.05 ) . Patients with RVA pacing had a significant increase in the incidence of myocardial perfusion defects ( p < 0.05 ) and a decrease in LVEF ( p < 0.01 ) between 6 and 18 months , but patients with RVOT pacing did not ( p > 0.05 ) . CONCLUSIONS This study demonstrates that preserved synchronous ventricular activation with RVOT pacing prevents the long-term deleterious effects of RVA pacing on myocardial perfusion and function in patients implanted with a permanent pacemaker This study compares LV performance during high right ventricular septal ( RVS ) and apical ( RVA ) pacing in patients with LV dysfunction who underwent His-bundle ablation for chronic AF . We inserted a passive fixation pacing electrode into the RVA and an active fixation electrode in the RVS . A dual chamber , rate responsive pulse generator stimulated the RVA through the ventricular port and the RVS via the atrial port . Patients were r and omized to initial RVA ( VVIR ) or RVS ( AAIR ) pacing for 2 months . The pacing site was reversed during the next 2 months . At the 2 and 4 month follow-up visit , each patient underwent a transthoracic echocardiographical study and a rest/exercise first pass radionuclide ventriculogram . We studied nine men and three women ( mean age of 68 + /- 7 years ) with congestive heart failure functional Class ( NYHA Classification ) : I ( 3 patients ) , II ( 7 patients ) , and III ( 2 patients ) . The QRS duration was shorter during RVS stimulation ( 158 + /- 10 vs 170 + /- 11 ms , P < 0.001 ) . Chronic capture threshold and lead impedance did not significantly differ . LV fractional shortening improved during RVS pacing ( 0.31 + /- 0.05 vs 0.26 + /- 0.07 , P < 0.01 ) . RVS activation increased the resting first pass LV ejection fraction ( 0.51 + /- 0.14 vs 0.43 + /- 0.10 , P < 0.01 ) . No significant difference was observed during RVS and RVA pacing in the exercise time ( 5.6 + /- 3.2 vs 5.4 + /- 3.1 , P = 0.6 ) or the exercise first pass LV ejection fraction ( 0.58 + /- 0.15 vs 0.55 + /- 0.16 , P = 0.2 ) . The relative changes in QRS duration and LV ejection fraction at both pacing sites showed a significant correlation ( P < 0.01 ) . We conclude that RVS pacing produces shorter QRS duration and better chronic LV function than RVA pacing in patients with mild to moderate LV dysfunction and chronic AF after His-bundle ablation CONTEXT Implantable cardioverter defibrillator ( ICD ) therapy with backup ventricular pacing increases survival in patients with life-threatening ventricular arrhythmias . Most currently implanted ICD devices provide dual-chamber pacing therapy . The most common comorbid cause for mortality in this population is congestive heart failure . OBJECTIVE To determine the efficacy of dual-chamber pacing compared with backup ventricular pacing in patients with st and ard indications for ICD implantation but without indications for antibradycardia pacing . DESIGN The Dual Chamber and VVI Implantable Defibrillator ( DAVID ) Trial , a single-blind , parallel-group , r and omized clinical trial . SETTING AND PARTICIPANTS A total of 506 patients with indications for ICD therapy were enrolled between October 2000 and September 2002 at 37 US centers . All patients had a left ventricular ejection fraction ( LVEF ) of 40 % or less , no indication for antibradycardia pacemaker therapy , and no persistent atrial arrhythmias . INTERVENTIONS All patients had an ICD with dual-chamber , rate-responsive pacing capability implanted . Patients were r and omly assigned to have the ICDs programmed to ventricular backup pacing at 40/min ( VVI-40 ; n = 256 ) or dual-chamber rate-responsive pacing at 70/min ( DDDR-70 ; n = 250 ) . Maximal tolerated medical therapy for left ventricular dysfunction , including angiotensin-converting enzyme inhibitors and beta-blockers , was prescribed to all patients . MAIN OUTCOME MEASURE Composite 
    Output:
    
R and omized-controlled trials provided inconclusive results with respect to exercise capacity , functional class , quality of life , and survival .