***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 — Left ventricular ( LV ) pacing alone may theoretically avoid deleterious effects of right ventricular pacing . Methods and Results — In a multicenter , double-blind , crossover trial , we compared the effects of LV and biventricular ( BiV ) pacing on exercise tolerance and LV remodeling in patients with an LV ejection fraction ⩽35 % , QRS ≥120 milliseconds , and symptoms of heart failure . A total of 211 patients were recruited from 11 centers . After a run-in period of 2 to 8 weeks , 121 qualifying patients were r and omized to  LV followed by BiV pacing or vice versa for consecutive 6-month periods . The greatest improvement in New York Heart Association class and 6-minute walk test occurred during the run-in phase before r and omization . Exercise duration at 75 % of peak VO2 ( primary outcome ) increased from 9.3±6.4 to 14.0±11.9 and 14.3±12.5 minutes with LV and BiV pacing , respectively , with no difference between groups ( P=0.4327 ) . LV ejection fraction improved from 24.4±6.3 % to 31.9±10.8 % and 30.9±9.8 % with LV and BiV pacing , respectively , with no difference between groups ( P=0.4530 ) . Reductions in LV end-systolic volume were likewise similar ( P=0.6788 ) . The proportion of clinical responders ( ≥20 % increase in exercise duration ) to LV and BiV pacing was 48.0 % and 55.1 % ( P=0.1615 ) . Positive remodeling responses ( ≥15 % reduction in LV end-systolic volume ) were observed in 46.7 % and 55.4 % ( P=0.0881 ) . Overall , 30.6 % of LV nonresponders improved with BiV and 17.1 % of BiV nonresponders improved with LV pacing . Conclusion — LV pacing is not superior to BiV pacing . However , nonresponders to BiV pacing may respond favorably to LV pacing , suggesting a potential role as tiered therapy . Clinical Trial Registration — URL : http://www . clinical trials.gov . Unique identifier : NCT00901212 BACKGROUND Biventricular ( BiV ) stimulation is the preferred means of delivering cardiac resynchronization therapy ( CRT ) , although left ventricular (LV)-only stimulation might be as safe and effective . B-LEFT HF is a prospect i ve , multicenter , r and omized , double-blind study aim ed to examine whether LV-only is noninferior to BiV pacing regarding clinical and echocardiographic responses . METHODS B-LEFT HF r and omly assigned 176 CRT-D recipients , in New York Heart Association class III or IV , with an LV ejection fraction < or = 35 % and QRS > or = 130 milliseconds , to a BiV ( n = 90 ) versus LV ( n = 86 ) stimulation group . Clinical status and echocardiograms were analyzed at baseline and 6 months after CRT-D implant to test the noninferiority of LV-only compared with BiV stimulation . RESULTS The proportion of responders was in line with current literature on CRT , with improvement in heart failure composite score in 76.2 % and 74.7 % of patients in BiV and LV groups , respectively . Comparing LV versus BiV pacing , the small differences in response rates and corresponding 95 % CI indicated that LV pacing was noninferior to BiV pacing for a series of response criteria ( combination of improvement in New York Heart Association and reverse remodeling , improvement in heart failure composite score , reduction in LV end-systolic volume of at least 10 % ) , both at intention-to-treat and at per- protocol analysis . CONCLUSIONS Left ventricular-only pacing is noninferior to BiV pacing in a 6-month follow-up with regard to clinical and echocardiographic responses . Left ventricular pacing may be considered as a clinical alternative option to BiV pacing OBJECTIVES We compared the effects of triple-site versus dual-site biventricular stimulation in c and i date s for cardiac resynchronization therapy . BACKGROUND Conventional biventricular stimulation with a single right ventricular ( RV ) and a single left ventricular ( LV ) lead is associated with persistence of cardiac dyssynchrony in up to 30 % of patients . METHODS This multicenter , single-blind , crossover study enrolled 40 patients ( mean age 70 + /- 9 years ) with moderate-to-severe heart failure despite optimal drug treatment , a mean LV ejection fraction of 26 + /- 11 % , and permanent atrial fibrillation requiring cardiac pacing for slow ventricular rate . A cardiac resynchronization therapy device connected to 1 RV and 2 LV leads , inserted in 2 separate coronary sinus tributaries , was successfully implanted in 34 patients . After 3 months of biventricular stimulation , the patients were r and omly assigned to stimulation for 3 months with either 1 RV and 2 LV leads ( 3-V ) or to conventional stimulation with 1 RV and 1 LV lead ( 2-V ) , then crossed over for 3 months to the alternate configuration . The primary study end point was quality of ventricular resynchronization ( Z ratio ) . Secondary end points included reverse LV remodeling , quality of life , distance covered during 6-min hall walk , and procedure-related morbidity and mortality . Data from the 6- and 9-month visits were combined to compare end points associated with 2-V versus 3-V. RESULTS Data eligible for protocol -defined analyses were available in 26 patients . No significant difference in Z ratio , quality of life , and 6-min hall walk was observed between 2-V and 3-V. However , a significantly higher LV ejection fraction ( 27 + /- 11 % vs. 35 + /- 11 % ; p = 0.001 ) and smaller LV end-systolic volume ( 157 + /- 69 cm(3 ) vs. 134 + /- 75 cm(3 ) ; p = 0.02 ) and diameter ( 57 + /- 12 mm vs. 54 + /- 10 mm ; p = 0.02 ) were observed with 3-V than with 2-V. There was a single minor procedure-related complication . CONCLUSIONS Cardiac resynchronization therapy with 1 RV and 2 LV leads was safe and associated with significantly more LV reverse remodeling than conventional biventricular stimulation AIMS To investigate whether biventricular ( BIV ) pacing  preserves left ventricular ejection fraction ( LVEF ) and reduces LV dyssynchrony when compared with st and ard dual-chamber right ventricular ( RV ) pacing in consecutive patients with high- grade atrioventricular block during 3 years of pacing . METHODS AND RESULTS Fifty patients were r and omized to RV pacing or BIV pacing .  LVEF was measured using three-dimensional echocardiography . Tissue Doppler imaging was used to quantify LV dyssynchrony in terms of the st and ard deviation of the time-to-peak velocity ( Ts-SD ) . LVEF differed significantly between the two groups during 3 years of pacing ( ANOVA : P=0.003 ) . LVEF in the RV group decreased from 59±5 % at baseline to 53±11 % ( P=0.01 ) , while LVEF remained unchanged in the BIV group ( 57±7 % at baseline vs. 58±10 % ( P=0.40 ) . After 3 years of follow-up , we observed no difference in LV dyssynchrony , LV remodelling or measurements of clinical heart failure ( N-terminal pro-brain natriuretic peptide , walking test , and New York Heart Association functional class ) between the two groups . However , in the RV group , but not in the BIV group , dyssynchrony increased significantly ( P=0.005 ) during follow-up . Furthermore , adverse LV remodelling was observed in the RV group with increased systolic volume and thinning of the LV septum . CONCLUSION BIV pacing preserves LVEF and minimizes LV dyssynchrony during long-term follow-up . Adverse remodelling observed during 3 years of RV pacing was prevented by BIV pacing . However , the adverse impact of RV pacing on LV function was not reflected in measures of clinical heart failure . CLINICAL TRIAL REGISTRATION www . clinical trials.gov ( identification number : NCT00228241 ) 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 assessed the clinical efficacy of single-site left ventricular ( LV ) pacing and determined the impact of baseline conduction delay severity on the magnitude of benefit . BACKGROUND Multisite biventricular pacing can improve heart failure ( HF ) symptoms in patients with an intraventricular conduction delay by resynchronizing abnormal ventricular contractions and improving LV systolic function . METHODS Eighty-six patients with at least New York Heart Association functional class II HF , chronic LV systolic dysfunction , normal sinus rhythm , and a QRS interval over 120 ms were implanted for atrial-synchronized LV pacing . The single-blinded , r and omized , controlled , crossover study stratified patients 1:1 by the baseline QRS interval into long ( QRS > 150 ms ) and short ( QRS 120 to 150 ms ) groups , which were compared during a three-month period of active ( univentricular ) pacing and a three-month period of inactive ( ventricular inhibited ) pacing . The primary end point was peak oxygen consumption ( VO(2 ) ) followed by anaerobic threshold , distance walked in 6 min , and quality -of-life question naire score . PATIENTS Twelve patients were withdrawn before r and omization and 17 could not complete both study periods . The short QRS group did not improve in any end point with active pacing . For the long QRS group , peak VO(2 ) increased 2.46 ml/min/kg ( p < 0.001 ) , the anaerobic threshold increased 1.55 ml/min/kg ( p < 0.001 ) , the distance walked in 6 min increased 47 m ( p = 0.024 ) , and the quality -of-life score improved 8.1 points ( p = 0.004 ) . CONCLUSIONS Left ventricular pacing significantly improves exercise tolerance and quality of life in patients with chronic HF , LV systolic dysfunction , and a QRS interval over 150 ms BACKGROUND Cardiac resynchronization therapy ( CRT ) is most commonly performed with biventricular ( BiV ) pacing . Left ventricular ( LV ) only pacing is an alternative pacing configuration for CRT , but comparative studies with BiV pacing have shown inconsistent results . This may be due to differences in LV activation pattern , which could be differentially affected by atriventricular ( AV ) programming or atrial pacing ( AP ) . OBJECTIVE The purpose of this study was to compare AV optimization and the effect of atrial overdrive pacing on 
    Output:
    
TriV CRT is an attractive alternative to univentricular or BiV pacing for heart failure patients .