title: Diminished left ventricular dyssynchrony and impact of resynchronization in failing hearts with right versus left bundle branch block.
abstract: OBJECTIVE
  We compared mechanical dyssynchrony and the impact of cardiac resynchronization therapy (CRT) in failing hearts with a pure right (RBBB) versus left bundle branch block (LBBB).
  
  
  BACKGROUND
  Cardiac resynchronization therapy is effective for treating failing hearts with conduction delay and discoordinate contraction. Most data pertain to LBBB delays. With RBBB, the lateral wall contracts early so that biventricular (BiV) pre-excitation may not be needed. Furthermore, the magnitude of dyssynchrony and impact of CRT in pure RBBB versus LBBB remains largely unknown.
  
  
  METHODS
  Dogs with tachypacing-induced heart failure combined with right or left bundle branch radiofrequency ablation were studied. Basal dyssynchrony and effects of single and BiV CRT on left ventricular (LV) function were assessed by pressure-volume catheter and tagged magnetic resonance imaging, respectively.
  
  
  RESULTS
  Left bundle branch block and RBBB induced similar QRS widening, and LV function (ejection fraction, maximum time derivative of LV pressure [dP/dt(max)]) was similarly depressed in failing hearts with both conduction delays. Despite this, mechanical dyssynchrony was less in RBBB (circumferential uniformity ratio estimate [CURE] index: 0.80 +/- 0.03 vs. 0.58 +/- 0.09 for LBBB, p < 0.04; CURE 0-->1 is dyssynchronous-->synchronous). Cardiac resynchronization therapy had correspondingly less effect on hearts with RBBB than those with LBBB (i.e., 5.5 +/- 1.1% vs. 29.5 +/- 5.0% increase in dP/dt(max), p < 0.005), despite similar baselines. Furthermore, right ventricular-only pacing enhanced function and synchrony in RBBB as well or better than did BiV, whereas LV-only pacing worsened function.
  
  
  CONCLUSIONS
  Less mechanical dyssynchrony is induced by RBBB than LBBB in failing hearts, and the corresponding impact of CRT on the former is reduced. Right ventricular-only pacing may be equally efficacious as BiV CRT in hearts with pure right bundle branch conduction delay.
authors.affiliation: Department of Medicine, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
authors.forename: Melissa J
authors.initials: MJ
authors.lastname: Byrne
authors.forename: Robert H
authors.initials: RH
authors.lastname: Helm
authors.forename: Samantapudi
authors.initials: S
authors.lastname: Daya
authors.forename: Nael F
authors.initials: NF
authors.lastname: Osman
authors.forename: Henry R
authors.initials: HR
authors.lastname: Halperin
authors.forename: Ronald D
authors.initials: RD
authors.lastname: Berger
authors.forename: David A
authors.initials: DA
authors.lastname: Kass
authors.forename: Albert C
authors.initials: AC
authors.lastname: Lardo
mesh_terms: D000818:Animals; D002037:Bundle-Branch Block / Q000503:physiopathology* / Q000628:therapy*; D002304:Cardiac Pacing, Artificial; D004285:Dogs; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D018754:Ventricular Dysfunction / Q000503:physiopathology*