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CLINICAL RESEARCH

Diminished Left Ventricular Dyssynchrony and Impact of Resynchronization in Failing Hearts With Right Versus Left Bundle Branch Block

Melissa J. Byrne, PhD^_*, Robert H. Helm, MD^_*, Samantapudi Daya, MD^_*, Nael F. Osman, PhD^,||, Henry R. Halperin, MD, MA, FAHA^_*,,, Ronald D. Berger, MD, PhD^_*, David A. Kass, MD, FAHA^_*, and Albert C. Lardo, PhD, FACC, FAHA^_*,,,,_*

^_* Department of Medicine, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, Maryland
Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland
Department of Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland
Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland
^|| Department of Electrical and Computer Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland

Manuscript received December 15, 2006; revised manuscript received May 15, 2007, accepted July 1, 2007.

^_* Reprint requests and correspondence: Dr. Albert C. Lardo, Johns Hopkins School of Medicine, Division of Cardiology, 720 Rutland Avenue, 1042 Ross Building, Baltimore, Maryland 21205. (Email: al{at}jhmi.edu).

Objectives: 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 01 is dyssynchronoussynchronous). 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.

Abbreviations and Acronyms   BiV = biventricular   CRT = cardiac resynchronization therapy   CURE = circumferential uniformity ratio estimate   dP/dtmax = maximum time derivative of left ventricular pressure   EF = ejection fraction   HF = heart failure   LBBB = left bundle branch block   LV = left ventricle/ventricular   MRI = magnetic resonance imaging   RBBB = right bundle branch block   RV = right ventricle/ventricular

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