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Background: RV pacing induces conduction delay (CD), mechanical dyssynchrony, and increased morbidity in patients with HF. CRT improves HF symptoms and survival, but sparse data exist on its direct effect on chronically RV-paced HF patients., Objectives: To assess the benefit of cardiac resynchronization therapy (CRT) in chronically right ventricle (RV)-paced heart failure (HF) patients., Methods: We studied 12 consecutive patients with class III HF who had a previously implanted pacemaker or implantable cardioverter-defibrillator. These individuals were chronically RV paced and referred for upgrade to a biventricular device by their primary cardiologists. Tissue Doppler and strain rate imaging (TDI and SRI, respectively) were performed immediately before each upgrade and 4-6 weeks afterward to quantify changes in regional wall motion and synchrony with CRT., Results: CRT significantly reduced the mean QRS duration (205 ms to 156 ms; P<.0001), and it increased the ejection fraction (30.7%+/-5.1% to 35.8%+/-5.1%; P<.01). Left ventricular end-systolic and end-diastolic dimensions were also significantly reduced. Clinically, patients improved by an average of one New York Heart Association (NYHA) functional class after upgrade (P = .006). The parameter exhibiting greatest improvement was the coefficient of variation (CoV: standard deviation/mean) of time to peak systolic strain rate, a marker of ventricular dyssynchrony, which decreased from 34.3%+/-13.0% to 19.0%+/-6.6% (P<.01). Reduction in CoV of time to peak systolic strain rate was maximally seen in the midventricle (38.2%+/-19.6% to 16.5%+/-9.7%; P<.01)., Conclusions: Upgrading chronically RV-paced HF patients to CRT improves global and regional systolic function. TDI and SRI provide compelling evidence that this benefit parallels that seen in HF patients with CD unrelated to RV pacing, which implies that biventricular pacing synchronizes mechanical activation in different myocardial regions in patients upgraded from RV pacing as well.