AKYOL, A., ALPER, A.T., CAKMAK, N., HASDEMIR, H., EKSIK, A., OGuz, E., ERDINLER, I., ULUFER, F.T. and GURKAN, K. Long-Term Effects of Cardiac Resynchronization Therapy on Heart Rate and Heart Rate Variability. Tohoku J. Exp. Med., 2006, 209 (4), 337-346 Conuestive heart failure is characterized by significant autonomic dysfunction. Development of left bundle branch block in congestive heart failure is a predictor of worse outcome. There are several lines of evidence that cardiac resynchronization therapy (CRT), by biventricular stimulation in patients with severe heart failure and left bundle branch block, improves autonomic functions which can be quantified by measuring heart rate variability. The aim of the present study was to assess the effect of CRT on autonomic functions quantified by heart rate variability and mean heart rate (HR) in patients with advanced heart failure and left bundle branch block in short and long-term follow-up. A total of 35 patients with systolic heart failure and left bundle branch block (mean-age 60 11 years; 24 male and 11 female; mean left ventricular ejection fraction [EF]: 22.3 +/- 3 %) were enrolled. Clinical assessment and echocardiographic examination were performed at baseline and every three months. Continuous electrocardiographic monitorization by 24-hour Holter recordings was performed pre-implantation, 3 months and 2 years after implantation. Mean HR and one of the time-domain parameters of heart rate variability, standard deviation of the R-R intervals (SDNN) were measured. CRT was associated with a decrease in the mean duration of QRS, and an increase in diastolic filling time, the rate with which the left ventricular pressure rises (dP/dt), and left ventricular ejection fraction. Decrease in mean heart rate and increase in SDNN were statistically significant in the third month and second year recordings when compared to baseline recording (p values were < 0.001 for both). In conclusion, CRT with biventricular pacing provides sustained improvement in autonomic function in patients with advanced heart failure and left bundle branch block. - Congestive heart failure; biventricular pacing; heart rate variability (c) 2006 Tohoku University Medical Press.