Background: Left ventricular (LV) systolic function during atrial fibrillation (AF) is positively correlated with an index of ventricular cycle- length irregularity, the ratio of preceding R-R interval to pre-preceding R-R interval (RR (subscript p)/RR (subscript pp)). This study was designed to elucidate the relationship between the RR (subscript p)/RR(subscript pp) ratio and LV function, and to identify factors which influence the relationship.Methods: Beat-to-beat variation of LV function was determined by biplane Simpson's ejection fraction (EF) over 20 beats in 120 patients with chronic AF. The relationship of normalized EF (y) versus the RR (subscript p)/RR (subscript pp) ratio (x) were analyzed by the regression equation y = a + bx. The relation-slope b describes the steepness of the relationship and is a measure of the sensitivity of LV function to ventricular cycle-length irregularity.Results: Mean EF and mean heart rate were factors affecting the relation slope. Negative correlations between mean EF and the relation slope were similarly present at faster heart rates (≧ 80/min， r=-0.81, p < 0.001) and slower heart rates (< 80/min, r = -0.85, p < 0.001). A steeper relationship with faster heart rates was present in patients with normal LV function (r = 0.67, P < 0.001) but not in patients with LV dysfunction (r= 0.23, P = 0.09).Conclusion: Beat-to-beat variation of LV systolic function (LV EF) is more sensitive to ventricular cycle-length irregularity during AF in patients with lower EF or higher heart rate.