Abstract: AIM: To investigate the effects of traditional right ventricular apical pacing and right ventricular outflow tract septum pacing on cardiac functions and the relationship between different right ventricular apical pacing ratio and left ventricular size. METHODS: Sixty-six cases of patients with arrhythmia were treated with slow pacing therapy,46 cases with right ventricular apical pacing and 20 cases with ventricular septal pacing. Left atrial and left ventricular internal diameters and left ventricular ejection fraction were evaluated by ultrasound before surgery as well as 6 and 12 months postoperatively. Right ventricular apical pacing rates were divided into two groups,with pacing rate ≥30% and < 30%,and left atrial and left ventricular internal diameter and left ventricular ejection fraction were also examined. RESULTS:No significance differences were observed in left atrial diameter,left ventricular end-diastolic diameter and left ventricular ejection fraction between right ventricular apex group and right ventricular septal group before surgery as well as 6 months and 12 months postoperatively. There were no significant differences in left atrial diameter and left ventricular ejection fraction between groups of apical pacing rates≥30% and < 30%. Howeveer,significant difference was found in left ventricular end-diastolic ventricular diameter changes 6 and 12 months postoperatively and the left ventricular end-diastolic diameter increased with the increases of pacing rate. CONCLUSION: There is no significant difference in left ventricular structure and function 12 months after surgery and right ventricular apical pacing with a high ratio( ≥30%)can cause an increase in left ventricular end-diastolic diameter.