BACKGROUND AND OBJECTIVES: To evaluate the influence of left bundle branch block (LBBB) on the coronary flow pattern of the left anterior descending coronary artery (LAD). SUBJECTS AND METHODS: 91 patients (M: F=33: 58, mean age: 64+/-14yrs) were divided into the LBBB (n=42), RV pacing (n=26) and control groups (n=23). All patients were examined by a surface ECG, and transthoracic Doppler echocardiograms performed. The QRS duration, the left ventricular ejection fraction (LVEF) and the left ventricular end diastolic volume (LVEDV) were measured. The coronary flow was measured in the distal LAD, with transthoracic Doppler echocardiography, and the percent of the diastolic flow duration (%DD) was expressed as the percentage of the diastolic duration of coronary artery flow divided by the R-R interval. RESULTS: The %DD of the LAD was significantly shorter in patients of the LBBB than the RV pacing group (59.3+/-7.6% in control group, 43.9+/-12.4% in LBBB group, 60.3+/-7.3% in RV pacing group. p<0.01) and the same in patients with a LVEF> or =50% only (58.6+/-6.0% in control group, 49.3+/-10.5% in LBBB group, 59.6+/-5.4% in RV pacing group. p<0.01). In the LBBB group, the %DD of the LAD was positively correlated with the LVEF (p<0.05, r=0.50), negatively correlated with the QRS duration (p<0.05, r=-0.41) and negatively correlated with the LVEDV (p<0.05, r=-0.57). CONCLUSION: Differing from RV pacing, LBBB can itself disturb the coronary flow of the LAD through the shortening of the diastolic duration. Furthermore, systolic dysfunction potentiates the shortening effect of the diastolic flow duration caused by LBBB. However, whether the left ventricular systolic dysfunction is the result of a coronary flow disturbance caused by LBBB, or vice versa, needs further investigation.