Background: Prolongation of ventricular depolarization time (QRS duration) particularly in left bundle branch block (LBBB) is commonly associated with many cardiac diseases. Electrocardiographic study of QRS duration and electrical axis may be predictors of sever LV systolic dysfunction.

 Materials and methods: In this prospective study that was conducted in CCU and out-patient clinic of Shaheed Beheshti hospital in Babol (2000-2003), 150 patients with the diagnosis of LBBB were assigned in two groups (one QRS≥160 and the other with QRS<160 millisecond.). Then relationship between QRS-duration, left axis deviation (LAD) and echocardiographic LV ejection fraction (EF) were analysed using SPSS software and T-test, chi-square and linear regression analysis in step wise manner.

 Results: There was no significant correlation between age and gender with duration of QRS and LAD among patients with LBBB (NS). The mean ± standard deviation of EF in the patients with a QRS duration ≥160 millisecond was significantly lower than the mean of EF in the patients with a QRS duration <160 millisecond (%(54.50±10.55) vs. %(23.89±5.47) p<0.001). Pearson correlation between EF and LAD was lower than its value between EF and QRS duration (r=0.378 p=0.006 vs. r=0.926 p<0.001, respectively).

Conclusion: Evaluating QRS duration in the presence of LBBB has a significant inverse relation with EF and when its value is greater than 160 millisecond, it could be a simple and appropriate marker of significant left ventricular dysfunction.