Abstract:Different from left anterior fascicular block(LAFB) complicated with right bundle branch block(RBBB), the lesion degree of cardiac conduction system of masquerading bundle branch block(MBBB) is more serious and usually inclined to progressive deterioration. The diagnosis of MBBB relies on electrocardiogram(ECG), yet yields high omission diagnostic rate. Combing with clinical examples, analyze and discuss the problem based on the essence of bundle branch block as well as the expressive characteristics of vectorcardiogram. When conduction disturbance occurs in some fiber of the left posterior branch, it represents in the vectorcardiogram as dacryon denseness at the middle part of QRS ring while such graphic expression is much more manifest than ECG duration of intrinsicoid deflection (or called ventricular activation time). Therefore, vectorcardiogram could help in clinically discovering MBBB which is hard to be diagnosed by ECG in early phase.