General information on intraventricular conduction disorders
The bundle branch block corresponds to a slowdown (incomplete block) or an interruption (complete block) of the path of the ventricular excitation wave in the conduction pathways below the bifurcation between the right branch and left branch.
The impact of a branch block on the electrical pattern was initially studied in animal models: the sectioning of one the branches induces a prolongation of QRS duration, a delayed intrinsicoid deflection in the leads relative to the ipsilateral ventricle and secondary repolarization disorders. The compression followed by the decompression of an unsectioned branch yields an incomplete bundle branch block with intermediate electrical patterns between complete block and the normal pattern.
Certain elements allow understanding the electrical pattern observed in patients with a bundle branch block:
- one of the 2 ventricles is activated in a delayed manner (the ventricle ipsilateral to the conduction block), the total duration of the ventricular depolarization process is therefore prolonged, thus explaining the prolongation in QRS duration; by definition, in adults, when the QRS duration is between 100 and 120 ms, it is deemed an incomplete block; when the duration exceeds 120 ms, it is deemed a complete block; in children under 8 years old, a QRS-complex longer than 100 ms is indicative of a complete block;
- the intrinsicoid deflection corresponds to the activation delay of the wall recorded by the lead; it is delayed in the right precordial leads for a right block and in the left precordial leads for a left block;
- the left bundle branch divides early in the middle third of the left septal surface culminating into an anterior fascicle and a posterior fascicle which travel to the base of the anterior and posterior pillars; the right bundle branch is a very long cord-like structure, which has no divisions prior to reaching the right ventricular endocardium at the level of the anterior papillary muscle near the septum; right bundle branch blocks have a relatively similar pattern whether the interruption is proximal or more distal; in contrast, a left troncular block is characterized by a proximal lesion whereas hemiblocks are characterized by a more distal lesion;
- in the right bundle branch block, activation of the left ventricle is normal and the initial septal activation remains unchanged; only the end of the QRS-complex corresponding to the delayed activation of the right ventricle is altered; in the left bundle branch block, septal depolarization is reversed and the entire QRS-complex is altered;
- the electrocardiographic pattern of left hemiblocs essentially consists of an axis deviation; the first vectors do not have a normal direction: they are deviated downward and to the right in anterior fascicular blocks, and upward and to the left in posterior fascicular blocks; the activation of the left ventricular free walls, rather than occurring synchronously, exhibits a slight delay due to the conduction time of the activation of the Purkinje network of the intact hemibranch to that of the blocked hemibranch; this conduction time is relatively short in the order of 20 ms, which explains the lack of significant increase in QRS duration.