In normal cardiac conduction, an electrical impulse is transmitted through atrioventricular (AV) node to the ventricles via the His bundle, which divides into the right and left bundle branches in the intraventricular septum. Depolarization in the ventricle is accomplished by the progression of the electrical impulse via the right and left bundles. The left bundle further divides into the anterior and posterior fascicles. The left anterior fascicle becomes the Purkinje fibers of the anterior and left free ventricular wall. The left posterior fascicle fans out and terminates in the Purkinje fibers of the inferior and posterior walls of the left ventricle (Figure 10.1).

The ventricular phase of cardiac conduction can be divided into two phases: the first phase involves the depolarization of the intraventricular septum via the left bundle branch, which progresses in an anterior and rightward direction. The second phase follows rapidly, involving the near‐simultaneous depolarization of the left and right ventricles. Mechanical contraction of the ventricles occurs immediately after electrical depolarization of the ventricular myocardium.

A bundle branch block (BBB), or intraventricular conduction abnormality (Box 10.1), occurs when this organized transmission through the normal ventricular conduction system is hindered. When a block occurs, there are characteristic changes in the morphology and deflection of the QRS complex. The type and degree of morphologic changes depend on where in the conduction system the blockage occurs. Complete conduction block of either the left or the right bundle branches typically causes a widening of the QRS complex (>0.12 s) leading to either a left bundle branch block (LBBB) or right bundle branch block (RBBB), respectively. Hemiblocks – a blockade of either the anterior or the posterior fascicle of the left bundle branch – typically demonstrate a lesser degree of QRS complex prolongation (0.08–0.12 s). Finally, non‐specific intraventricular conduction abnormalities may occur in cases where there is a blockade of the usual ventricular conduction system but do not fulfill the criteria of either RBBB or LBBB pattern.

The Bundle Branch Blocks

RBBB presents when an electrical impulse is transmitted normally through the bundle of His as well as through the left bundle and its anterior and posterior fascicles to the left side of the heart; yet during conduction, the right bundle is non‐functional and transmission of the impulse is “blocked.” Therefore, the right ventricle must depolarize via cardiac cell‐to‐cell transmission. This inefficient pathway gives rise to the characteristic ECG changes that are noted in RBBB (Figure 10.2).

In RBBB, the QRS complex has altered morphology with the deflection representing right ventricular (RV) activation being delayed and becoming very prominent, resulting in a broad R wave in lead V1. This broadened R wave may take any of the following morphologies: monophasic R, biphasic RSR′ (“M”‐shaped or “rabbit ears”), or QR formation. In leads I and V6, early intrinsicoid deflection (time to peak deflection and the onset of the R′ wave) and either a wide, shallow S wave or RS wave are seen. QS complexes are often encountered in the inferior leads. The QRS complex duration is prolonged, usually greater than 0.12 s. Marked ST segment changes are often seen including ST segment depression and T wave inversion in the right precordial leads (leads V1–V3), associated with the predominantly positive QRS complexes. Further, the ST segment in inferior and left precordial leads is frequently elevated with an upright T wave, with a greater degree of ST segment elevation seen in the inferior leads. These changes represent a “new normal” in the patient with RBBB (Figure 10.3), and do not necessarily indicate an acute ischemic event.