ECG Changes in Right and Left Bundle Branch Blocks
In right bundle branch block (RBBB), normal depolarization of the right ventricle is interrupted. In this case, initial depolarization of the ventricular septum (which is stimulated by a branch of the left bundle) is unaffected so that the normal small R wave in lead V1 and small Q wave in lead V6 are recorded. As the wave of depolarization spreads down the septum and into the left ventricular free wall, the sequence of depolarization is indistinguishable from normal, because left ventricular forces normally outweigh those of the right. However, by the time the left ventricle has almost fully depolarized, slow cell-to-cell spread has finally reached the “blocked” right ventricle and depolarization of that chamber begins, unopposed by left ventricular activity (because that chamber has nearly fully depolarized). This prolonged depolarization process widens the QRS complex and produces a late depolarization current in the direction of the anteriorly situated right ventricle. Since the terminal portion of the QRS complex in RBBB represents these right ventricular forces acting alone, the ECG records an abnormal terminal upward deflection (known as an R′ wave) over the right ventricle in lead V1 and a downward deflection (S wave) in V6 on the opposite side of the heart. The appearance of the QRS complex in lead V1 in RBBB (upward R, downward S, then upward R′) is often described as having the appearance of “rabbit ears.”
Left bundle branch block (LBBB) produces even more prominent QRS abnormalities. In this situation, normal initial depolarization of the left septum does not occur; rather, the right side of the ventricular septum is first to depolarize, through branches of the right bundle. Thus, the initial forces of depolarization are directed toward the left ventricle instead of the right (see Fig. 4-19B; see also Fig. 4-30). Therefore, an initial downward deflection is recorded in V1, and the normal small Q wave in V6 is absent. Only after depolarization of the right ventricle does slow cell-to-cell spread reach the left ventricular myocytes. These slowly conducted forces inscribe a widened QRS complex with abnormal terminally upward deflections in the leads overlying the left ventricle (e.g., V5 and V6),
