Complete right bundle branch block
This patient presents a complete right bundle branch block in an otherwise healthy heart. Since electrical conduction is interrupted in the right branch, activation of the left ventricle is normal, while that of the right ventricle is delayed. The QRS complex can be divided into two parts: the first part corresponds to the activation vectors of the left ventricle and the second part to the activation vectors of the right ventricle.
During right bundle branch block, ventricular activation initiates in the middle portion of the left side of the septum; the ensuing vectors are also normal given the predominance of left ventricular mass activation; on the other hand, the delayed activation of the right ventricle adds abnormal vectors directed to the right at the end of the QRS complex which considerably deform the terminal portion of the QRS.
The left side of the interventricular septum is therefore activated first, at its median and upper aspect, as in normal ventricular activation. The septum is normally activated from left to right. There is an initial positive deflection in V1 (r wave) and an initial negative deflection in V6 (q wave). Activation of the left ventricle is not altered relative to normal and immediately follows septal activation. It propagates from the endocardium to the epicardium with a wide positive deflection in V6 (R wave) and a negative deflection (S wave) in V1 which follows the initial positive deflection of the normal QRS complex.
The transit through the septum via the muscle is often prolonged and contributes to the widening of the QRS. Activation of the right ventricle is therefore achieved by slow muscular conduction. This delayed activation of the right ventricle is reflected on the electrocardiogram by a positive wave in V1 (R') often of high amplitude and a negative wave in V6 (S), since there is no longer any opposition by the left ventricular electrical potentials whose activation is completed.
The characteristic electrocardiographic pattern of the right bundle branch block combines:
- A widened QRS complex of 120 ms or more in adults, a QRS greater than 100 ms in children under 8 years of age;
- A delayed onset of intrinsicoid deflection (ventricular activation time) in the right precordial leads: this is due to the delayed depolarization of the affected ventricular wall; this represents a major and fundamental criterion of right bundle branch block; intrinsicoid deflection is normal in left-sided precordial leads since the left ventricle is normally activated;
- A characteristic QRS pattern in V1 and V2: there is a late positive deflection typically of high amplitude; the first recorded deflection is positive (R or r wave) and corresponds to septal depolarization directed from left to right in the direction of the electrode; it is followed by a negative wave (S or s wave) when the impulse travels through the left ventricle while fleeing the electrode; the last deflection (R' wave), representing the intrinsicoid deflection, corresponds to the delayed activation of the right ventricle: it is directed towards the electrode; the activation of the right ventricle is no longer in opposition to the depolarization of the left ventricle, the result being a high amplitude potential with R'> R; one thus observes either a rsR' or rSR' or rR' pattern;
- A characteristic pattern in V6: a q wave represents normal septal depolarization (it may be absent depending on the differences in morphology and orientation of the heart) as well as a wide and slurring S wave corresponding to the late depolarization of the right septum and the right ventricular cavity;
- A modified ST segment and T wave in V1, V2: a ST segment depression and an inverted T wave can be observed; the depolarization being altered, there are associated repolarization disorders; positive T wave in V6;
- In the frontal plane, the first part of the QRS displays a normal morphology and orientation while the second part has an axis orientation deviated to the right. The electrical axis of the QRS is a function of the relatively divergent direction of the resulting vectors from the first half and the second half of the QRS. It is most often normal or slightly deviated to the right. The right bundle branch block patterns departing from the usual type with major axis deviation are usually associated with the adjunction of another factor such as ventricular hypertrophy, a vertical heart position or an associated left posterior fascicular block.