Intraventricular block is conduction block below the His bundle. The intraventricular conduction system consists of the right bundle branch, left anterior fascicle, and left posterior fascicle. Lesions in this system can affect one, two, or all three branches.
Right bundle branch block (RBBB) is more common and often clinically insignificant. Left bundle branch block (LBBB) often occurs with acute myocardial infarction, cardiomyopathy, or drug toxicity causing diffuse myocardial damage. Left anterior fascicular block is more common than left posterior fascicular block.
Monofascicular and bifascicular blocks usually have no clinical symptoms. Occasionally, splitting of the first and second heart sounds may be heard. Complete trifascicular block presents similarly to complete atrioventricular block.
Electrocardiographic features
Right bundle branch block
QRS duration is ≥ 0.12 seconds. In lead V1, there is an rsR' pattern with a broad R' wave; in leads V5 and V6, there is a qRS or RS pattern (wide S wave). The T wave is opposite the main direction of the QRS complex. Incomplete RBBB has a similar pattern, but the QRS duration is < 0.12 seconds.
Left bundle branch block
QRS duration is ≥ 0.12 seconds. In leads V5 and V6, there is an R wave with notched or broadened peak, without a preceding q wave. In leads V1 and V2, there is a QS or rS pattern with a deep, wide S wave. The T wave is opposite the main direction of the QRS complex. Incomplete LBBB has a similar pattern, but the QRS duration is < 0.12 seconds.
Figure 1 Complete right bundle branch block and complete left bundle branch block
A. Complete right bundle branch block: Sinus rhythm, QRS duration of 0.16 seconds, rsR' pattern in lead V1, qRs pattern with terminal QRS widening in leads V5 and V6
B. Complete left bundle branch block: Sinus rhythm, QRS duration of 0.18 seconds, notched R wave in leads V5 and V6, rS pattern with a deep and wide S wave in lead V1
Left anterior fascicular block
The mean frontal QRS axis is leftward at -45° to -90°. Leads I and aVL show a qR pattern, while leads II, III, and aVF show an rS pattern, with QRS duration < 0.12 seconds.
Left posterior fascicular block
The mean frontal QRS axis is rightward at +90° to +120° (or +80° to +140°). Lead I shows an rS pattern, while leads II, III, and aVF show a qR pattern, with R in lead III greater than R in lead II, and QRS duration < 0.12 seconds.
Bifascicular and trifascicular block
Bifascicular block involves block in any two of the three fascicles, while trifascicular block involves all three, often presenting complete AV block. Various ECG patterns can occur depending on the number, degree, and intermittent nature of the blockages. The most common is RBBB in combination with left anterior fascicular block. RBBB with left posterior fascicular block is rare. Alternating RBBB and LBBB indicates bilateral bundle branch block.
Treatment
Patients with chronic unilateral bundle branch block who are asymptomatic do not require treatment. Bifascicular and incomplete trifascicular blocks can progress to complete AV block, but the timing and certainty of this progression are unpredictable, so routine prophylactic pacemaker therapy is not necessary. However, if syncope or Adams-Stokes syndrome occurs, early consideration of cardiac pacing is warranted.