Right Bundle Branch Block on ECG

Place in the classification of conduction disorders

Pathologies of electrical impulse conduction in the heart are classified according to the level of the conduction system affected. Right bundle branch block belongs to the broad group of intraventricular blocks, in which the anatomical obstacle to the signal is located below the atrioventricular (AV) node.

To precisely localize the conduction delay, an intracardiac electrophysiological study (EPS) is used. It identifies three levels relative to the bundle of His:

• Proximal (above the bundle, in the AV node) — manifests as prolongation of the AH interval on EPS.
• Medial or intrahisian (within the bundle) — characterized by an increased duration of the His bundle spike.
• Distal or infrahisian (below the site of spike recording) — accompanied by prolongation of the HV interval.

A clear distinction should be made between conduction disorders (blocks) and abnormalities in impulse formation itself. Conditions such as sinus tachycardia, extrasystoles, fibrillation, and paroxysmal tachycardias are classified as arrhythmias rather than blocks, although they are often discussed within the same diagnostic section.

Associated AV blocks

Right bundle branch damage is often assessed in conjunction with AV junction conduction. AV blocks that may accompany RBBB are graded by severity:

1. First-degree AV block: conduction is delayed, but all atrial impulses successfully reach the ventricles.
2. Second-degree AV block:
3. Type I (Mobitz I): progressive prolongation of the P-Q interval prior to a dropped complex (known as Wenckebach periodicity).
4. Type II (Mobitz II): sudden blockage of an impulse without preceding deterioration in conduction.
5. Third-degree AV block: complete cessation of conduction, resulting in complete dissociation of atrial and ventricular rhythms.

Complex and combined forms (bi- and trifascicular blocks)

The left bundle of His divides into anterior and posterior fascicles, making isolated block of only one of them possible.

When RBBB is combined with a block of one of the left fascicles, a bifascicular block is formed. Its ECG features include the classic criteria for right bundle branch block and a deviation of the electrical axis of the heart (EAH) specific to the particular blocked left fascicle.

The most threatening combined variant is a trifascicular (three-fascicle) block. It is diagnosed when prolongation of the P-Q interval (first-degree AV block) is added to a bifascicular block. The pathophysiological interpretation of this ECG pattern is unequivocal: it represents damage to all three bundles of the conduction system. This condition is associated with an extremely high risk of progression to complete transverse block (third-degree AV block).

RBBB in pulmonary embolism

The appearance of a complete right bundle branch block is of paramount clinical importance when pulmonary embolism (PE) is suspected.

In clinical practice, typical ECG changes serve as a marker of severe cases of PE. These include:

• Complete right bundle branch block (CRBBB).
• Low voltage of complexes in standard leads.
• Formation of the classic S1Q3T3 pattern.

Indications for pacing and extended monitoring

Management strategy depends on whether the block is chronic or newly onset.

In patients with pre-existing complete RBBB, early permanent pacing is indicated if conduction disorders progress or new conduction failures occur after medical procedures (ESC class of recommendation IIa B).

By comparison, if a patient newly develops complete left bundle branch block with a QRS duration greater than 150 ms or a PR interval greater than 240 ms, and there is no progression 48 hours after transcatheter aortic valve implantation, the strategy is different. In this case, multi-day ECG monitoring or EPS is recommended (ESC class IIa C).

Class and exam questions

What else to study on this topic

• Anatomical structure of the ventricular conduction system
• ECG criteria for left bundle branch fascicular blocks
• Mechanism of the S1Q3T3 pattern formation in pulmonary embolism
• Differential diagnosis of second-degree AV blocks (Mobitz I and Mobitz II)

Other topics in the "Additional Topics" section

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