Left bundle branch block on ECG

Position of LBBB in the general classification of blocks

For the correct interpretation of the electrocardiogram, it is necessary to understand that all conduction disorders are divided into major groups depending on the site of the lesion.

First, sinoatrial (SA) block is distinguished. In this condition, impulse conduction from the sinus node to the atria is impaired (this pathology belongs to the broad group of sinus node dysfunctions).

Second, there is atrioventricular (AV) block. In this case, conduction is impaired at the level of the AV junction, preventing the impulse from passing from the atria to the ventricles. It can be partial or complete.

Third, intraventricular blocks are distinguished. Left bundle branch block belongs precisely to this category. In such blocks, an obstacle arises below the AV node, localized in the right or left bundle branch, as well as in their peripheral branches.

Levels of conduction delay

In cardiological practice, especially during intracardiac electrophysiological study (EPS), it is critically important to precisely determine the level of signal delay in relation to the bundle of His itself.

• Proximal level. Localized above the bundle of His (directly in the AV node). On EPS, this manifests as prolongation of the AH interval.
• Medial (intrahisian) level. The damage is located within the bundle of His itself. The main criterion here is an increase in the duration of the specific His bundle spike.
• Distal (infrahisian) level. The block is located below the spike recording site. It is diagnosed by the characteristic prolongation of the HV interval.

Left bundle branches and variants of intraventricular blocks

Intraventricular disturbances have their own anatomical features. Block of both the right and left bundle branches can be complete or incomplete. However, the left bundle branch has a unique structure: it anatomically divides into two separate branches — anterior and posterior.

Due to such branching, isolated block of only one of the branches of the left bundle is frequently encountered in clinical practice. Such a condition is called a fascicular block. Thus, damage to the left bundle can be not only total, but also fragmentary.

Combined bundle lesions

Multifascicular lesions always attract special attention from cardiologists, as they carry serious risks for the patient.

1. Bifascicular block. Represents a combination of right bundle branch block (RBBB) with a block of one of the branches of the left bundle. On the ECG, this looks like a combination of RBBB criteria and deviation of the electrical axis of the heart (EAH) specific to the lesion of the corresponding left branch.
2. Trifascicular block. Is a combination of an existing bifascicular block with prolongation of the P-Q interval (i.e., first-degree AV block is added). Clinically, this is interpreted as extensive damage to all three branches of the conduction system, which is associated with an extremely high risk of progression to complete AV block.

Classification of AV blocks for differential diagnosis

To avoid confusing intraventricular blocks with atrioventricular delays, it is necessary to remember the classification of the latter by severity:

• First-degree AV block — conduction is slowed, but absolutely all atrial impulses successfully reach the ventricles.
• Second-degree AV block — is subdivided into two types. Type I (Mobitz I) proceeds with progressive prolongation of P-Q before a dropped complex (Samoylov-Wenckebach periodicity). Type II (Mobitz II) is characterized by sudden blocking of the impulse without preliminary deterioration of conduction.
• Third-degree AV block — is a complete block characterized by total dissociation of atrial and ventricular rhythms.

Blocks and arrhythmias: what is the difference?

Separately, it must be remembered that a rhythm disorder and a conduction disorder are completely different pathological processes. Impairments of impulse formation (sinus tachycardia, extrasystole, atrial or ventricular fibrillation and flutter, various paroxysmal tachycardias) belong to arrhythmias. Blocks (SA, AV, or intraventricular), on the other hand, are conduction defects. Although these conditions are often discussed in the same section of diagnostics, mixing their concepts is unacceptable.

Class and exam questions

What else to study on this topic

• ECG criteria for right bundle branch block (RBBB)
• Sinus node dysfunction and sinoatrial blocks
• Differential diagnosis of second-degree AV blocks (Mobitz I and Mobitz II)
• Paroxysmal tachycardias and impulse formation disorders

Other topics in the "Additional Topics" section

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