Left anterior fascicular block

Place in the classification of conduction disorders

The cardiac conduction system can malfunction at different levels. All impulse conduction disturbances (not to be confused with impulse formation disorders, i.e., arrhythmias) are broadly divided into three categories:

• Sinoatrial (SA) blocks: a problem at the level of impulse exit from the sinus node to the atrial tissue.
• Atrioventricular (AV) blocks: delay or complete cessation of the impulse at the border between the atria and ventricles (grades I, II, and III).
• Intraventricular blocks: pathology of conduction pathways below the AV node, i.e., at the level of the bundle of His trunk, its right or left branches, or their individual fascicles.

During intracardiac electrophysiological study (EPS), the level of delay can be determined with maximum precision. Left bundle branch fascicular blocks belong to the distal (infra-Hisian) level. On EPS, this manifests as prolongation of the HV interval, since the obstacle is located below the site of the His bundle spike recording.

Isolated and combined fascicular blocks

An anatomical feature of the left bundle branch is its division into fascicles. Therefore, on the ECG, we can see not only a complete block of the entire branch, but also an isolated fascicular block, when only the anterior or only the posterior fascicle is affected.

Damage to the anterior fascicle is often combined with other conduction disturbances, forming complex and prognostically unfavorable patterns:

• Bifascicular block: occurs with simultaneous damage to the right bundle branch (RBBB) and one of the left bundle branch fascicles (e.g., the anterior). On the ECG, this appears as a combination of RBBB criteria with a marked deviation of the electrical axis of the heart (EAH) specific to the blocked fascicle.
• Trifascicular block: an extremely dangerous condition. It is a combination of bifascicular block with prolongation of the P-Q interval (first-degree AV block). This picture is interpreted as simultaneous damage to all three fascicles of the conduction system.

Impact of block on the diagnosis of other pathologies

The presence of an anterior hemiblock requires caution when interpreting the electrocardiogram, as it can distort the signs of other diseases.

A striking example is the diagnosis of right ventricular hypertrophy (RVH). One of the additional ECG criteria for RVH in the left chest leads ($V_5-V_6$) is the predominance of the S wave amplitude over the R wave (when the $R/S < 1.0$ ratio is present).

Crucial condition: this diagnostic sign is valid and reliable only in the absence of scar changes in the anterior wall and absence of left anterior hemiblock. If this fascicle is blocked, RVH cannot be assessed using this criterion.

Diagnostic workup and management in conduction disturbances

If the ECG shows a 1:1 rhythm with a normal PR interval, but non-specific intraventricular delays or an isolated anterior hemiblock are present, the patient requires:

1. Thorough evaluation of clinical symptoms.
2. Additional diagnostics for the underlying heart disease that caused damage to the conduction pathways.

Of particular concern is the detection of alternating bundle branch block. This is a condition in which QRS complexes on a single ECG show alternating morphology of left and right bundle branch block. Such a pattern indicates severe, significant disease of the conduction system. The risk for the patient is extremely high: alternating block can suddenly progress to complete heart block with the development of a slow escape rhythm or asystole.

Role of intraventricular blocks in CRT prescription

In severe heart failure (HF), intraventricular conduction disturbances exacerbate the dyssynchrony of heart function. To solve this problem, cardiac resynchronization therapy (CRT) is used.

However, it is important to understand that an isolated anterior hemiblock is not a criterion for device implantation. CRT is indicated for patients with the following parameters:

• Symptomatic heart failure persisting despite optimal medical therapy.
• Sinus rhythm.
• Left ventricular ejection fraction $\le$ 35%.
• Presence of complete left bundle branch block (LBBB) with a QRS duration $\ge$ 150 ms.

Class and exam questions

What else to study on this topic

• Electrocardiographic criteria for the diagnosis of right ventricular hypertrophy (RVH)
• Secondary repolarization abnormalities: arched ST depression and negative T wave
• Patient selection and indications for cardiac resynchronization therapy (CRT)
• Intracardiac EPS: differentiation of proximal, medial, and distal levels of blocks
• Mechanisms of formation and ECG signs of Wenckebach periods (Mobitz type I AV block)

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.