Intraventricular Conduction Disorder

General classification of conduction disorders

Depending on the topographic level of the conduction system lesion, all disturbances in electrical impulse propagation in the heart are divided into three main groups:

• Sinoatrial (SA) block — the obstruction occurs on the pathway from the sinus node to the atrial myocardium. This pathology is traditionally considered within the framework of sick sinus syndrome (SSS).
• Atrioventricular (AV) block — the impulse is delayed or completely blocked at the border of the atria and ventricles (in the AV junction area). This block can be partial or complete and is strictly classified by severity grades.
• Intraventricular blocks — the disturbance is localized below the AV node level. The problem directly affects the bundle branches (right, left, or their individual fascicles).

Important note: conduction disorders must not be confused with impulse formation disorders. Extrasystoles, sinus tachycardia, atrial or ventricular fibrillation and flutter belong to the group of arrhythmias. Although they are often considered within adjacent diagnostic algorithms, they are pathophysiologically fundamentally different conditions.

Topographic diagnostics and EP study

To precisely determine the level of impulse conduction delay relative to the bundle of His, invasive cardiac electrophysiological study (EPS) is used.

Three levels of lesions are distinguished:

At proximal stages, AV blocks are more frequently diagnosed. They are divided into grade I (all atrial impulses reach the ventricles, but with a delay), grade II (includes Mobitz type I with progressive P-Q lengthening according to Wenckebach before a dropped complex, and Mobitz type II with a sudden block without warning lengthening), and grade III — complete dissociation of atrial and ventricular rhythms.

Types of intraventricular blocks

Intraventricular lesions depend on the number of damaged fascicles of the conduction system.

1. Right bundle branch block (RBBB). Can occur in complete or incomplete forms (codes I45.0–I45.1).
2. Left bundle branch block (LBBB). Also occurs as complete and incomplete. Anatomically, the left branch divides into anterior and posterior fascicles, making isolated fascicular block of one of them possible. The presence of LBBB morphology with a QRS duration ≥ 150 ms requires special attention (ESC recommendation class IA), as correction of this condition reduces symptoms, morbidity, and the risk of sudden death.
3. Bifascicular block (I45.2). Represents a combination of RBBB with a lesion of one of the left bundle fascicles. The ECG shows classic RBBB criteria supplemented by a deviation of the electrical axis of the heart (EAH) specific to the respective left bundle fascicle.
4. Trifascicular block (I45.3). The most severe variant. The picture of a bifascicular block is joined by P-Q interval prolongation (AV block of the first degree). This indicates total damage to all three fascicles of the conduction system with a critically high risk of progression to complete heart block.

Complications and clinical significance

The presence of intraventricular blocks can lead to a number of life-threatening conditions and complications requiring immediate medical intervention:

• Stokes-Adams attacks. Occur with the development of complete AV block (often as a consequence of progressive trifascicular block) and present as syncopal episodes due to severe bradycardia or asystole.
• Ventricular tachyarrhythmias. Patients with impaired intraventricular conduction have an increased risk of arrhythmias. One of the prognostic markers of this risk is the detection of ventricular late potentials on signal-averaged ECG.
• Hemodynamic disturbances. Pronounced intraventricular blocks cause asynchronous (uncoordinated) contraction of the ventricular walls. This leads to a decrease in cardiac output and, in some cases, serves as a direct indication for cardiac resynchronization therapy (CRT).
• Marker of exacerbation. If bundle branch block occurs acutely (suddenly), it is an alarming sign of an exacerbation of the underlying cardiac disease (e.g., ischemia). In such situations, priority treatment of the underlying heart pathology is essential.

Class and exam questions

What else to study on this topic

• Diagnostic criteria for fascicular blocks: anterior and posterior fascicles of the left bundle branch.
• Indications for cardiac resynchronization therapy (CRT) in hemodynamic disturbances.
• Methodology for recording late potentials on signal-averaged ECG as a marker of tachyarrhythmias.
• Differential diagnosis of second-degree AV block (Wenckebach periodicity and Mobitz type II).
• Management tactics for patients with Stokes-Adams attacks.

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