Atrioventricular Block on ECG

Classification by severity

The severity of the conduction disorder is assessed by how effectively electrical impulses reach the ventricles. There are three main degrees:

• First-degree AV block. Characterized by overall slowed conduction. Despite the delay, absolutely all atrial impulses successfully reach the ventricles. On the ECG, this presents as prolongation of the P–Q interval. This type of conduction disorder can be detected during instrumental diagnostics of various inflammatory processes, such as valvulitis.
• Second-degree AV block. In this pathology, some impulses are blocked and fail to reach the ventricles. It is divided into two types:
• Type I (Mobitz I). Characterized by Wenckebach periodicity (Samoylov–Wenckebach phenomenon). The cardiogram shows a gradual and progressive prolongation of the P–Q interval, ending with a dropped ventricular complex.
• Type II (Mobitz II). The impulse is blocked suddenly. There is no preceding deterioration in conduction or change in intervals.
• Third-degree AV block (complete). The signal fails to pass through the atrioventricular junction altogether. Complete dissociation of rhythms occurs: the atria and ventricles contract independently of each other.

Topographic Classification and EPS

Intracardiac electrophysiological study (EPS) is used for precise diagnosis of the lesion level. The level of conduction delay is assessed relative to the bundle of His. There are three localization variants:

Relationship with Intraventricular Blocks

AV blocks can be combined with damage to the underlying parts of the conduction system — the bundle branches.

Of particular clinical significance is trifascicular block. It represents a combination of bifascicular block (when the right bundle branch and one of the left bundle branch fascicles are blocked) and first-degree AV block (P–Q interval prolongation).

This condition is interpreted as severe damage to all three fascicles of the heart's conduction system. It is associated with an extremely high risk of progression to complete (third-degree) AV block.

The Role of ECG in Comprehensive Diagnosis

Performing a 12-lead ECG is recommended for all patients with suspected structural myocardial changes, such as dilated cardiomyopathy (DCM).

Although the cardiogram alone cannot confirm or exclude a diagnosis of DCM, it allows for the detection of associated disorders:

• Presence of sinoatrial, atrioventricular, and ventricular blocks.
• Morphology and duration of the QRS complex.
• Signs of myocardial hypertrophy or ventricular overload (systolic and diastolic).
• Myocardial ischemia.

Assessment of rhythm, heart rate (HR), and conduction helps determine indications for pacemaker (artificial pacemaker) implantation or cardiac resynchronization therapy (CRT).

Differentiation from Other Disorders

It is important to remember that rhythm disorders and conduction disorders are fundamentally different pathological processes, although they are often discussed in the same diagnostic section.

• Blocks (SA block, AV block, bundle branch blocks) are problems with the conduction of an already generated impulse.
• Arrhythmias (sinus tachycardia, extrasystoles, atrial fibrillation, paroxysmal tachycardias) are disorders of the impulse formation process itself.

For example, in pulmonary embolism (PE), the most typical rhythm disorder is atrial fibrillation, while among conduction disorders, right bundle branch block is more common.

Class and exam questions

What else to study on this topic

• ECG criteria for pulmonary embolism (S1Q3T3 syndrome and chest lead changes).
• Classification of intraventricular blocks (RBBB, LBBB, fascicular blocks).
• Bifascicular and trifascicular blocks: diagnostic criteria.
• The role of ECG in diagnosing dilated cardiomyopathy (DCM) and indications for pacemaker implantation.
• Instrumental diagnostics of valvulitis (Echocardiography and chest X-ray).

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