Complete Atrioventricular Block

Mechanism and ECG features in complete block

With the development of third-degree atrioventricular block, a complete dissociation of rhythms is observed. Electrical impulses generated in the sinoatrial node fail to cross the atrioventricular junction.

On the electrocardiogram, this manifests as two independent rhythms:

• Atrial activity: normal sinus P waves are recorded. They appear regularly and at an adequate rate (e.g., about 110 beats per minute), yet have no electrophysiological connection to ventricular contractions.
• Ventricular activity: maintained by the functioning of an ectopic (escape) pacemaker. The morphology of the QRS complexes and the ventricular rate are directly determined by the level of the conduction system at which the block occurs.

Levels of escape rhythm formation

The site of conduction system injury is critical for prognosis. Depending on the level of conduction block, two main types of escape rhythm are distinguished:

1. Proximal level: the impulse source is the atrioventricular junction (below the site of the block). In this case, the ventricular QRS complexes remain narrow or only slightly altered. The impulse generation rate is higher than in more distal lesions, being approximately 40 beats per minute.
2. Distal level: the rhythm source becomes the idioventricular center (Purkinje fibers or ventricular myocardium). The ECG shows wide and markedly deformed QRS complexes. The ventricular rate is critically low—usually less than 30 beats per minute (in some cases, e.g., 27 per minute).

Clinical presentation and prognostic risks

The hemodynamic consequences of complete AV block are caused by bradycardia and the lack of synchronous contraction of the atria and ventricles. A sharp drop in cardiac output leads to the development of Stokes-Adams attacks. These episodes are characterized by sudden loss of consciousness and require immediate medical intervention. The presence of such attacks is a direct indication for permanent cardiac pacing (pacemaker insertion).

An important risk factor for the sudden development of complete block (often accompanied by asystole or a very slow escape rhythm) is alternating bundle branch block. If the ECG alternately shows morphological features of right bundle branch block (RBBB) and left bundle branch block (LBBB), this indicates a profound and significant lesion of the entire cardiac conduction system.

Indications for diagnosing conduction disorders

For the timely detection of blocks, including those preceding the development of third-degree block (e.g., isolated first-degree AV block or fascicular blocks), a thorough 12-lead ECG is indicated. This baseline examination is mandatory for the following patient categories:

• Patients with suspected bradycardia: a baseline assessment is performed followed by an algorithmic workup. If a standard resting ECG is uninformative, the diagnostic search is continued using ambulatory monitoring or exercise stress tests.
• Patients with chronic heart failure or dilated cardiomyopathy: to assess rhythm, QRS duration, detect scar changes, and determine indications for cardiac resynchronization therapy (CRT).
• Patients with suspected myocarditis or active carditis: in these cases, prolongation of the P-Q interval or the appearance of intraventricular blocks may be the only objective sign of myocardial injury.
• Individuals with infective endocarditis, sarcoidosis, abdominal aortic aneurysm, or hemorrhagic fever with renal syndrome (HFRS) who have auscultatory abnormalities—in order to rule out concomitant involvement of the conduction pathways.

Differences from lower-grade AV blocks

Unlike complete block, other degrees of impaired AV conduction preserve at least partial communication between the atria and ventricles:

• First-degree AV block: all atrial impulses reach the ventricles, with only a slowing of conduction noted (the P-Q interval exceeds 0.21 s, the rhythm is regular without dropped beats).
• Second-degree AV block (Mobitz type II): there is a sudden dropping of the QRS complex without prior P-Q prolongation. The level of such a block is most often distal (below the AV node), and this condition is associated with a high risk of rapid progression to complete third-degree AV block, requiring vigilance and additional diagnostics of the underlying heart disease.

Class and exam questions

What else to study on this topic

• ECG criteria for first-degree AV block and the use of the Kiesel-Jones threshold
• Comparative characteristics of second-degree AV blocks (Wenckebach periodicity and Mobitz type II)
• Morphological features of right and left bundle branch block in chest leads
• Differential diagnosis of bundle branch blocks and ventricular extrasystoles
• Indications for thrombolytic therapy in newly onset left bundle branch block in the setting of myocardial infarction
• Management algorithm for alternating bundle branch block

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