Atrial Fibrillation with Complete Heart Block: The Physiology of “Regularized” Atrial Fibrillation

Atrial fibrillation (AF) is classically characterized by an irregularly irregular ventricular rhythm due to chaotic atrial depolarization and random conduction through the atrioventricular (AV) node. However, in rare instances, AF can produce a regular ventricular rhythm—a phenomenon that occurs when there is complete heart block (CHB).

The Normal AV Node: A Gatekeeper of Conduction

The AV node plays a critical role in regulating the transmission of atrial impulses to the ventricles. Here’s a closer look at its physiology:

 1. Location and Anatomy:

• The AV node resides in the interatrial septum near the coronary sinus.
• It acts as the only electrical bridge between the atria and ventricles (except in accessory pathway conditions).

 2. Intrinsic Properties:

The AV node has a long refractory period, which prevents the ventricles from being overwhelmed by rapid atrial rates, as seen in conditions like AF or atrial flutter.

• Conduction through the AV node is influenced by autonomic tone:
  • Sympathetic stimulation shortens the refractory period and increases conduction velocity.
  • Parasympathetic (vagal) stimulation prolongs the refractory period and slows conduction.

 3. Filtering Irregular Impulses in AF:

During AF, chaotic atrial depolarizations bombard the AV node at rates of 300–600 bpm. The AV node selectively conducts a fraction of these impulses to the ventricles, producing the hallmark irregularly irregular ventricular rhythm.

What Happens in Complete Heart Block?

In complete heart block (CHB), the AV node or the His-Purkinje system fails entirely to transmit atrial impulses to the ventricles. The result is electrical isolation of the atria and ventricles.

 1. Atrial Activity in CHB:

• Atrial impulses, including the chaotic depolarizations of AF, continue but are completely blocked at the AV node or lower conduction system.
• As a result, the ventricular rhythm is no longer influenced by the atrial activity.

 2. Escape Rhythm:

• In the absence of atrial input, the ventricles are paced by an escape rhythm originating from a lower pacemaker:
  • Junctional escape rhythm (40–60 bpm) if the escape focus is at the AV junction.
  • Ventricular escape rhythm (20–40 bpm) if the escape focus is within the Purkinje system or ventricles.
• This escape rhythm is regular because it is generated by a single, autonomous pacemaker with a consistent rate.

Thus, the ventricular rhythm in AF with CHB is regularized and completely independent of the chaotic atrial activity.

ECG Features of AF with Complete Heart Block

The diagnosis of AF with CHB relies on recognizing its distinct ECG pattern:

 1. Chaotic Atrial Activity:

• Irregular, fibrillatory waves are seen, particularly in leads V1 and II.
• There are no P waves, consistent with AF.

 2. Regular Ventricular Rhythm:

• A regular QRS rhythm occurs due to the escape pacemaker. The rate depends on the location of the escape focus:
  • 40–60 bpm for junctional escapes (narrow QRS complexes).
  • 20–40 bpm for ventricular escapes (broad QRS complexes).

 3. Lack of Atrioventricular Communication:

• The atria and ventricles are electrically independent.
• There is no relationship between the fibrillatory waves and QRS complexes.

Image from my good friends at ECGguru.com!

Physiology Behind “Regularized” AF

The regularity of the ventricular rhythm in this scenario stems from the loss of AV nodal conduction. Normally, the AV node’s variable conduction properties result in irregular ventricular activation during AF. However, in CHB, the AV node no longer participates in conduction, and the ventricles are governed by the intrinsic rhythm of the escape pacemaker.

This “regularization” of AF underscores the complete electrical isolation of the atria and ventricles. Despite the chaotic atrial activity, the escape pacemaker operates independently and consistently, creating a predictable ventricular rhythm.

Key Takeaways

Atrial fibrillation with complete heart block produces a regular ventricular rhythm because the ventricles are governed by an escape pacemaker, independent of the chaotic atrial activity.

Hope this helps! Take care.

-Reid

ECG Lectures with Reid