Why Atrial Fibrillation with Ventricular Pacing Can Appear Regular

Atrial fibrillation (AF) is classically described as an irregularly irregular rhythm, defined by chaotic atrial activity and a ventricular response that lacks any predictable pattern. However, a unique exception occurs in patients who have ventricular pacing, especially those with complete AV block and a functioning pacemaker. In these patients, the ventricular rhythm can appear surprisingly regular, despite the ongoing atrial fibrillation. This post explores the physiological mechanisms behind this phenomenon.

Check out another Example of Regular AF.

Atrial Fibrillation: The Baseline Mechanism

AF is the result of disorganized atrial electrical activity, typically due to multiple reentrant circuits or ectopic firing, especially around the pulmonary veins. This leads to:

• Absent P waves on ECG

• Chaotic atrial depolarizations (>350–600 bpm)

• Irregular AV conduction, depending on the AV node's refractory properties

In a patient with an intact AV node, the fibrillatory waves bombard the AV node at rapid rates, and ventricular conduction becomes highly variable, as the AV node selectively filters impulses based on its refractory state. This is why AF is typically irregularly irregular.

Enter the Pacemaker: Restoring Regularity

In patients with a ventricular pacemaker, especially single-chamber VVI or dual-chamber DDD devices operating in VVI mode, the rhythm of ventricular depolarization becomes pacing-dependent. Here's what changes:

1. Complete AV Block or Pacemaker Override

• In many cases, the patient has complete AV block, or the AF impulses fail to conduct through the AV node.

• The pacemaker becomes the sole driver of ventricular rhythm, usually firing at a set rate (e.g., 60 bpm).

• Since the pacemaker is insensitive to atrial activity, it paces the ventricles at a fixed interval, regardless of what the fibrillating atria are doing.

2. Fixed Output = Fixed Rhythm

• The pacemaker monitors the ventricles.

• If it does not detect intrinsic ventricular activity within its preset interval (i.e., no spontaneous QRS), it delivers a pacing spike.

• This results in consistent ventricular pacing, often appearing as a regular rhythm on ECG — despite atrial chaos.

Why It Matters: Reframing “Irregularly Irregular”

In clinical medicine, AF is almost always taught as an irregular rhythm. But this changes when we factor in artificial pacing:

• AF with a regular ventricular rhythm should immediately raise suspicion for complete AV block with AF.

• This scenario often implies pacemaker dependence, or in some cases, junctional escape rhythm if not paced.

ECG Features to Look For

• No P waves (consistent with AF)

• Regular R-R intervals (suggests pacing or complete AV block)

• Pacing spikes preceding each QRS (in paced rhythms)

• Wide QRS complexes (if pacing from the RV apex)

You see an ECG with regular wide QRS complexes at 60 bpm, no P waves, and clear pacing spikes. The patient has AF and a VVI pacemaker — the ventricular rhythm is regularly paced, while the atria remain in fibrillation.

Here is the full ECG... (there's a PVC at the end)

Key Takeaway

Atrial fibrillation does not always produce an irregular rhythm. When a ventricular pacemaker is driving the rhythm — particularly in complete AV block — the ventricular rate becomes fixed and regular, creating a paradox: AF with a regular rhythm.

Always correlate the ECG findings with clinical context and device interrogation if applicable. Recognizing this pattern prevents misdiagnosis and highlights the elegant interface between artificial pacing and intrinsic cardiac electrophysiology.

You can learn more about the anatomical basis of ECG in my course ECG in 21 Days.

Reid