Rate dependent bundle branch block

We have previously discussed physiologic ventricular aberration with atrial ectopy or atrial fibrillation with a rapid ventricular response.

There is another type of ventricular aberration, which is slower and represents abnormal bundle branch physiology.

This is a rate dependent bundle branch block.

In this situation as the sinus rate increases, a block occurs in one of the bundle branches and often called a tachycardia dependent bundle branch block.

Here is another example, which is a left bundle branch block:

 Features:

• P wave before each complex and an identical PR interval.
• On Holter monitoring, the rate difference between the two sets of QRS complexes is usually minor as in these examples.
• Rates are slower than with physiologic aberrant ventricular conduction.
• There are lesions in the bundle branch which result in altered conduction and are rate dependent.
• Can be either the left or right bundle branch.
• Precursor to a fixed block.
• At least for the left bundle, the reported prognosis of a rate dependent bundle branch block can be poor as it is often associated with septal ischaemia.
• I am unsure if a right rate dependent bundle branch block has any significance.

There are a number of ECG facts to remember:

As part of the clinical spectrum, an apparent “fixed” bundle branch block may actually demonstrate normal conduction at very slow rates:

The bundle branch block can lead to a serious misdiagnosis during tachycardia.

Note the P wave following the ectopic pause (red highlight) confirming sinus tachycardia rather than ventricular tachycardia.

A “bradycardia dependent bundle branch block” is a rare ECG finding. You may see a glimpse of it after a pause.

This is a tracing from this week: Bundle branch block only at very slow rates

There are a number of important differential diagnoses:

The most important is an idioventricular rhythm

Features:

• No AV association and P waves move in and out of the rhythm (red highlight).
• The rate of the idioventricular rhythm is usually fixed and almost the same rate as the sinus rhythm, which varies with sinus arrhythmia and exertion.
• Look for fusion beats.

Fusion beat have the characteristics of their parents and there is a gradual progression toward the ventricular rhythm as the sinus P waves are swallowed up.

Another tricky differential is ventricular pacing. With single chamber pacing (VVI), the appearance is identical to idioventricular rhythm with the ectopic ventricular focus moving in and out of the sinus rhythm again with fusion beats.

With DDD pacing, the appearance is the same as rate dependent bundle branch block, but the PR interval or AV delay differ with the conducted beats always being shorter than the paced beats.

There is an exception to this rule, we recently discussed (fun with ECGs 16).

Just to complete the picture, there is another related rhythm disturbance.

Isorhythmic AV dissociation.

Features:

• Bradyarrhythmia
• P waves wandering in and out of the QRS
• The QRS complexes remain the same and there is no bundle branch block.

This time, the escape rhythm is junctional although an idioventricular rhythm could also be called isorhythmic AV dissociation as it follows the same principles.

Its all in the timing.

Harry Mond