What Do You Think 55

Dr. Les from the Austin Hospital in Melbourne sent me this ECG.

What do you think?

‍

To some people this ECG appears impossible to interpret.

As always, we need to break the tracing down to its elements.

• Look at the rhythm strip first.
• Is it sinus rhythm?
• Is there AV conduction.
• Atrial rhythms or ectopy.
• Ventricular rhythms and ectopy.

Let us start with the rhythm strip.

• Sinus rhythm 94 bpm (red arrows).
• First degree AV block (280 ms).
• There are two different conducted QRS complexes with the full PR interval (red highlight).

On the 12-lead ECG (red highlight):

• The first QRS is narrow with a long coupling interval of 940 ms from the previous QRS.
• The second QRS has a left bundle branch block and a shorter coupling interval of 680 ms.

The differing coupling intervals and the widening of the QRS suggest a rate dependent bundle branch block.

Because of underlying pathology in the bundle branches, the concept of a block is often regarded as permanent and irreversible. There are, however, “functional” bundle branch blocks, which are intermittent and rate dependent. This can be seen with physiologic aberrant ventricular conduction, (phase 3 block) which is tachycardia related and involves a block usually in the right bundle branch, particularly with atrial fibrillation with a rapid ventricular response.

There is another type of rate (or cycle length) dependent bundle branch block, this time usually involving the left bundle branch. This pathologic block (again phase 3 block) may occur as the rate increases and is referred to as “tachycardia dependent”or if the rate decreases, “bradycardia dependent” (phase 4 block). Both may be a precursor to a fixed block resultant from disease or degeneration of the bundle branch. Because the mechanisms differ, both bradycardia and tachycardia rate dependent bundle branch blocks can occur in the same patient

Tachycardia dependent bundle branch block.

The term tachycardia merely refers to the rate of the bundle branch block being faster than the rate of the normal conduction.Rather than a true tachycardia, the development of the block often occurs at a rate about 50 to 70 bpm. The changes generally occur in the setting of structural heart disease and usually results in a left bundle branch block. It may occur with only minor changes in the sinus cycle length. Hence, I prefer to use the general term rate dependent bundle branch block, whereas others call it phase dependent bundle branch block.

A left bundle branch block develops as the rate increases.

The block resolves as the rate decreases.

Not surprisingly, many of these cases are in patients believed to have a fixed left bundle branch block and normal conduction appears during a bradycardia or pause such as a compensatory pause.  

Atrial ectopic compensatory pause.

Sinus rhythm with a right bundle branch block (red highlight). An atrial ectopic (blue highlight) results in a compensatory pause with the next sinus beat conducting with a narrow QRS (yellow highlight).

Why does the block occur usually in the left bundle branch and rarely in the right?

I suspect the reason is that the right bundle branch is a rounded discrete structure, whereas the left bundle branch is flat broad and fan-like.Disease in the right bundle is more likely to result in a fixed block, whereas with the broad left bundle it is more likely to progress through a rate dependent period as the disease progresses.

Ventricular ectopic compensatory pause.

Sinus rhythm with a left bundle branch block (red highlight). A ventricular ectopic (blue highlight)results in a compensatory pause with the next sinus beat conducting without a bundle branch block (yellow highlight).

In a patient with normal conduction at rest, transition to a rate dependent bundle branch block may occur at a normal heart rate but only detected during sinus tachycardia and misdiagnosed as ventricular tachycardia.

Rate dependent bundle branch block with transition at 60bpm.  With sinus tachycardia, the rhythm was mistaken for ventricular tachycardia. An ectopic (red highlight) resulted in a pause with a P wave (P).  Note that the heart rate changes are minute, which is not uncommon.

As most of these blocks involve the left bundle branch, it seems intuitive that rate dependent left fascicular blocks should also occur.

Sinus rhythm, normal QRS axis and inferior T wave changes(red highlight). Left anterior fascicular block (yellow highlight) occurs with minimal changes in rate.

Idioventricular rhythm is an important differential diagnosis. Unlike rate dependent bundle branch blocks, there is AV dissociation.

Regular sinus rhythm (red highlight, red arrow). The P waves move in and out of the ventricular escape rhythm.  

Bradycardia dependent bundle branch block.

A bradycardia dependent bundle branch block is a rare ECG finding with the mechanism related to phase 4 of the action potential.

Following a sinus pause, the next sinus beat is conducted with a wide QRS.

A bradycardia dependent bundle branch block may follow a Wenckebach AV pause.

Sinus rhythm with first degree AV block (red highlight). With Wenckebach AV block pauses, a left bundle branch block occurs (yellow highlight).

Another example of a subtle rate change.

2:1 AV block with a ventricular rate ~40 bpm. Conversion from left bundle branch block to normal conduction occurs without any measurable rate change. Is this bradycardia or tachycardia dependent bundle branch block?  

Let us return to our ECG.

There are also two types ventricular ectopics (highlight).

‍

On the 12-lead ECG.

• Two premature ventricular ectopics (yellow highlight) with the next sinus P wave, concealed and non-conducted (red stippled arrows).
• Three late ventricular ectopics (blue highlight) and fall in the next PR interval (end diastolic). They differ in configuration from other ventricular ectopics because they fuse with the propagating conducted beat and thus are fusion beats.  

Tell me about end diastolic ventricular ectopics and fusion beats!

If a very late ventricular ectopic occurs after the on set of the next sinus P wave and lies in the PR interval it is referred to as “enddiastolic”. It results in a fusion beat where ventricular depolarization is an amalgam initially from the ventricular ectopic and then normal AV conduction.  Although very late, the ventricular ectopic is still premature as it lies before the onset of the conducted QRS but there is usually no compensatory pause.

Sinus rhythm with end diastolic ventricular ectopics (red highlight) commencing in the PR interval of the next sinus beat.

Depending on the timing, end diastolic ventricular ectopic fusion beats will differ in appearance depending on their contribution to theQRS.  

Sinus rhythm with a run of end diastolic ventricular bigeminy. The first ventricular ectopic occurs within the sinus P wave and is not fused (red highlight). The second and third ventricular ectopics occur within the PR interval and have different levels of fusion (yellow, blue highlight).

The resultant fusion beat may have a narrow QRS resulting in it being missed or called a junctional or fascicular escape beat (red highlight).

Although the QRS is narrow in some leads, in others it is wider (red highlight).

So an ECG that looked impossible was not difficult at all, using basic interpretive principles, arrows and highlight.

To summarize:

• Sinus rhythm 94 bpm.
• First degree AV block (280 ms).
• Rate dependent bundle branch block.
• Ventricular ectopics – either prior to the nextP wave (concealed and non-conducting) or end diastolic with fusion.

‍

Harry Mond