ECG Blog #130 (AV Block – ABBB – RBBB – LBBB)

The 3 successive lead MCL-1 rhythm strips that are shown in Figure-1 were obtained from a 56-year old man with dyspnea, but no chest pain.

• How would you interpret the rhythm?

Figure-1: Lead MCL-1 showing 3 successive rhythm strips from a patient with dyspnea. Can you explain what is happening? 

Although there is slight distortion of some QRS complexes, and the ECG grid is not well seen — this is a fascinating tracing!

PEARL: As we have done with several of our recent ECG Blog posts — We begin by noting 3 Helpful Steps for facilitating interpretation of complex arrhythmias: 

• i) Look first for an underlying rhythm; then, 
• ii) Use calipers (as by far the fastest, easiest, and most accurate way to seek out atrial activity and determine relationships between P waves and neighboring QRS complexes); and;
• iii) On a copy of the rhythm strip (so that you do not write on the original tracing) — Mark the presence of sinus P waves that you can clearly see. We have done this in Figure-2:

Figure-2: We have numbered the beats in the middle (Panel B) and lower (Panel C) tracings, and marked (with RED arrows) the presence of sinus P waves that we clearly saw in Figure-1 (See text).

Interpretation: Use of calipers makes it readily apparent that regularly occurring sinus P waves are present throughout this tracing (RED arrows). There is some conduction. That said, there are 2 different QRS complexes, and the PR interval is not the same in front of all conducting beats ...

• Start with What You Know — Focusing on the middle and lower tracings in Figure-2 (Panels B and C) — beats #1, 2, 12, 13 and 14 are all preceded by a similar-looking P wave with a constant PR interval. This tells us that these beats are clearly being conducted.
• Unfortunately, the ECG grid is not clear. There is also no 12-lead ECG on this patient — which means that our assessment of QRS morphology is limited to this single right-sided MCL-1 monitoring lead. That said, the QRS complex for all beats on this tracing looks to be widened. The predominantly negative rS configuration of beats #1,2,12,13 and 14 is consistent with LBBB (Left Bundle Branch Block).
• Beats #3,4,5,6,7,8,9,10 and 11 also appear to be conducted — as the PR interval preceding these beats looks to be constant. However, QRS morphology of these beats in this right-sided MCL-1 lead suggests a change to RBBB (Right Bundle Branch Block) conduction. If confirmed on a 12-lead — this would mean there is ABBB (Alternating Bundle Branch Block).
• There is also 2:1 AV Block in some parts of Figure-2. Interestingly — 2nd-Degree AV Block with 2:1 AV conduction occurs in association with the QRS complexes manifesting LBBB (ie, beats #1,2,12,13 and 14). In contrast — 1:1 AV conduction occurs in association with the QRS complexes manifesting RBBB (ie, beats #3,4,5,6,7,8,9,10 and 11). The question is why?

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The Parts of this Tracing We are Not Certain About …

There are some additional confounding findings on this tracing. These relate to a highly unusual pattern of variation in the PR interval that was not apparent to us on initial assessment of this tracing (Figure-3).

Figure-3: Caliper measurement reveals a highly unusual pattern of PR interval variation, which we have color coded for clarity (See text).

Explanation of Figure-3: It turns out that the PR interval preceding all beats with LBBB morphology is the same (short RED horizontal lines). The PR interval preceding all beats with RBBB morphology is also constant (short YELLOW horizontal lines) — however, this PR interval (the yellow lines) is slightly longer than the PR interval preceding LBBB beats (red lines). To add to the complexity — the PR interval preceding the 1st RBBB beat in a run inexplicably has an even longer PR interval (BLUE horizontal lines in Figure-3).

• The fact that other than this 1st RBBB beat in Panels A and B — the PR interval preceding each RBBB beat is the same (yellow lines) clearly indicates that these RBBB-pattern beats are being conducted. However, we cannot explain the changing PR interval relationship over the course of Figure-3 that we just described, as it is not what would be expected with simple Wenckebach conduction, dual AV nodal pathways, or vagotonic AV block.

BOTTOM Line: This is a fascinating tracing that we admittedly cannot completely explain. That said, we can state the following:

• There is 2nd-Degree AV Block, with intermittent 2:1 AV conduction. This is associated with an unusual pattern of variation in the PR interval, as shown by the color-coded PR intervals in Figure-3.
• There is significant bradycardia during 2:1 AV conduction (the ventricular rate drops down to the 40s).
• There appears to be ABBB (Alternating Bundle Branch Block). It is rare to see true alternating bundle branch block. When this phenomenon does occur, it almost always indicates severe His-Purkinje disease. Given the associated AV block with significant bradycardia — it is highly likely that a pacemaker will be needed.

Concluding NOTE:

This case illustrates how even when we are unsure of certain aspects in our interpretation — we can still arrive at the appropriate next step in management.

• Additional comments are welcome!
• P.S. — I think between the comment below by Jan S and my reply (7/5/2016) — that we may have arrived at reasonable explanation for the unusual variation in PR intervals ...

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Acknowledgment: — My thanks to Bady Hanna Adly (from Asyut, Egypt) for his permission allowing me to use this case and ECG.

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