Thursday, April 6, 2023

RBBB and LAFB. Is it trifascicular block? The complexities of Wenckebach, with Ken Grauer analysis.

This patient presented with complications of his dialysis fistula.

Medical history:

History of Ischemic Heart Disease

History of Congestive Heart Failure

Diabetes Mellitus requiring treatment with insulin

Pre-operative serum creatinine >2 mg/dL

He had this ECG recorded:

What do you think?

There is AV 2nd degree AV Block (Mobitz I, 
Wenckebach) with RBBB and LAFB

The physicians recognized the RBBB and LAFB, but not the Wenckebach. They compared to a previous ECG and thought they were identical.  Here is the previous ECG from 2 months prior for pre-op for cataract surgery

This appears to be atrial fibrillation with RBBB and LAFB.  I don't see P waves and of course Wenckebach requires P-waves.  

See Ken's interpretation below.

Are these ECGs of similar risk?

I could not find a single case report of Wenckebach with RBBB and LAFB, so I don't know the risk.

However, 1st degree AV block with RBBB and LAFB carries the misnomer of "trifascicular block" and portends complete (3rd degree) AV block, and mandates admission if the patient presents with syncope.

If the patient does not present with syncope, then outpatient management is probably OK, with referral to cardiology.

Interestingly, the patient presented multiple times in the next months with these Wenckebach ECGs which were not recognized:

This was interpreted as "First Degree AV Block".  But it is Wenckebach.

This was interpreted as "First Degree AV Block" by a cardiologist.  But it is Wenckebach.

This was interpreted as "Atrial Fibrillation".  But it is Wenckebach.

This was interpreted as "Atrial Fibrillation".  But it is Wenckebach.

This was interpreted as "First Degree AV Block" by Cardiology.  But it is Wenckebach.


MY Comment, by KEN GRAUER, MD (4/6/2023):


I found today's case presented a number of important concepts regarding the clinical occurrence and significance of combined conduction system disorders. The patient in question is a presumably older adult, for whom an ECG was obtained following cataract surgery ( = ECG #1 in Figure-1). 
  • prior ECG was on file from 2 months earlier ( = ECG #2). 

  • Are these 2 ECGs really different?  

Figure-1: Comparison between the 2 ECGs in today's case.

MY Thoughts on ECG #1 in Figure-1:
As per Dr. Smith — the "capsule" interpretation of ECG #1 is that it shows bifascicular block (ie, RBBB/LAHB) + AV Wenckebach. Taking a Closer LOOK:
  • There is a fairly marked sinus arrhythmia (RED arrows in Figure-2). This point becomes very important when we get to comparing ECG #1 with ECG #2 (which I'll do momentarily).

  • PEARL #1: It is common with 2nd- and 3rd-degree AV block to see a "ventriculophasic" sinus arrhythmia. Usually with ventriculophasic sinus arrhythmia, the P-P intervals that "sandwich" QRS complexes tend to have shorter P-P intervals than those P-P intervals that do not contain a QRS between them. The physiologic reason for this — is thought to be the result of momentarily increased circulation from mechanical contraction arising from the "sandwiched in" QRS complex.
  • In Figure-2 — We do not consistently see this P-P duration relation (ie, the shortest P-P interval is between beats #2-3, which do not contain a QRS — whereas the P-P interval between the first 2 P waves on this tracing is relatively much longer [ie, 780 msec.]).

  • PEARL #2 (Beyond-the-Core): A possible reason for the marked and unpredictable P-P interval variation seen in Figure-2 could be that this patient has SSS (Sick Sinus Syndrome). As a result — obtaining a 24-hour Holter Monitor would be important not only looking to see if there is ever complete AV block — but also looking for "telltale" pauses beyond 2.5-to-3.0 second that might document underlying SSS.

  • PEARL #3: Awareness of the marked and unpredictable nature of this patient's sinus arrhythmia will become very important momentarily when we look more closely at ECG #2.

PEARL #4: Whenever you see Mobitz I, 2nd-degree AV Block — it is important to carefully scrutinize ECG findings on the rest of the tracing, looking for a potential reason WHY the patient may have AV Wenckebach. Given the prevalence of "silent infarction" among older individuals — recognition of AV Wenckebach should prompt close attention to the possibility of recent prior inferior (or infero-postero) infarction.
  • While small in size — the Q waves in leads I and aVL of ECG #1 are wider-than-expected. These Q waves could be a marker for prior high lateral infarction.
  • The shape of the ST-T wave in lead V2 of ECG #1 (with terminal T wave positivity) could be consistent with recent posterior infarction. To EMPHASIZE: ECG #1 is in no way by itself diagnostic of OMI. But these subtle ECG findings in association with bifascicular block + AV Wenckebach merit a careful history and perhaps a repeat tracing.

Figure-2: For clarity — I've labeled P waves from ECG #1 with RED arrows, adding RED arrows also to simultaneously-recorded leads when I wanted to verify that smaller (partially hidden) deflections in the long lead II rhythm strip truly represented sinus P waves. I've also carefully measured all P-P intervals in milliseconds — to illustrate the surprisingly marked sinus arrhythmia.

LADDERGRAM Illustration of the Mobitz I:
The providers who saw ECG #1 recognized the bifascicular block — but failed to recognize that there was also Mobitz I ( = AV Wenckebach) 2nd-Degree AV Block. The laddergram in Figure-3 clarifies the presence of typical Wenckebach conduction.
  • PEARL #5: The KEY for facilitating recognition of AV Wenckebach — is to know when to look for it! Stepping back a little bit from the tracing in Figure-3 — there is group beating (ie, 3 "groups" of beats there are shorter R-R intervals of approximately equal duration — separated by 3 longer R-R intervals of approximately equal duration)To Emphasize — The "shorter" R-R intervals between beats #1-2; 6-7; and 8-9 are almost the same duration. The "longer" R-R intervals between beats #2-3; 5-6; and 7-8 are also almost the same duration. This is unlikely to occur by chance!

  • PEARL #6: Although many of the P waves in Figure-3 are at least partially hidden — the KEY is to recognize that the PR interval at the end of each of the relative pauses (ie, the PR interval before beats #3, 6 and 8) — is the same (ie, 0.24 second). This is definitely not by chance!

  • P.S.: When considering the possibility of Mobitz I vs Mobitz II — a 1st-degree AV block for the 1st conducted beat in each group is much more commonly seen with Mobitz I.

Figure-3: Laddergram illustration to clarify the presence of 2nd-Degree AV Block, Mobitz Type I ( = AV Wenckebach) — here with several groups of 3:2 AV conduction, and 1 group with 4:3 AV conduction.

Taking Another LOOK at ECG #2:
I will emphasize that there is a tremendous amount of artifact in ECG #2. As a result — I fully acknowledge that I can not be certain of all that is going on in this tracing. That said — I strongly suspect that instead of AFib — that this patient was in the same 2nd-Degree, Mobitz I AV Block on the pre-operative ECG, that was done 2 months earlier ( = ECG #2 in Figure-4).
  • Rather than the complete random occurrence of QRS complexes — there is a semblance of group beating in the long lead II rhythm strip in Figure-4. That said — there are more R-R intervals of similar duration that I would expect by chance.
  • But the strongest CLUE (in my opinion) to the presence of AV Wenckebach in ECG #2 — is highlighted by RED arrows in the long lead II rhythm strip. I believe that despite the heavy baseline artifact — that we still can see a more prominent "hump" preceding the QRS complex of beats #4710 and 11 — with each of these RED-arrow sinus P waves suggesting the same prolonged PR interval (0.24 second) as was seen in ECG #1.
  • While fully acknowledging marked artifact in all 12 leads — I believe additional RED arrows that I've placed in selected other simultaneously-recorded leads confirm that the RED arrows that I've drawn in the long lead II of ECG #2 are indeed sinus P waves with a long PR interval.
  • I believe there is an extra "something" (ie, beyond artifact) under most of the spots where I've placed PINK arrows in the long lead II of ECG #2. Isn't the cadence (ie, amount of R-R interval variability) that we identified in ECG #1 consistent with the cadence of R-R variability highlighted by PINK and RED arrows in ECG #2?

  • Finally — ECG #2 in today's case was obtained 2 months prior to ECG #1. Given lack of any significant change in QRS and ST-T wave morphology between these 2 tracings — Isn't it more logical that this patient has been in AV Wenckebach the whole time? — rather than to postulate that 2 months ago this patient spontaneously (and asymptomatically) converted from AFib to a sinus rhythm that now manifests Mobitz I conduction?

  • BOTTOM LINE: As I emphasize above — there is simply too much artifact to be 100% certain of my theory. But the KEY Teaching Points for my delving into so much detail regarding these 2 tracings are: i) That the truly sharp emergency provider will hopefully grasp the subtle-but-very-important clues I provide to facilitate recognition of complex AV conduction disturbances; and, ii) That the Lesson-to-be-Learned when you see artifact that prevents valid interpretation — is to repeat the ECG.

Figure-4: Taking another LOOK at the comparison between ECGs #1 and #2. I suspect this patient was in AV Wenckebach in both tracings (See text).

ADDENDUM: Is There Tri-Fascicular Block?

The term, “trifascicular” block — implies impaired conduction in all 3 of the major conduction fascicles: i) the right bundle branch; ii) the left anterior hemifascicle; andiii) the left posterior hemifascicle.

  • That said — the term, “trifascicular block” is no longer recommended (Surawicz et al, Circulation — AHA/ACCF/HRS Recs, 2009). This is because of “the great variation in anatomy and pathology producing this pattern” — as well as the fact that one will usually not be able to make a definitive diagnosis of trifascicular block from the surface ECG. We simply can not tell IF PR interval prolongation or Mobitz I in a patient with bifascicular block (as occurs in today's case) is due to AV nodal disease or disease in the remaining conducting fascicle.
  • Rarely, one may be able to diagnose involvement in all 3 conduction fascicles — if for example, there is RBBB and LAHB that alternates with LPHB. But even in this circumstance — Current recommendations favor clarity in description by avoiding the term “trifascicular block”, and instead noting each of the conduction defects that are present (ie, in Figure-4  there is Mobitz I, 2nd-degree AV block + RBBB + LAHB).

  • Final Thought: What then to do for today's patient with Mobitz I + RBBB + LAHB? As per Dr. Smith — as long as this patient has been asymptomatic (ie, no syncope or presyncope) — outpatient management with referral to cardiology seems appropriate. Along the way — I'd order a 24-hour Holter monitor for the reasons I specified earlier (ie, to ensure no episodes of transient complete AV block — and no prolonged pauses or other ECG findings suggestive of SSS).


Dr. Smith added 5 tracings to the end of today's case. Each tracing shows a variation of AV Wenckebach block ( = Mobitz Type I 2nd-degree AV block) that was not detected by the interpreting physicians. The message is clear:

  • "Birds of a feather flock together". This English proverb has served me well in the field of arrhythmias. By this I mean — that when I encounter a part of a complex ECG that I do not understand — if everything else on the tracing points to a certain phenomenon — then the chances are high that the part of the tracing that I don't understand is also a manifestation of that phenomenon.
  • In today's case — there are multiple variations of AV Wenckebach. As a result — the chances are excellent that additional initially unclear rhythms will also turn out to be some form of Wenckebach.

  • Classic AV Wenckebach is EASY to recognize. The PR interval progressively increases until a beat is dropped — and then the cycle begins again with shortening of the PR interval after the brief pause.
  • That said — there are multiple potential variations of AV Wenckebach, including phenomena such as junctional escape beats, PACs that reset the cycle, echo beats, dual AV nodal pathways (each with it's own AV conduction properties) — and others. Suspecting that some form of AV Wenckebach is present — aided by awareness of these potential variations on the Wenckebach "theme" plus a set of calipers (to quickly confirm PR interval duration) — will usually be all that is needed to confirm the diagnosis.

  • Failure by multiple providers to consider the "Birds of a feather" truism in today's case — is what led to overlooking the diagnosis of AV Wenckebach on multiple occasions.

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