October 16, 2015

A patient with palpitations and dizziness. What could cause this "bizarre" ECG pattern?

ECG case presented by Dr Bojana Uzelac:

(ECG strips courtesy of Dr. Stefan Savić) These are tracings from 78yo man, c/o palpitations and dizziness (previously history of HTA and stable AP). What do you think about rhythm here? Any thoughts and what could cause different QRS morphologies ?

(Case posted in ECG Rhythm FP page 10.10.2015 by Dr. Bojana Uzelac - https://www.facebook.com/ecgrhythms/posts/623608487741802




This was one of the most challenging rhythms I’ve analyzed lately, so I needed more time than usual for conclusion. I have a great help from Arnel Carmona in this.

People who love (complex) ECG would like to say that every decent arrhythmia has more than one explanation. As Moutaz Elkadri said in his comment, “sometimes it’s not about the diagnosis but the mental process of forming a differential diagnosis”.

So, I will give a few possible explanations together with the correct one, by my opinion.
On the surface ECG (Picture 1), several findings stand out. First, it is important to notice that atrial activity here is regular; this is sinus rhythm of 75 bpm. Next thing you should see is the repetitive pattern of grouping beats. They are grouped in two or in three QRS complexes and after every group there is a dropped (non-conducted) P wave or there is a 4:3 and 3:2 pattern of conduction.

Picture 1. Grouping beats in red circles

So, there must be some sort of AV block here, right?
If we just focus with the 3:2 AV conduction pattern, then the first possible interpretation is sinus rhythm with AV Wenckebach (second degree AV block type I or Mobitz I).
Wenckebach with a twist
 If we analyze further, there are three different types of QRS complexes.

Picture 2. Beat labeled with I- normally conducted beat; II-beat with RBBB morphology; III-beat with LBBB morphology. 

The first QRS complex (in the group) is conducted with a PR interval of 200 ms; but otherwise it is the narrow beat with a high voltage in precordial leads suggestive for LVH morphology.

QRS # 2 shows RBBB morphology: it is 160 ms in duration, with rSR morphology in V1 and wide S in leads I and V6. This QRS is always second in the group and has longer PR interval (320 ms).

If we look at the group with 4:3 AV conduction ,the last QRS shows LBBB morphology. It is 180 ms in duration; has monophasic R in leads I, aVL and V5-V6 (also showing mid-QRS notching in these leads) and QS in lead V1. The PR interval is very close to PR interval of the first beat in a row, approximately 200 ms.

Picture3. First in a group is always normally conducted beat (red circle); second is showing RBBB shape (blue circle) and third QRS has LBBB morphology
Going back to the Wenckebach theory, this is second degree AV block type I. (Picture 3) The second beat in a group (meaning #4,#7 and #9) is aberrantly conducted. This can be considered “as Ashman's phenomenon because the preceding RR interval is long which would cause the refractory period of beats 3, 6 and 8 to lengthen, resulting in the beats 4, 7 and 9 to land on the tail end of the lengthened refractory period of the bundle branches causing aberrant conduction, which is what Ashman's phenomenon is after all.”

For beats #2 and #5 “handle as PVCs”. K. Wang

The LBBB shape beat in this case represents VPB.

Picture 4. Regular atrial activity (sinus rhythm) shown with red circle. Non-conducted beats after every group is marked with black star. Green dashes are for PR interval prolongation. PVB- premature ventricular beat.

Dual AV node conduction theory

The second explanation includes dual-level AV nodal conduction:

Picture 5. Schematic presentation of dual AV node physiology associated with presuming 2nd degree AV block type II

(Picture 5) Normally conducted beats (#1 and #4) and beats with LBBB morphology (#3) have similar (equal) PR interval of 200 ms, while RBBB shape (#2 and #5) beats have different PR interval of 320 ms. It could indicate that there is dual AV node physiology and existence of block at two levels in the AV nodal conduction system. We could say that beats #1,#3,#4 and #6 are going down fast pathway (FP) and beats #2 and #5 via slow pathway (SP). Non-conducted P waves after beats #3 and #5 in that case could indicate AV block 2nd degree 4:3 and 3:2 type II or a block in the fast and slow pathway. The change in QRS morphology could be due to change in HP system effective refractory period due to variable previous RR interval.

Bilateral Bundle Branch Block

In arrhythmia interpretation, a complex-looking or a "funky" arrhythmia must have a unifying explanation. This means that what we see on the surface ECG can due to one possible explanation and not multiple electrophysiologic phenomenon. The strip presented behaved like one of the patterns in Bilateral Bundle Branch Block (Bilateral BBB).

In Chapter 18 of  Electrocardiography of Clinical Arrhythmias by Fisch and  Knoebel (2000), "AV-delay or block caused by BBB was demonstrated by Scherf and Shookhoof in 1925. By sectioning one bundle branch and compressing the contralateral branch, they were able to elicit a spectrum of AV block ranging from prolongation of AV conduction to  complete AV block.... As a rule, AV conduction delay of block can be assumed to be caused by BBBB only in the presence of an alternating RBBB and LBBB with a changing PR interval. Not, infrequently, the bilateral BBB is due to acceleration-dependent aberration".

The concept of existing bilateral BBB assumes that “conduction in both branches is only partially interrupted and in which changes in the comparative degree of block in the two branches allow the patterns of right and left bundle branch block to appear alternately or intermittently in the same patient.” (By M. Rosenbaum, 1955.)

M. Halpern supports this concept and illustrate that an area of AV block, even with the classic Wenckebach structure, is not necessarily always located in the AV node or His bundle (meaning it could be infra-hisian). The different degrees of abnormal conductivity could affected the bundle branches unequally, and even in some cases be so severe enough as to produce complete impedance in propagation from atria to ventricles. This theory was proven by Scherf’s experiments in the dog’s hearts: the depressed area was located in the bundle branches themselves.

E. Lepeschkin (1964.) categorized various combinations of 1st, 2nd, and 3rd degree bilateral BBB. In his classification one type of a block could exists in one branch and same or other type/degree could be present in the other bundle branch. He was using the same terminology as in the case of A-V block.
”First-degree block" corresponds to decrease of conduction velocity without complete interruption of conduction.

 In "second-degree block" only a part of the impulses are conducted while the remainder is not conducted at all. In "type I" second degree block the first few conducted impulses after the blocked one show a progressive decrease of conduction velocity; while in "type II" block the conduction velocity in this ease remains constant.

In "third-degree block" no impulses are conducted during the period before a ventricular pacemaker takes over.

I’ll skip this complex classification of bilateral BBB and explain one of the sub-types: first-degree block in one bundle branch and second-degree block in the other.

If the conducted beats in the branch with second-degree block have a conduction time longer than that in the branch with first-degree block, the conduction will result in simple prolongation of P-R interval with a BBB pattern. However, if the conducted beats have a shorter conduction time, they will result in alternation between RBBB and LBBB patterns, one of these being constantly associated with a shorter PR. For example, the second degree block could be localized in the left bundle branch and resulting in a right bundle branch block pattern with a shorter PR interval or vice versa.

Picture 6. Conduction trough the right (R) and left (L) bundle branches: + represents presence of conduction; 0 is for absent conduction.
So, in our case the most likely cause of conduction abnormality is 2nd degree type I block associated with RBBB and 1st degree block associated with LBBB; this meaning presence of bilateral bundle branch block and infra-hisian site of the block.

Picture 7. Schematic conduction of bilateral bundle branch block: green dash is for a block in conduction

This patient ended with permanent pacemaker and his outcome was good.


  1. Thanks for going through the ECG with such detail. I was following the explanation well until almost the end. However, if you have just 1st degree block in one of the bundle branches, then you shouldn't have P waves that doesn't conduct. For the theory to work you need at least second degree block in each of the bundles, but then it will become really difficult to interpret?


  2. Thanks for going through the ECG with such detail. I was following the explanation well until almost the end. However, if you have just 1st degree block in one of the bundle branches, then you shouldn't have P waves that doesn't conduct. For the theory to work you need at least second degree block in each of the bundles, but then it will become really difficult to interpret?


  3. Yes, this is truly a fascinating and complex tracing. CREDIT to Bojana Uzelac for her detailed and meticulous explanation. That said — I’d propose that rather than concern by the non-electrophysiologist about uncovering a fail-proof “mechanism” for this exceedingly complex arrhythmia — appreciation of the general concepts should suffice.

    So, paraphrasing what Bojana eloquently described — the atrial rhythm is regular throughout. There are groups of beats with a repetitive pattern (albeit with slight variation). There is variation in the PR interval, but NOT is simple Wenckebach fashion. However, within groups, we consistently see either a long or short PR interval — and the same QRS morphology follows consistently after either the long or short PR interval.

    KEY — QRS morphology (looking at leads V1 and V6) strongly suggests alternating RBBB and LBBB. Putting together ALL of the above observations — We should come to the essential BASIC concept that ALL beats on this tracing ARE being conducted!

    Looking further — there IS some AV block. It is NOT 3rd degree (because some beats ARE conducted. albeit others are not …) — so therefore some type of 2nd Degree AV block — but not simple Wenckebach (because the PR sometimes gets shorter after lengthening before the beat is dropped).

    BOTTOM LINE: There is obviously VERY severe conduction system disease because there is alternating bundle branch block PLUS some type of 2nd degree AV block. Even though the overall ventricular rate is not excessively slow — there would seem to be high likelihood of progression to more severe AV block — therefore pacemaker implantation is clearly indicated.

    GREAT tracing! Wonderful explanation and illustration by Bojana Uzelac! — :)


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