Wednesday, September 14, 2022

ECG Blog #332 — 3rd-Degree with Double BBB?

The ECG in Figure-1 — is from a woman in her 60s, who presented with “chest tightness” over several days. This tracing was diagnosed as showing complete AV block.

  • Do YOU agree with the interpretation? OR — Is this “trifascicular" block?
  • Is there evidence of infarction on this tracing?

Figure-1: 12-lead ECG and long lead II rhythm strip — obtained from a woman with "chest tightness" over several days. Is this complete AV block? — Is it "trifascicular" block? (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:
As always – I favor beginning interpretation with assessment of the long lead rhythm strip — using the Ps, Qs & 3R Approach to recall the KEY Parameters (See ECG Blog 185). I find it easiest (and most productive) to delay assessing the 12-lead ECG until after I’ve had a chance to look at the rhythm.

PEARL #1: As I have often emphasized — It does not matter in what sequence you choose to assess the KEY Parameters — and I often START with whichever of these parameters is easiest to assess.
  • Today's tracing is extremely complicated. On initial inspection of this rhythm — it simply is not immediately evident: i) Why QRS morphology is changing? (ie, Bundle branch block or ventricular escape beats?); ii) Why the ventricular rate is changing? — and, iii) What kind of AV block is present?

  • As a result of the above complexities — My "eye" focused on beat #4 in Figure-1 because: i) This beat occurs earlier-than-expected; ii) The QRS complex of beat #4 is not as wide as the QRS in the first 3 beats; and, iii) The PR interval preceding beat #4 = 0.18 second, which makes it the only one of the 6 beats on this tracing that is preceded by a PR interval with a reasonable chance of conducting. 
PEARL #2: When a complex form of AV block is present — my favorite CLUE that a beat is likely to be conducted — is IF you see a beat that occurs earlier-than-expected.
  • In addition to its earlier-than-expected occurrence — further support that beat #4 in Figure-1 is probably conducted is forthcoming from: i) The fact that QRS morphology changes from that of the 3 preceding beats; ii) A look at simultaneously-recorded lead V1 for beat #4 suggests a very typical RBBB (Right Bundle Branch Block) morphology — in the form of an rSR' (with S wave descending below the baseline in lead V1 — and a terminal "taller right rabbit ear" R' deflection); and, iii) Beat #4 is the only beat preceded by a normal PR interval — whereas the none of the other PR intervals preceding the remaining 5 beats on this tracing seem likely to conduct.

  • NOTE: For more on HOW to tell if a P wave is (or is not) likely to conduct — Check Out the Audio Pearl in the ADDENDUM below.

PEARL #3: As I often emphasize — the simple step of labeling P waves is amazingly helpful for: i) Determining IF there is an underlying regular (or almost regular) atrial rhythm; andii) Facilitating assessment as to whether some (or all) of the P waves you identify are (or are notRelated to neighboring QRS complexes.
  • In Figure-1 — I began my “Search for P Waves” — by labeling those P waves that I was certain are present. I have done this with RED arrows in Figure-2.
  • Accounting for slight variation in the P-P interval that is so often seen — it seems logical to anticipate that 2 additional sinus P waves are likely to be hiding within the QRS of beats #1 and 2 (PINK arrows in Figure-2).

PEARL #4: It is common to see slight variation in the atrial rate when there is 2nd- or 3rd-degree AV block. This phenomenon is known as ventriculophasic sinus arrhythmia — and is thought to be due to better perfusion for those P waves that "sandwich" a QRS complex (with resultant slight shortening of the P-P interval). There may then be slight lengthening for P-P intervals that do not contain a QRS between them.
  • To Emphasize: The above relationship showing slight shortening of the P-P interval for those P waves that "sandwich" a QRS complex between them — does not always hold. That said — we do for the most part see this relationship hold true for most P-P intervals shown in Figure-2.

  • Doesn't the labeling of P waves with ARROWS in Figure-2 — facilitate determining which of the P waves in today's tracing are likely to have a chance to conduct?

Figure-2: I have added RED arrows over those P waves that I am certain are present in the long lead II rhythm strip.

Is There Complete AV Block in Figure-2?
Ideally — we would have a much longer period of monitoring for determining the rhythm in Figure-2. That said — I find it helpful to break down our assessment of the rhythm into several Parts:
  • Part-1: Look at the first 3 beats in Figure-2. The QRS complex for these first 3 beats is wide — with a morphology potentially consistent with LBBB (Left Bundle Branch Block).
  • The 6 P waves associated with these first 3 beats appear completely unrelated to the QRS (ie, with a constantly changing PR interval). The 1st, 3rd and 5th P waves in the long lead II rhythm strip ( = the first 3 RED arrows) have clearly had more than adequate opportunity to conduct — yet failed to do so. This suggests that at the least — there is high-grade 2nd-degree AV Block.

  • Part-2: As we have already deduced (ie, in Pearls #1 and #2 above) — the degree of AV block for the rhythm in Figure-2 can not be "complete" — because beat #4 is conducted! We know this — because beat #4 occurs earlier-than-expected — beat #4 is preceded by a normal PR interval and QRS morphology of this beat #4 changes from the LBBB-like appearance of the first 3 beats in Figure-2 — to a QRS morphology consistent with RBBB conduction.

  • Part-3: The remaining 2 beats in Figure-2 ( = beats #5 and 6) are not conducted. We know this because: i) None of the P waves associated with these last 2 QRS complexes occur at a point in the cardiac cycle when they would be expected to conduct; and, ii) The R-R intervals between beats #4-5 and beats #5-6 are equal to each other — and — these R-R intervals are significantly longer than the R-R interval preceding beat #4 that we know is conducting. This suggests that beats #5 and 6 must be "escape" beats that manifest (at least in lead II) a similar QRS morphology as conducted beat #4.

Figure-2: Putting It All Together
  • High-grade 2nd-degree AV Block is present in the long lead II rhythm strip in Figure-2 — because there are many P waves that clearly have more than adequate opportunity to conduct, yet fail to do so. But because at least 1 QRS complex is conducted (ie, beat #4) — the degree of AV block can not be complete!

  • The 1 beat in Figure-2 that is conducted ( = beat #4) — is conducted with an underlying RBBB (based on the typical rSR' morphology of this beat in lead V1 — as was highlighted in Pearl #2 above).
  • Because QRS morphology of "escape" beats #5 and 6 in the long lead II rhythm strip is similar to QRS morphology of conducted beat #4 — this suggests that the site of these escape beats is from the AV Node. The wide terminal S wave in simultaneously-recorded lateral leads V5 and V6 for beat #6is consistent with RBBB conduction.
  • Presumably — the first 3 beats in this tracing, that manifest AV dissociation with more pronounced QRS widening (ie, beats #1,2,3) — are arising from an "escape" site below the AV Node (ie, either from the right bundle branch or from ventricular myocardium).

  • What do the measurements that I've added in Figure-3 show?

Figure-3: I've measured the R-R intervals between each of the beats in today's rhythm. I've also dropped 3 vertical BLUE lines to show where the QRS complex begins for beats #1, 4 and 6 in simultaneously-recorded leads.

Figure-3: What Do My Measurements Show?
When I first saw today's tracing — I wondered if beats #1,2,3 might be supraventricular, given the close resemblance of QRS morphology for these beats to LBBB conduction. As a result — I initially wondered if alternating BBB (Bundle Branch Block) might be present — with change from LBBB to RBBB conduction beginning with beat #4?
  • I found it interesting that caliper measurement reveals an equal R-R interval between beats #1-2 and beats #2-3 ( = 1480 msec.) — and — that this R-R interval is different than the R-R interval between beats #4-5 and beats #5-6 ( = 1530 msec.). The fact that these 2 R-R intervals are different — suggests there are 2 different "escape" rhythm sites.
  • The finding of similar (if not identical) QRS morphology for conducted beat #4 — and non-conducted "escape" beats #5,6 — essentially localizes the site of this "escape" focus to the AV Node.
  • I thought this finding made it much more likely that beats #1,2,3 represent a ventricular escape focus (possibly arising from the right bundle branch).

  • Finally — I dropped verticle BLUE lines passing through simultaneously-recorded leads for beats #1, 4 and 6. These lines begin just before the onset of the QRS complex. They explain why despite RBBB conduction — the QRS complex does not look overly wide for beats #4,5,6 in the long lead II rhythm strip (ie, the initial part of the QRS in lead II lies isoelectric with the baseline).

Is There Evidence of Infarction in Figure-3?
Common things are common. Given the presence of bradycardia + high-grade 2nd-degree AV block + underlying RBBB — recent infarction has to be strongly considered as the potential precipitating factor for these multiple conduction defects.
  • Assessment of ST-T wave changes during a ventricular rhythm is usually difficult (and often not possible). As a result — I did not think much could be said about the possibility of recent infarction from assessment of ST-T wave appearance of beats #1,2,3 in Figure-3 — because these first 3 beats in the rhythm presumably arise from a ventricular escape focus.
  • Beats #4,5,6 manifest RBBB conduction. While not definitive — I thought the appearance of ST-T waves in several of the chest leads leads in which this supraventricular RBBB conduction morphology appears was potentially suspicious for a recent (or acute) cardiac event. That is: i) Usually the ST segment in lead V1 with RBBB is depressed — and not at the baseline, as it is in Figure-3. There is also usually no terminal T wave positivity with RBBB in lead V1; ii) ST depression with RBBB is usually also seen in lead V2 — so the distinctly flat ST segment that we see in lead V2 of Figure-3 is not typical; and, iii) The T waves in leads V5,V6 appear larger-than-expected given R wave amplitude in these leads (ie, These T waves may be hyperacute).

  • To Emphasize: The above described ST-T wave changes are not definitive. Instead — they are subtle! But in the setting of the advanced conduction defects that we see in today’s tracing — these subtle ST-T wave changes are suspicious for a possible recent event — and — this suspicion justifies obtaining serial tracings and troponins, and possibly performing cardiac catheterization (depending on specifics of the clinical situation).

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.

  • The term, “trifascicular block” is no longer recommended (Surawicz et al — JACC: Vol. 53, No. 11, pp 976-981, 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 in a patient with bifascicular block is due to AV Nodal disease or disease in the remaining conducting fascicle.
  • The Exception: Rarely, one may be able to diagnose involvement in all 3 conduction fascicles — if for example, there is RBBB and LAHB that alternates with RBBB/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.
  • Regarding Today’s Tracing: As stated above — the conduction defects in today’s tracing include: i) High-grade 2nd-degree AV block (but not complete AV block!); and, ii) Underlying RBBB for the 1 conducted beat, and for the 2 AV Nodal escape beats. Since there is no indication of either left anterior or posterior hemiblock — there is no evidence in today's tracing for “trifascicular” block.

Final Confirmation of the Rhythm in Today's Case:
The BEST way to demonstrate the etiology of a complex cardiac rhythm — is by construction of a Laddergram — which I illustrate in Figure-4.

  • NOTE: For more on how to read (and/or draw) Laddergrams — Please check out our ECG Blog #188 (which includes teaching aids + LINKS to more than 50 illustrated laddergrams I have published).   

Figure-4: My proposed laddergram for explaining the mechanism of today’s rhythm (See below).

Explanation of the Laddergram in Figure-4:
  • The first 3 beats in the long lead II rhythm strip most likely represent a ventricular escape rhythm (which is why these beats originate from the bottom of the Ventricular Tier). None of the first 6 P waves (labeled "a"-thru-"f") are able to conduct to the ventricles — so there is AV dissociation during the first half of this tracing.
  • The KEY to interpreting today’s rhythm lies with recognition that beat #4 is conducted, albeit with RBBB. The fact that at least 1 on-time P wave (this being the P wave labeled “gis able to conduct — tells us that the degree of AV block is not complete.

  • Following beat #4 — there is again AV dissociation, as none of the next 5 on-time P waves (labeled "h"-thru-"l") are conducted to the ventricles.
  • While true that the on-time P waves labeled “i” and “k” do not have a reasonable “chance” to conduct (because they either occur with a PR interval that is too short to conduct — or they occur right after the QRS during the absolute refractory period) — the P waves labeled “h”, and especially “jshould be able to conduct, but fail to do so. This supports our conclusion that there is high-grade 2nd-degree AV block.
  • Since the R-R intervals between beats #4-5 and beats #5-6 are equal to each other, and clearly longer than the R-R interval preceding conducted beat #4 — and — since QRS morphology of beats #5 and 6 is similar (if not identical) to QRS morphology of conducted beat #4 — beats #5 and 6 presumably represent a junctional escape focus that also manifests RBBB conduction.

CASE Follow-Up:
Unfortunately, my follow-up to today’s case is limited. I know that troponin was normal — and that the patient did not have hyperkalemia. A permanent pacemaker was placed — and the patient was discharged from the hospital after several days without further complication.


Acknowledgment: My appreciation to 林柏志 (from Taiwan) for the case and this tracing.


Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation. (This block post also reviews the concept of "Trifascicular Block").
  • ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation

  • ECG Blog #188 — Reviews how to read (and/or drawLaddergrams (plus LINKS to more than 50 clinical examples of laddergrams I have drawn).

  • ECG Blog #203 — Reviews the ECG diagnosis of Axis and Hemiblocks.
  • ECG Blog #204 — Reviews a user-friendly approach for diagnosis of the Bundle Branch Blocks.

  • ECG Blog #186 — and ECG Blog #236 — for review on the basics of 2nd-degree AV Block.
  • ECG Blog #192 — Reviews the 3 Causes of AV Dissociation — and emphasizes why AV Dissociation is not the same thing as Complete AV Block. 
  • ECG Blog #191 — Emphasizes the difference between AV Dissociation vs Complete AV Block.
  • ECG Blog #202 — and ECG Blog #257 — Review cases regarding HOW to tell if there is (or is not) Complete AV Block.

  • ECG Blog #247 — Reviews a complex case with AV Dissociation.

ADDENDUM (9/14/2022): I've reviewed some KEY material related to today's case:
  • Audio Pearl on HOW to tell if a given P wave in an AV block tracing is likely to be conducting.
  • 3 figures below (from my ECG-2014-PB) — that review "My Take" on the ECG diagnosis of Bifascicular Block.

ECG Media PEARL #61 (5:45 minutes Audio) — Reviews HOW to Tell IF a P Wave is Conducting? Being able to answer this question is KEY for determining the etiology of complicated AV Block/AV Dissociation tracings.



Figure-6: RBBB/LPHB (cont.) — ECG examples of bifascicular block.


Figure-7: Trifascicular Block? — Isolated LPHB vs marked RAD.


  1. I am as always amazed and and learnt a lot

  2. Wanted to ask, if the rate of ventricular escape is 1480 and junctional escape is 1530, why the ventricle didn't takeover the rhythm at 1480 period after the sinus capture beat?

    1. Excellent question JJ! In theory — the ventricular escape pacemaker would have taken over — but keeping in mind that the ventricular escape pacemaker is the “lowest escape site” — it was “behaving” after seeing a sinus conducted beat. Most probably IF the junctional escape site had not “woken up” — then this ventricular escape site would have reappeared a little bit later at the 1480 escape interval. There may be a slight “delay” until you see a given escape focus. That is LESS likely to happen when you have AV dissociation by “usurpation” from an accelerated escape focus.

      Hope the above makes sense. It just isn’t always exactly as we think it “should be” (ie, “Why is the sky blue?”) — but I bet the ventricular pacemaker would have kicked in later had the AV nodal escape not reappeared … — :)