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.

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