ECG Blog #401 — What Kind of Block?

The ECG in Figure-1 was obtained from an elderly woman — who presented to the ED (Emergency Department) for dyspnea on exertion over recent weeks. 

• What are YOUR "Quick Thoughts" about this case?

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY "Quick Thoughts" on Today's CASE:

The ECG in Figure-1 is highly concerning — so it is indeed fortunate that this elderly woman came to the ED when she did!

• The rhythm in Figure-1 is complex — and defies precise interpretation without careful study. That said — obvious findings include: i) Marked bradycardia! — ii) Tiny-but-definitely present P waves in the long lead V1 rhythm strip — which at 1st glance look like they may (?) be regular! — iii) Non-conduction of a number of these regular P waves — suggesting at least some form of significant AV block; — and, iv) At least 3 different QRS morphologies (if not more) — which considering that the lead monitored is lead V1, may represent variations of normal, RBBB and LBBB conduction! (Right and Left Bundle Branch Block conduction).
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• PEARL #1: The goal of clinical ECG interpretation of a tracing such as this one is to expedite interpretation of "the essentials" — and to leave details of the ECG and rhythm strip shown in Figure-1 until later, when time is available. I therefore intentionally did not dive deeper than the  4 general conclusions noted above in the 1st bullet — which took me no more than seconds to arrive at! My Immediate Impression — was that this elderly woman with a several week history of symptoms would most likely leave the hospital with a pacemaker.

PEARL #2: Interpretation of the 12-lead ECG in Figure-1 is no easy task! This is because there are multiple QRS morphologies — and we do not yet know which are supraventricular (nor with what kind of conduction propoerties).  That said — LOOK to see if there is an underlying rhythm!

• Beats #5 and #7 look to be narrow and are both preceded by P waves with a constant and normal PR interval. This suggests that beats #5 and #7 are probably normal sinus-conducted beats. This means that we can assess ST-T wave morphology for ischemic changes in simultaneously-recorded leads V1,V2,V3 for beat #5 — and in leads V4,V5,V6 for beat #7.
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• There is deep, symmetric T wave inversion with a prolonged QTc interval in anterior leads V1,V2,V3. This suggests ischemia of uncertain duration.
• The ST-T wave appearance in leads V4,V5,V6 is less worrisome — with shallow T inversion in V4 — and ST segment flattening with slight depression in leads V5,V6 — but which does not look acute.
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• Beyond-the-Core: Advanced interpreters may already suspect that beat #2 in the long lead V1 rhythm strip is sinus-conducted with RBBB (given its rSR' morphology in lead V1 — and being preceded by the same PR interval before this beat as was seen for sinus beats #5 and 7). 
• Similarly — Advanced interpreters may also suspect that beat #4 in the long lead V1 is sinus-conducted with LBBB (given its all negative QRS in lead V1, and its monophasic positive morphology in lead aVL — also preceded by the same PR interval as for the other sinus-conducted beats).
• The "good news" — is that ST-T wave appearance in simultaneously-recorded leads I,II,III for beat #2 — and in leads aVR,aVL,aVF for beat #4 — does not look acute.

IMPRESSION of Today’s Tracing:

The main problem in today's case of this elderly woman with a several week history of dyspnea on exertion — is the markedly bradycardic rhythm with some form of AV block. 

• Given this patient's older age — if nothing "fixable" is found, she most likely has SSS (Sick Sinus Syndrome) and will need a pacemaker (See ECG Blog #342 for more on SSS).
• Given the history of dyspnea on exertion over a several week period (but no mention of chest pain) — and — the finding of deep, symmetric T wave inversion in the anterior leads (as per Pearl #2) — it is possible that the onset of her symptoms is the result of a "Silent MI" (See ECG Blog #228 for more on "Silent" MI). 
• KEY Point: As fascinating as today’s rhythm disorder is — a detailed explanation for the mechanism of this rhythm is not needed for optimal clinical management. Instead — the clinical points summarized in the above 2 bullets suffice! That said — I believe appreciation of the probable mechanism for today's rhythm is instructive, and reinforces our interpretation. For those with an interest in learning more — Follow along with me below!

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Looking Closer at Today's Rhythm:

Regardless of whether you recognized the advanced points I raised above under "Beyond-the-Core" — the KEY is to recognize the marked bradycardia with some form of significant AV block, with need for referral for possible (probable) pacemaker implantation.

• CREDIT if you also recognized the deep, symmetric anterior T wave inversion — suggestive of possible "silent MI" several weeks earlier at the time this patient's symptoms began.

Next Steps for Determination of the Rhythm:

• The easiest next step in interpretation — is to label P waves in the long lead V1 rhythm strip. It is because of the small amplitude of P wave deflections in this lead — that calipers so greatly facilitate (and speed up) detection of atrial activity.
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• PEARL #3: I've labeled with RED arrows in Figure-2 — the P waves in the long lead V1 rhythm strip. Some of these P waves are partially hidden within some of the T waves. This is precisely where calipers assist: — Select 2 P waves in a row that you definitely see. Then "walk out" this caliper setting throughout the entire lead V1 rhythm strip.
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• PEARL #4: Take another LOOK at the long lead V1 rhythm strip in Figure-2. Now that all P waves are labeled — Isn't it much easier to appreciate that the PR intervals before beats #1,2; 4; 5; 6; 7; and 8 are all the same! This tells us that despite the different QRS morphologies — each of these beats is sinus-conducted!

Figure-2: I've labeled all P waves in today's rhythm with RED arrows.

WHY so many QRS Morphologies?

I've already alluded to why there are multiple QRS morphologies. LOOK at Figure-3 — in which I've added colored labels to facilitate discussion.

• RED arrows in Figure-3 highlight sinus-conducted P waves — because each of the QRS complexes that follow these P waves (ie, beats #1,2,4,5,6,7 and 8) is preceded by a constant and normal PR interval.
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• YELLOW arrow P waves are not conducted. It should be apparent that there is "high-grade" 2nd-degree AV block — because in the presence of a regular atrial rhythm, we see several instances in which consecutive YELLOW arrow P waves do not conduct despite adequate opportunity to do so (See ECG Blog #399 — for more on "high-grade" AV block).
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• I distinguish the remaining P wave with a BLUE arrow — because this P wave has a longer PR interval than all of the RED arrow P waves — and is followed by a non-conducted (ie, YELLOW arrow) P wave after beat #3. This suggests that beats #2 and #3 form a 3:2 cycle of AV Wenckebach (ie, Mobitz I 2nd-degree AV block).

 

I have also labeled the different QRS morphologies in Figure-3:

• PEARL #5: The beauty of having a 12-lead ECG with a simultaneously-recorded long lead rhythm strip below it — is that this provides us with "4 looks" at QRS morphology for each of the 8 beats in today's rhythm.
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• Beats #1,4,6 and 8 — all look similar in the long lead V1 rhythm strip (ie, they all manifest a wide negative QRS complex). The reason that I know this is consistent with LBBB conduction — is that we see the typical all-positive, wide upright QRS for beat #1 in simultaneously-recorded lead I — for beat #4 in lead aVL — and for beat #8 in lead V6. QRS morphology for beat #6 is also consistent with LBBB conduction — because the QRS is predominantly negative in each of the anterior leads.
• Beat #2 is consistent with RBBB conduction — because it manifests an rSR' in the long lead V1 — with a wide terminal S wave in simultaneously-recorded lateral lead I.
• I also labeled beat #3 as consistent with RBBB conduction — because of its similarity to the QRS appearance of beat #2 in the long lead V1 rhythm strip.
• Beat #5 is not wide, but manifests a small r' deflection in the long lead V1 — so this beat most probably reflects incomplete RBBB conduction.
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• PEARL #6: Indication for permanent pacing is justified in today's case on multiple grounds, including: i) This elderly patient with marked bradycardia has been symptomatic for weeks; ii) Her rhythm is high-grade 2nd-degree AV block; and, iii) There is alternating BBB (Bundle Branch Block) — which of itself is indication of severe conduction system disease at risk of developing ventricular standstill. 

Figure-3: I've added colored labels of P waves and QRS complexes to facilitate discussion of conduction properties (See text).

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Laddergram Illustration:

At this point — I needed to work out, and then draw a laddergram that I could then verify to ensure a plausible mechanism for today's arrhythmia.

• Sequential legends over the next 4 Figures illustrate my thought process. (See ECG Blog #188 for review on how to read and/or draw Laddergrams).
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• To EMPHASIZE: Today's laddergram was challenging. I fully acknowledge needing several attempts until I could finally derive a plausible mechanism involving dual-level block out of the AV node. That said — my hope is that even readers with limited experience with laddergrams will be able to follow the mechanism I propose in my final Figure-7.

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Beginning the Laddergram:

Figure-4: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (as per the RED lines drawn directly below the onset of each of the P waves — as shown here by the large GREEN arrows). Note that these RED lines in the Atrial Tier are nearly vertical — since conduction of sinus P waves through the atria is rapid. Note also that the P-P interval between successive P waves (vertical RED lines) is similar, albeit not quite equal (ie,There is slight sinus arrhythmia).

Figure-5: The most challenging part of most laddergrams is construction of the AV Nodal Tier — so I typically save that for last. Therefore, after drawing in all P waves into the Atrial Tier — It's easiest to next add in the Ventricular Tier the indication of all QRS complexes that are conducted. Perhaps the most complex aspect of today's rhythm — is realization that despite the changing QRS morphologies — each of the 8 beats in the long lead V1 rhythm strip are sinus-conducted beats! The large GREEN arrows show that I use the onset of each QRS complex as my landmark for entering beats #1-thru-8 into the Ventricular Tier.

Figure-6: As noted — each of the 8 beats in today's rhythm are sinus-conducted beats, albeit with different conduction properties! To better illustrate normal, RBBB or LBBB conduction — I've added light BLUE and PINK butt ends into the Ventricular Tier to schematically show the conduction defects. With completion of the Atrial and Ventricular Tiers — I was ready to begin solving the laddergram — which entailed postulating which of the P waves from the Atrial Tier would be able to make it through the AV Nodal Tier to be conducted to the ventricles. 

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PEARL #7: Because of the complexity of today's rhythm — I went through some trail-and-error before arriving at the laddergram I propose below in Figure-7. Two KEY principles figured in my arriving at this laddergram:

• Principle #1: It is extremely unusual for a patient to alternate between Mobitz I and Mobitz II. As a result — IF there is clear evidence of 2nd-degree AV block, Mobitz Type I somewhere on the tracing you are interpreting (or somewhere in other recently-obtained telemetry monitoring rhythm strips) — then it becomes highly likely that some form of Wenckebach conduction is responsible for other unknown forms of AV block that you see on that patient's ECG. 
• As noted earlier — the increasing PR interval from beat #2-to-beat #3, followed by the non-conducted (YELLOW arrow) P wave after beat #3 — is consistent with a 3:2 cycle of AV Wenckebach (ie, Mobitz I 2nd-degree AV block). This makes it highly likely that the 2:1 block of beats #1,4,5,6,7 and 8 are also a manifestation of Wenckebach conduction.
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• Principle #2: Simple AV Wenckebach results in non-conduction of a single on-time P wave. But as per the YELLOW arrow P waves in Figure-7 — we see consecutive on-time, non-conducted P waves within several of the R-R intervals — which strongly suggests there is dual-level block out of the AV node! (See ECG Blog #347 for more on dual-level AV Wenckebach).

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Putting It All Together:

• As discussed in ECG Blog #347 — the DOTTED line in the AV Nodal Tier in Figure-7 schematically illustrates the 2 levels of exit block out of the AV Node.
• We know that beats #2 and 3 are conducted to the ventricles in a 3:2 AV Wenckebach cycle.
• We know that beats #1,4,5,6,7 and 8 are also all conducted beats, albeit with alternating bundle branch block.
• This leaves us with having to postulate a path of transmission for each of the non-conducted (YELLOW arrow) P waves — in which the 3 R-R intervals that contain consecutive YELLOW arrow P waves — must manifest block of 1 of these P waves at each of the 2 levels within the AV Nodal Tier (which I schematically show in Figure-7).

Figure-7: My proposed laddergram for today's complex rhythm.

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CASE Follow-Up:

Following assessment of this patient's initial rhythm — she was admitted to a monitored floor with plans for pacemaker placement. Unfortunately, before this could be accomplished — the patient went into cardiac arrest. She was successfully resuscitated — with a post-arrest rhythm similar to that seen in Figure-1.

• Cardiac cath did not reveal significant coronary disease! So although I initially suspected that the deep anterior T wave inversion in the 12-lead tracing shown in Figure-1 (in association with the several week history of dyspnea) might reflect the occurence of a "silent" infarction at the time this patient's symptoms began — the negative cath argues against this. Instead — the negative cath favors more of a pure history of SSS (Sick Sinus Syndrome) in this elderly woman (See ECG Blog #342 for more on SSS — and ECG Blog #228 for more on "Silent" MI).
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• A permanent pacemaker was placed.

As always — I welcome questions and/or comments on today's ECG Blog post! THANK YOU for your interest! 

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Acknowledgment: My appreciation to Hans Helseth  (from Minnesota, USA) for the case and this tracing (and CREDIT to Hans for his well-drawn laddergram that I have slightly modified).

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
• ECG Blog #185 — Reviews the Ps, Qs, 3R Approach to Rhythm Interpretation.
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• ECG Blog #188 — Reviews how to read and draw Laddegrams (with LINKS to more than 90 laddergram cases — many with step-by-step sequential illustration).
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• ECG Blog #192 — The 3 Causes of AV Dissociation.
• ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.
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• ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".
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• ECG Blog #259 — Reviews the concept of Dual-Level AV Block.
• The October 25, 2021 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) reviews my approach to another case of a Dual-Level Wenckebach block. 
• ECG Blog #226 — Works through a complex Case Study (including an 11:00 minute ECG Video Pearl that walks you through step-by-step in the construction of a laddergram with Wenckebach conduction and dual-level block within the AV node).
• ECG Blog #243 — Reviews a case of AFlutter with Dual-Level Wenckebach out of the AV Node.

 

ADDENDUM (10/28/2023):

This 15-minute ECG Video (Media PEARL #52) — Reviews the 3 Types of 2nd-Degree AV Block — plus — the hard-to-define term of "high-grade" AV block. I supplement this material with the following 2 PDF handouts.

• Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks.
• Section 20 (54 pages = the "long" Answer) from my ACLS-2013-Arrhythmias Expanded Version provides detailed discussion of WHAT the AV Blocks are — and what they are not!

• ECG Media Pearl #4 (4:30 minutes Audio) — The AV Blocks & When to Suspect Mobitz I  — See ECG Blog #186 —

—  —

ECG Media PEARL #71 (5:45 minutes Audio) — Reviews the phenomenon of Dual-Level Wenckebach out of the AV Node (HOW to recognize this phenomenon — and how to distinguish it from Mobitz II).