ECG Blog #399 — Which Laddergram is Correct?

The ECG in Figure-1 was obtained from a woman in her 60s — who was seen in the ED (Emergency Department) as part of her evaluation for trauma following a motor vehicle accident. She was hemodynamically stable — and did not have chest pain, lightheadedness or syncope.                  

QUESTIONS:

• HOW would you interpret the rhythm in Figure-1?
• Is this "high-grade" AV block?
• Is a pacemaker needed?

Figure-1: The initial ECG in today's case.

MY Thoughts on the ECG in Figure-1:

I routinely begin assessment of each 12-lead ECG I encounter — with interpretation of the rhythm. To do this — I apply the Ps, Qs, 3R Approach (See ECG Blog #185 — for review of my system).

• The long lead rhythm strip at the bottom of the 12-lead ECG in Figure-1 — shows a very slow and irregular rhythm, albeit the 5 beats that we see appear to manifest group beating!
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• The QRS complex is wide — but with very typical morphology for RBBB (Right Bundle Branch Block) conduction (ie, typical triphasic rSR' pattern in lead V1 —  with wide terminal S waves in lateral leads).
• P waves are present — with what appears to be some repetitive PR intervals, which suggests that there is at least some conduction of P waves!

PEARL #1: After considering the 5 KEY Parameters — the EASIEST next step for determining the mechanism of a complex rhythm — is to label the P waves. I do this with RED arrows in Figure-2.

• With P waves labeled — Isn't it now much easier to appreciate that the atrial rhythm is quite regular (with no more than a slight sinus arrhythmia)?

QUESTIONS:

• Are the PR intervals in front of beats #1, 3 and 5 the same?
• How about the PR intervals before beats #2 and 4?
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• KEY Point: Why is the answer to these questions important for determining the mechanism of today's rhythm?
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• HINT: Unless you used calipers — You are likely to get the answer to at least one of the above questions wrong!

Figure-2: I've labeled P waves in the long lead V1 with RED arrows. Doesn't this make it easier to see that the atrial rhythm is regular?

PEARL #2: The reason it is important to determine IF there are any PR intervals that repeat — is that if there is, then it is very likely that those beats are sinus-conducted.

• In Figure-3 — I have very carefully measured the PR intervals and the R-R intervals of the 5 beats in today's tracing. Note that the PR intervals that precede beats #1, 3 and 5 are all normal and equal (ie, = 0.17 second). This strongly suggests that these 3 beats are sinus-conducted, albeit with RBBB.
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• NOTE: Before I used calipers to measure — I thought that the PR intervals before beats #2 and 4 were also equal. However, as shown in Figure-3 — there is a slight-but-real difference in the PR interval before beat #2 ( = 0.36 second) — compared to the PR interval before beat #4 ( = 0.34 second).

PEARL #3: The fact that the PR intervals before beat #2 and beat #4 are not the same — makes it less likely (albeit not impossible) that these 2 beats are sinus-conducted.

• NOTE: Although the PR intervals before beats #2 and #4 are not the same — the R-R intervals that precede these 2 beats are exactly the same! ( = 2150 msec.). This is unlikely to be by chance — and, I believe strongly suggests that beats #2 and #4 are escape beats.

PEARL #4: If beats #2 and #4 are indeed "escape" beats — the fact that QRS morphology of these beats is identical to the QRS morphology of sinus-conducted beats #1,3,5 tells us that the escape site is within the ventricular conduction system (ie, either coming from the AV Node or the Bundle of His).

• The fact that the R-R interval preceding beats #2 and #4 is more than 10 large boxes on ECG grid paper (ie, corresponding to a rate of slightly less than 30/minute) — suggests that the escape site is probably below the AV Node (since the usual junctional "escape" rate would be faster, typically between ~40-60/minute).

Figure-3: I have measured PR and R-R intervals for today's tracing.

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Putting It All Together:

The precise mechanism of today's arrhythmia is complex and difficult to determine. For those with a special interest in cardiac arrhythmias — READ ON! — as I explore the probable mechanism of today's rhythm with laddergrams below. 

For those with less interest in complex cardiac arrhythmias:

• KEY Point: We have already addressed the key elements in today's case needed to determine appropriate clinical management. Even if we stopped here — We could conclude the following:
• There is marked bradycardia in today's rhythm (ie, Heart rate in the low 30s). There are also twice as many P waves as QRS complexes — so at the least, there is 2nd-degree AV block. As a result — IF no "fixable" cause is found (ie, ischemia/infarction — electrolyte disturbance — rate-slowing medication) — then because of the AV block and very slow heart rate, this patient will probably need a pacemaker.
• Given the History in today's case (ie, trauma following a motor vehicle accident) — special attention should be given to physical examination to rule out the possibility of cardiac contusion as the cause of the conduction disturbance (RBBB, AV block and bradycardia are among the more common disturbances seen in those patients with cardiac contusion — as per My Comment at the bottom of the page in the August 6, 2022 post in Dr. Smith's ECG Blog).
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• However — IF there are no signs of chest trauma from this patient's motor vehicle accident — then the nonspecific ST segment flattening seen in multiple leads, with slight ST elevation in leads aVR and V1 — may represent diffuse subendocardial ischemia from longstanding underlying coronary disease as a potential cause of this patient's conduction defects.
• Finally — If today's patient does not have significant underlying coronary disease — then her bradycardia with AV block may be the result of SSS (Sick Sinus Syndrome). 
• Clinical correlation to sort this all out will clearly be needed.

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A Closer LOOK at Today's Rhythm:

Further explanation of today's rhythm is probably best accomplished by means of a Laddergram.

• I strongly favor the laddergram illustration in Figure-4 — as the most logical explanation for today's rhythm.
• That said, complex arrhythmias often stimulate discussion and consideration of more than a single mechanism — which is why I ofter an alternative (albeit less likely) mechanism for today's rhythm in Figure-5.

Figure-4: This is the laddergram I favor for illustrating the mechanism of today's rhythm.

Explanation of the Laddergram in Figure-4:

For those in search of a review on reading and/or drawing Laddergrams — Please check out my ECG Blog #188. Regarding today's case — my explanation for the laddergram in Figure-4 is as follows:

• What We KNOW: Beats #1, 3 and 5 are sinus-conducted because (as stated above) — the PR interval before each of these beats is identical and of a reasonable duration for normal conduction ( = 0.17 second — for the PR interval between each of the RED arrows and its neighboring QRS).
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• NOTE: The colored ARROWS in Figure-4 represent sinus P waves. There is slight irregularity in the occurrence of these P waves (which can be seen from the slight irregularity in the vertical lines representing spread of the impulse through the Atrial Tier). This is consistent with a slight sinus arrhythmia. 
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• PEARL #5: It could be EASY to overlook the fact that the P wave rhythm is not regular — IF one did not use calipers. That there is slight sinus arrhythmia is clinically relevant to the mechanism of today’s rhythm — because it accounts for the variation in R-R intervals that we see between sinus-conducted beats #2-3 ( = 1760 msec.) — and between beats #4-5 ( = 1700 msec.). It also accounts for the difference in PR intervals before beats #2 and #4 — which I believe is the KEY Clue to the likely etiology of today’s arrhythmia! (ie, 0.36 ≠ 0.34 second! — as per PEARL #3).
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• PEARL #6: My “strategy” for the sequence in which I draw a laddergram — is to first draw those elements of the rhythm that are most likely to be true. In Figure-4 — I thought it virtually certain that none of the YELLOW arrow P waves are conducted to the ventricles (which is why I drew the butt-ends in the AV Nodal Tier representing non-conduction to the ventricles of these impulses).
• PEARL #7: Note that we have just established that some form of 2nd-degree AV block is present in Figure-4 — because despite more than adequate “opportunity” to conduct, none of the YELLOW arrow P waves are conducted to the ventricles (since none of these YELLOW arrow P waves occur anywhere near a QRS complex with a reasonable chance to be conducted).
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• This left me with only having to account for the final path of the PINK arrow P waves. As emphasized above in PEARLS #3 and #4 — the fact that despite the slight-but-real difference in PR intervals before beats #2 and #4 — the R-R intervals before beats #2 and #4 are precisely equal ( = 2150 msec.). This strongly suggests that beats #2 and #4 are escape beats (ie, before these PINK P waves have a chance to conduct — escape beats arising either from the AV Node, or more likely from the His terminate the pauses in the rhythm between beats #1-2 and between beats #3-4).
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• NOTE: We do not know IF the PINK arrow P waves would have conducted if escape beats #2 and #4 would not have occurred.
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• PEARL #8: In general, I reserve the term, "high-grade" AV block for when 2 or more P waves in a row that should conduct, fail to do so. Technically, the Figure-4 laddergram does not satisfy this definition — because the occurrence of escape beats #2 and #4 prevent us from knowing if the PINK P arrow waves might have conducted with a prolonged PR interval if given a chance to do so.
• The above said, this semantic distinction is not clinically important — since regardless of whether or not the definition of "high-grade" AV block is satisfied — the combined conduction defects of marked bradycardia, with at least 2:1 AV block plus RBBB — would satisfy the need for pacing IF no "fixable" cause is found.

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A 2nd Theory = Dual AV Nodal Pathways:

When I first saw today's tracing — I immediately suspected that the mechanism for this arrhythmia was going to be the result of dual AV nodal pathways because: i) There appeared to be group beating, suggestive of AV Wenckebach; and, ii) There appeared to be 2 "families" of PR intervals that repeated — with a large PR interval "increment" between shorter PR intervals (ie, before beats #1,3,5) — and the longer PR intervals (ie, before beats #2,4).

• The presence of 2 functioning AV nodal pathways, each with its own PR interval and conduction capabilities — is a logical explanation for why we may sometimes see a large difference between the PR interval of 2 families of conducting beats.
• I schematically illustrate this mechanism in Figure-5 — in which the RED lines within the AV Nodal Tier represent conduction over the faster AV nodal pathway (with the shorter PR intervals before beats #1,3,5).
• The BLUE lines within the AV Nodal Tier represent conduction over the slower AV nodal pathway (with the longer PR intervals before beats #2 and 4).
• Both AV nodal pathways are blocked for those P waves that are not followed by any QRS complex.
• The reason we see a big "jump" in the PR interval difference before beats #1 and 2 — and before beats #3 and 4 — is that the faster pathway is intermittently blocked, such that the P waves in front of beats #2 and 4 are conducted over the slower pathway (with the longer PR interval).
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• The Problem with Figure-5: As I noted above — caliper measurements show that the PR intervals before beats #2 and 4 are not equal (ie, 0.36 vs 0.34 second — as shown in Figure-3). While not impossible for the slower AV nodal pathway to conduct with slightly different PR intervals — I think this is much less likely than the mechanism proposed in Figure-4, in which beats #2 and 4 represent escape beats. 

Figure-5: An alternative laddergram for today's arrhythmia — suggesting the presence of dual AV nodal pathways (See text).

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Acknowledgment: My appreciation to Sam Ghali  (from Jacksonville, Florida — @EM_RESUS) for the case and this tracing.

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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 Laddergrams (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 #186 — Highlights the importance of group beating — and reviews when to suspect the Mobitz I form of 2nd-Degree AV Block ( = AV Wenckebach).

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• ECG Blog #251 — Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach (Please check out the Audio Pearl in this blog post that focuses on these concepts).
• ECG Blog #164 — Reviews a case of typical Mobitz I 2nd-Degree AV Block (with detailed discussion of the "Footprints" of Wenckebach).
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• ECG Blog #259 — and ECG Blog #347 – 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 of dual-level AV block.
• ECG Blog #243 — Reviews a case of AFlutter with Dual-Level Wenckebach out of the AV Node.
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• ECG Blog #267 — Reviews a case with dual AV nodal pathways.

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ADDENDUM (10/14/2023):

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 (This is a free download).
• 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! (This is a free download).

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