ECG Blog #295 — Why is the 1st Beat Different?

The ECG in Figure-1 was obtained from an older patient with new chest pain.

• What is the cause of the rhythm?
• WHY is the 1st beat always different?

Figure-1: ECG obtained from an older patient with new chest pain.

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NOTE #1: This tracing is a smart phone photo that was sent to me for my comment. This is the reason for the slanting and slight distortion. That said — I believe the image still is good enough for accurate interpretation.

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NOTE #2: Some readers may prefer at this point to listen to the 3:45 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-41a).

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Today's ECG Media PEARL #41a (3:45 minutes Audio) — Reviews HOW to recognize within seconds (!!!) a Mobitz I (AV Wenckebach) 2nd-degree AV Block, in which there is ongoing Inferior STEMI.

MY Thoughts on the ECG in Figure-1:

There are 2 parts to this tracing: i) The cardiac rhythm (as shown in the long lead II rhythm strip); and, ii) The 12-lead ECG above it. I generally like to start my initial assessment by taking at least a brief look at the long lead rhythm strip — though sometimes (as in today’s case) — I need to switch my attention to the simultaneously-recorded 12-lead ECG in order to complete assessment of the cardiac rhythm.

• My initial impression on looking at the long lead II rhythm strip in Figure-1 — was that there is group beating! That is — we see 3 groups, each with 4 beats — and each with similar timing.
• Of Note — the 1st beat in each group (ie, beats #1, 5, 9 13) looks different than other beats in the group! 
• Also of Note — is that the duration of each of the short pauses in this rhythm (ie, the R-R intervals between beats #4-5; 8-9; and 12-13) are the same. It should be clear that this pattern of rhythmicity is not by chance!

Continuing my approach to assessment of the rhythm — I next considered the 5 KEY parameters (See ECG Blog #185 for review of the Ps, Qs, 3R Approach that I favor for Rhythm Interpretation):

• There are P waves! Of note — at least some of these P waves are related to neighboring QRS complexes (ie, the PR intervals preceding beats #1, 5, 9 and 13 are equal — therefore these beats are “Related” — in that these P waves are conducted to the ventricles).
• The QRS complex is narrow everywhere! To verify this — I looked at all beats in each of the 12 leads in the ECG above the long-lead rhythm strip.
• The remaining 2 parameters are Rate and Regularity. We have already addressed “regularity” — in that there is a “regular irregularity” to the rhythm (ie, a “pattern” — in the form of group beating). 
• As to the parameter of "Rate" — the ventricular Rate varies. In order to determine IF the atrial Rate is regular — we need to first complete our search for P waves.

PEARL #1: I’ll emphasize that one of the most helpful things to do for assessment of complex arrhythmias — is to label P waves. This is usually EASY to do with use of calipers. Simply set your calipers to the P-P interval between any 2 P waves in a row that you are sure about — and then see IF you are able to “walk out” P waves throughout the rest rest of the tracing.

• In Figure-2 — we can definitely distinguish the first 2 P waves in the long lead rhythm strip (ie, the first 2 RED arrows in Figure-2). Note how each of the subsequent RED arrows falls on a deflection resembling a P wave (even when this occurs during the ST-T wave) — throughout the entire rhythm strip. Thus, the atrial rhythm is regular at a Rate of ~100/minute! 
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• Now that we have labeled all P waves in today's rhythm with RED arrows — Isn’t it much easier to assess the potential relation that each of these P waves might have with its neighboring QRS?

Figure-2: I've labeled the P waves with RED arrows. The atrial rhythm is regular at ~100/minute. QUESTION: Is there evidence on the 12-lead ECG of acute inferior MI? (See discussion below!).

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PEARL #2: The finding of group beating in a rhythm in which the atrial rhythm is regular should immediately suggest the possibility of some form of Wenckebach conduction.

• The likelihood of AV Wenckebach ( = 2nd-degree AV Block of the Mobitz I type) — becomes much greater when group beating in a rhythm with a regular atrial rhythm occurs in the setting of acute inferior MI.
• For clinical examples of acute inferior MI with Mobitz I 2nd-degree AV Block ( = AV Wenckebach) — See ECG Blog #55 — ECG Blog #154 — ECG Blog #224 — and ECG Blog #232.

NOTE: At this point in my interpretation — I shifted my attention from the long lead II rhythm strip, in order to focus on the 12-lead ECG. 

• I did this because IF today's patient (who presented with new chest pain) was in the process of evolving an acute inferior MI — then even before completing assessment of the complex rhythm in Figure-2 — statistical likelihood that this rhythm will end up representing some form of AV Wenckebach will be greatly increased (and knowing will make it much easier for me to find confirmation of this when I return for final interpretation of this complex rhythm)!

QUESTION:

• Do YOU see evidence on the 12-lead ECG in Figure-2 of evolving acute inferior MI?

ANSWER:

We have already noted the following regarding today's ECG: i) There are 2 different QRS morphologies in this tracing that are seen in this tracing; ii) All beats on the 12-lead ECG are narrow — which means that all beats are supraventricular; and, iii) The fact that the PR interval preceding each of the beats that end one of the short pauses is the same (ie, the PR interval before beats #1, 5, 9 and 13 is the same!) — means that these 4 beats are definitely conducted!

• Focus your attention on the ST-T waves for beat #1 in simultaneously-recorded leads II and III — and on beat #5 in simultaneously-recorded leads aVF and aVL. Each of the ST segments in the 3 inferior leads for these beats is elevated — with the picture of mirror-image reciprocal ST depression in lead aVL. This is diagnostic of acute inferior MI.
• I found it interesting that this ECG evidence of acute inferior MI is much more easily appreciated for beats #1 and 5 — than for beats #2,3,4 and 6.
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• In the chest leads — beat #9 in simultaneously-recorded leads V1,V2,V3 suggests ongoing acute posterior MI (ie, early transition with a tiny isoelectric QRS in lead V1, and predominant R waves already by lead V2 — with associated ST depression in lead V2, but not in lead V1).
• Because the 12-lead ECG is abruptly cut off after the QRS complex of beat #13 — we do not see the ST-T wave of this last beat well in the lateral chest leads. But it's clear that there is abnormal ST depression in leads V4, V5, V6 for beats #11 and 12.
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• Putting It All Together: I interpreted the 12-lead ECG in Figure-2 as diagnostic of acute infero-postero MI. Lack of any ST depression in lead V1 suggested associated acute RV involvement — which strongly suggests that the "culprit" artery is the proximal RCA (Right Coronary Artery — See ECG Blog #190, including the Audio Pearl in this post for more on the ECG recognition of acute RV MI).

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NOTE: The results of Cardiac Catheterization of today's patient confirmed my clinical suspicion. There was diffuse disease of the RCA (including proximal, mid- and more distal segments) — with total occlusion, confirming this as the "culprit" vessel. In addition — there was multi-vessel disease in the form of an 80% mid-LAD lesion and 70-80% LCx lesion.

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NOTE: At this point in my interpretation — I shifted my attention back to the long lead II rhythm strip — in order to complete my assessment of the cardiac rhythm.

• PEARL #3: "Armed" with the knowledge that the 12-lead ECG showed acute inferior infarction — I virtually knew that the group beating with regular atrial rhythm that we see in the long lead II rhythm strip was going to represent some form of AV Wenckebach.
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• The next 4 Figures that I show illustrate by means of colored arrows my deductive process. I'll emphasize that total time it took me to work through what I show in the next 4 Figures was less than 15 seconds!
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• PEARL #4: The only way I could arrive at my final rhythm diagnosis so fast — was to use calipers. In my opinion — it is impossible to time-efficiently interpret complex rhythms such as this one without using calipers.
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• For clarity — I illustrate the mechanism of today's rhythm in the Laddergram shown in Figure-7. For those interested — I provide links to posts which review step-by-step description of how to read (and/or draw) laddergrams — with numerous detailed laddergrams that I have shown in prior posts on my ECG Blog #188.

Figure-3: RED arrows highlight our observation that the PR interval preceding each of the beats that end one of the short pauses is the same (ie, the PR interval before beats #1, 5, 9 and 13 is the same!). This tells us that these 4 beats are conducted.

Figure-4: Note that the 2nd PR interval in each group of 4 beats is slightly longer (YELLOW arrow) — than the PR interval that precedes the 1st beat in each group (RED arrow).

Figure-5: BLUE arrows highlighting the 3rd P wave in each group continue to show progressive lengthening of the PR interval within groups of beats.

Figure-6: Further PR interval lengthening (GREEN arrows) — followed by a non-conducted P wave (BLACK arrows) — confirms the diagnosis of 2nd-Degree AV Block of the Mobitz I Type ( = AV Wenckebach) — with 5:4 AV conduction (ie, 4 of the 5 P waves in each group are conducted — with the 5th P wave blocked).

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Final QUESTION:

• The final issue to resolve in today's complex tracing is the question of WHY the 1st beat in each of the 4-beat groups looks different?

ANSWER = PEARL #5: What makes today's ECG especially challenging is having to account for WHY the QRS complex of beats #1, 5, 9 and 13 looks different from other QRS complexes on this tracing. 

• Return for a moment to the 12-lead ECG shown in Figure-2. The reason beats #1, 5, 9 and 13 all look different — is that these beats all conduct with LAHB (Left Anterior HemiBlock) aberration, as they manifest predominant negativity in each of the inferior leads. But unlike the overwhelming majority of aberrantly-conducted beats — it is relative bradycardia (ie, slight slowing of the heart rate) rather than tachycardia that precipitates aberrant conduction. This is contrary to what one would anticipate — since improved conduction (not LAHB) is normally expected following a longer preceding R-R interval. 
• The mechanism of this paradoxical form of bradycardia-dependent block (sometimes called “Phase 4” block) has been described by Marriott. The mechanism is complex (Marriott HJL, Boudreau Conover MH: Advanced Concepts in Arrhythmias, Mosby, St. Louis, 1983, pp 164-166). Suffice it to say that this form of bradycardia-dependent aberrant conduction is not common — and that when it does occur, it is usually associated with significant underlying heart disease. This was borne out in today's case — in which cardiac catheterization showed acute RCA occlusion with a background of severe multi-vessel coronary disease.

I schematically illustrate the mechanism of today's arrhythmia in the Laddergram shown in Figure-7.

• For those interested — I presented a step-by-step laddergram illustration of another case with a similar form of bradycardia-dependent AV Wenckebach in the August 17, 2020 post of Dr. Smith's ECG Blog.

Figure-7: Laddergram illustration of today's arrhythmia. The dotted BLUE lines in the Ventricular Tier for beats #1, 5, 9 and 13 schematically illustrate that the reason the QRS complex of these beats looks different — is that these beats conduct with LAHB aberration.

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Acknowledgment: My appreciation to Arshad Ali (from Karachi, Pakistan) for the case and this tracing.

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Relevant LINKS to Today's Case:

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Additional Relevant Material to Today's Case:

• See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to rhythm interpretation.
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• Inferior STEMI + AV Wenckebach — See ECG Blog #154 —
• Mobitz I + Acute MI — See ECG Blog #55 —
• Inferior MI with Mobitz I — See ECG Blog #224 —
• Recent Inferior MI with Mobitz I — See ECG Blog #232 —
• ECG Diagnosis of acute RV MI — See ECG Blog #190 —

How to Draw a Laddergram (Step-by-Step Demonstration) 

• See ECG Blog #69 — for a Step-by-Step description on drawing a Laddergram.
• See ECG Blog #188 — for a brief ECG Video review on the basics of what a Laddergram is — with LINKS at the bottom of the page to more than 50 ECG blog posts in which I review illustrative laddergrams.
• See ECG Blog #164 — for a user-friendly rhythm-solving approach to AV Wenckebach, followed by Step-by-Step construction of the Laddergram.
• ECG Blog #236 — Reviews in our 15-minute Video Pearl #52 how to recognize the 2nd-Degree AV Blocks (including "high-grade" AV block). 
• ECG Blog #186 — Reviews when to suspect 2nd-Degree, Mobitz Type I.
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• CLICK HERE — to DOWNLOAD my Free PowerPoint Laddergram STENCIL for your use as desired.

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