ECG Blog #429 — Mobitz I or Mobitz II?

The 12-lead ECG and long lead II rhythms shown in Figure-1 — was obtained from an older man with a recent history of “easy fatiguability” and a presyncopal episode.

QUESTIONS:

• How would YOU interpret the ECG in Figure-1?
• Is the group beating due to the Mobitz I or Mobitz II type of 2nd-degree AV block?

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

MY Initial Thoughts on Today’s CASE:

For clarity, given the above clinical history — I present my initial thoughts regarding clinical interpretation of today’s initial ECG in the sequence I used for assessment of this case. 

• To EMPHASIZE: One of my goals in developing this ECG Blog — is to help clinicians to optimize their time efficiency. For example, in today's case — I completed my initial assessment of the tracing in Figure-1 in less than 1 minute. I outline my approach for doing so below. 

MY Approach:

When presented with a 12-lead ECG and accompanying long-lead rhythm strip — I favor spending a few quick seconds first taking a look at the long lead rhythm strip — before I look at the 12-lead. This is especially relevant when either the QRS is wide and/or the rhythm is not regular (and in today’s case — both of these conditions are present!).

• By the Ps, Qs, 3R Approach (See ECG Blog #185) — the rhythm in the long-lead II of Figure-1 is not Regular. Instead — there is a pattern consistent with “group beating” (ie, groups of 2 beats — each with similar spacing between these 2 beats — and — similar spacing during the slight pause between groups).
• The QRS complex in the long lead II appears to be wide (ie, more than half a large box on ECG grid paper — and in Figure-1, at least 0.12 second).
• P waves are present in front of each of the 13 beats in the long-lead II rhythm strip (RED arrows in Figure-2). These P waves are all upright in this lead II — with a constant and normal PR interval (ie, all P waves are Related to the QRS closest to them — in that the PR interval remains the same before all beats). There is therefore an underlying sinus rhythm in today's tracing.
• The 5th parameter in the Ps,Qs,3R Approach is the 3rd R = Rate — which is difficult to determine because of the irregularity in the rhythm. That said — we can say that the rate is not overly fast.
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• To EMPHASIZE: Assessment of today’s rhythm by the approach outlined in the above bullets should take less than 30 seconds! The conclusion I came to (within these 30 seconds) — was that today’s rhythm manifests conduction of sinus beats at a reasonable heart rate — with a wide QRS and with group beating.

Figure-2: I've labeled sinus P waves with RED arrows.

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PEARL #1: For as interesting as the rhythm in today’s case looks to be (ie, with group beating and a wide QRS) — I’ll suggest that optimal time efficiency for clinical assessment and management — would be to next figure out WHY the QRS complex is wide!

• As noted above — there is sinus conduction in today's rhythm, with a constant and normal PR interval. As a result — there is no WPW. This means the cause of the wide QRS in today's rhythm is due to some type of conduction defect.
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• As reviewed below in today's ADDENDUM — attention to 3 KEY leads ( = right-sided lead V1 — and left-sided leads I and V6) — allows you to distinguish between the 3 principal types of conduction defects within seconds!
• Normally — the QRS is predominantly negative in right-sided lead V1. Therefore — the combination of the widened RR' complex seen in lead V1 of Figure-2 — in association with the wide terminal S waves seen in left-sided leads I and V6 — is diagnostic of RBBB (Right Bundle Branch Block) as the explanation for QRS widening in today's case (See ECG Blog #282 for review on ECG diagnosis of the BBBs).

PEARL #2: The ECG Video in today's ADDENDUM also reviews quick assessment of ST-T wave changes in bundle branch block. The simple rule that can be assessed within seconds, is that with either RBBB or LBBB — the ST-T wave in the 3 KEY leads (leads I, V1, V6) will be oppositely directed to the last QRS deflection in these 3 leads.

• Since the last QRS deflection in lead V1 of Figure-2 is positive (ie, the R' ) — the ST-T wave in this lead should be negative. It is.
• Since the last QRS deflection in lateral leads I and V6 is the negative S wave — the T wave in these leads should be positive. It is.
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• Looking next at neighboring leads in Figure-2 — there appear to be no acute ST-T wave changes.

Next — A closer look at the Rhythm ... 

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PEARL #3: Despite the title of today’s blog post (which is, "Mobitz I or Mobitz II?" ) — We can tell at a glance that today’s rhythm does not represent any form of AV block — because the atrial rate is not regular!

• Using calipers tremendously expedites the process. With 2nd- or 3rd-degree AV block — the atrial rate should be regular (or at least almost regular — if there is an underlying sinus arrhythmia) — but it should be clear from the timing of the RED arrows in Figure-2 — that no on-time P wave occurs between beats #1-2; #3-4; #5-6; #7-8; #9-10; #11-12.
• 
  
• For completeness, I'll note that SA Block can produce the bigeminal pattern of beats that we see in Figure-2 . That said, as emphasized in ECG Blog #312 — SA block is rare! I count on my fingers and toes the number of times I've seen true SA block over my decades of diligently looking for this rhythm disorder. 
• 
  
• Instead, as suggested in PEARL #4 — there is a much more common explanation for today's arrhythmia that we should look for first!

PEARL #4: Perhaps my favorite mantra in arrhythmia interpretation is the following statement that I learned from Marriott: "The most common cause of a pause is a blocked PAC."

• In clinical practice, when confronted with a "pause" in the rhythm — the finding of blocked PACs is far more common than any form of AV block. 
• To emphasize, that by a "pause" — I mean any even slightly longer-than-expected interval, which clearly is present in Figure-2 — because the interval between the 2-beat groups is much longer than anticipated given the P-P interval between the RED arrow sinus P waves in this tracing.
• As a result, as soon as I saw the bigeminal rhythm in today's case (ie, with brief pauses between each of the 2-beat groups) — I considered the possibility of blocked PACs.

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Now — Take another LOOK at Figure-2.

• Carefully examine the ST-T wave of every odd-numbered beat in Figure-2 — and compare these ST-T waves with the the ST-T waves of each even-numbered beat. 
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• Is there any difference?

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I illustrate my ANSWER in Figure-3:

Figure-3: Is there any difference in the ST-T waves?

Figure-3 Explained:

The challenge in looking for blocked PACs — is distinguishing between subtle differences in ST-T waves that are "real" vs artifact vs slight variations that can normally be seen in parts of the tracing.

• Isn't the ST-T wave of every even-numbered beat in Figure-3 straighter (above the slanted BLUE lines) — than the ST-T wave of every odd-numbered beat (above the slanted RED lines)? That's because there is a non-conducted PAC that is hidden within the ST-T waves of every odd-numbered beat — and this is what causes this slight-but-real distortion of the ST segment! These blocked PACs serve to "reset" the SA node — and thereby produce the slight pause that we see between each of the 2-beat groups!
• 
  
• PEARL #5: It's important to appreciate that PACs that arise from a site close to the SA node may look similar in morphology to sinus P waves. For this reason — I always look at simultaneously-recorded leads for subtle differences in P wave morphology that might not be readily apparent on a single long lead rhythm strip. NOTE: YELLOW arrows in Figure-3 show that the difference in ST-T wave morphology between sinus-conducted beats and the beats with blocked PACs is most easily seen in leads V2 and V3.

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The Lesson I Learned a Long Time Ago ...

• As emphasized earlier in PEARL #4: The most common cause of a pause is a blocked PAC. Blocked PACs are much common than any form of AV block.
• Until I became aware of the fact that blocked PACs are the most common cause of a pause — I never saw blocked PACs.
• Once I began to look for blocked PACs — I found them everywhere!
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• Remember that the biggest challenge in accurately diagnosing blocked PACs — is distinguishing between differences in ST-T waves that are "real" vs artifact vs slight variations that can normally be seen in parts of the tracing. 
• KEY Point: In today's case — the reason I know that the differences in ST-T wave morphology between odd- and even-numbered beats is real — is because it is seen with every beat in the long lead rhythm strip in Figure-3.

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Acknowledgment: My appreciation to Hafiz Abdul Mannan Shahid (from Lahore, Pakistan) for the case and these tracings.

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 Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Reviews the Ps, Qs and 3R Approach to Systematic Rhythm Interpretation.
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• ECG Blog #232 — Reviews the causes of Bigeminy.
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• ECG Blog #164 — Reviews a case of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
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• ECG Blog #252 — for a similar case as today (with LINKS to more examples of blocked PACs).

 

 

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ADDENDUM (5/11/2024): I've added below material regarding a user-friendly approach to the ECG diagnosis of the Bundle Branch Blocks

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ECG Media Pearl #22 (13:15 minutes Video) — Reviews a user-friendly approach that allows diagnosis of the Bundle Branch Blocks in less than 5 seconds. 

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• CLICK HERE — for FREE download PDF of this 26-page file on BBB (from my ECG-2014-ePub) — with review on the Basics for ECG diagnosis of the Bundle Branch Blocks (including diagnosis of acute MI & LVH with BBB).

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