ECG Blog #397 — An Unusually Long Cycle?

You are asked to interpret the ECG in Figure-1.

• What is the rhythm in ECG #1?

Figure-1: You are asked to interpret this tracing. What is the rhythm?

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 the rhythm to be Regular at a Rate of ~90/minute — with the exception of a slight pause at the end of the tracing.
• The QRS complex is narrow in all 12 leads of this tracing — which tells us that the rhythm is supraventricular.
• P waves are present. 
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• The 5th Parameter in the Ps,Qs,3R Approach is the 3rd "R" — which is to assess whether the P waves that are present are Related to "neighboring" QRS complexes. At first glance — Doesn’t it look as if the PR interval is constant (albeit with a slightly prolonged PR interval) — for most of this tracing?

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 in Figure-2.

• Note that I also number the beats — since this instantly allows everyone involved to ensure we are all talking about the same part of the tracing. 

Take another LOOK at today’s tracing in Figure-2.

• Do the RED arrows facilitate assessment of today’s Rhythm?

Figure-2: I've numbered the beats — and have highlighted P waves with RED arrows.

What is the Cause of the Pause in the Rhythm?

The simple step of labeling all of the obvious P waves — is surprisingly helpful in evaluating the rhythm.

• Isn’t it now more obvious that P waves remain regular throughout the entire long lead rhythm strip?
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• KEY Point: When most beats in an arrhythmia are regular, but some are not — the best CLUE to interpretation often resides in looking for a “break” in the rhythm (such as is seen between beats #13 and 14 in Figure-2).

PEARL #2:  The most common cause of a pause in an otherwise regular rhythm — is a blocked PAC (See ECG Blog #66 and ECG Blog #147). To emphasize — blocked PACs are a much more common cause of one or more relative pauses in a rhythm than AV block!

• That said — the reason we know that the cause of the pause in today’s rhythm is not a blocked PAC — is that the atrial rhythm remains regular throughout the entire tracing — which means that there is no premature P wave at the onset of the short pause between beats #13-to-14 (RED arrows in Figure-2).

This raises the question as to whether the rhythm disturbance in Figure-2 could be the result of AV Wenckebach? (with "AV Wenckebach" being a synonym for 2nd-degree AV block, Mobitz Type I).

• We typically think of AV Wenckebach as that form of 2nd-degree AV block in which the PR interval progressively lengthens until a beat is dropped — after which another cycle begins, with shortening of the PR interval (See the Video Pearl and extra links below in the ADDENDUM for more on ECG diagnosis of the AV Blocks).
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• The above said — the majority of Mobitz I, 2nd-degree AV blocks consist of groups with a limited number of beats — until a P wave is non-conducted. As a result — it will almost always be possible to see progressive lengthening of the PR interval from one beat-to-the-next. However — We do not see progressive PR interval lengthening from one beat-to-the-next in Figure-2!

So — Is Today’s Rhythm a Type of AV Wenckebach?

On occasion — Mobitz I may present as a long Wenckebach cycle. Although this is not common — it does occur. The BEST way to recognize a long Wenckebach cycle when it does occur — is to look for at least of couple of the “Footprints of Wenckebach”. As discussed in the ECG Video in today’s Addendum — these “Footprints” include:

• A regular (or at least almost regular) atrial rhythm (ie, Slight irregularity may be seen if there is an underlying sinus arrhythmia).
• The presence of Group beating.
• An increasing PR interval until a beat is dropped — with shortening of the PR interval as the next group begins.
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• The pause containing the dropped beat is less than twice the shortest R-R interval (which typically occurs because the greatest increase in PR interval — typically occurs between the 1st and 2nd beats in any group).

Additional Associated Features of AV Wenckebach:

• AV Wenckebach ( = Mobitz I) — is commonly seen in association with acute inferior MI.
• Because Mobitz I generally occurs at a higher level in the conduction system than Mobitz II — the QRS is usually narrow (whereas with Mobitz II — the QRS is usually wide).
• It is unlikely for a patient to go back-and-forth between Mobitz I and Mobitz II — so IF you see clear evidence elsewhere on the tracing that the patient has Mobitz I — then the 2:1 block you are assessing is probably also Mobitz I.
• Mobitz I is much more common than Mobitz II (In my experience more than 95% of all 2nd-degree AV blocks that I have seen are Mobitz I, and not Mobitz II).
• Prognosis tends to be much better with Mobitz I than with Mobitz II — and those cases of Mobitz I arising in association with acute inferior MI often spontaneously resolve. But the reason to be alert to recognizing those rare cases of Mobitz II — is that a pacemaker is usually needed for Mobitz II.

CASE Conclusion: Laddergram Illustration

Because of minimal (negligible) change in the PR interval from one-beat-to-the-next in today's tracing — it's impossible to appreciate gradual PR interval prolongation.

• The Laddergram that I have drawn in Figure-3 reveals the solution — with the KEY being to focus on the short pause at the end of the rhythm strip (ie, between beats #13-and-14).
• As highlighted by the BLUE arrow — the PR interval at the end of the 13-beat run — is clearly longer than the PR interval that begins the next cycle (the RED arrow before beat #14).
• GO BACK to the beginning of this tracing. Note that the 1st beat in this long Wenckebach cycle ( = beat #1) — is preceded by a PR interval that is clearly shorter than that seen before beats #2-thru-13.
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• CONCLUSION: The rhythm in today's case is 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach) — with a very long Wenckebach cycle. The on-time P wave highlighted by the YELLOW arrow is non-conducted — after which the next group begins with shortening of the PR interval that we see before beat #14.
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• Final confirmation that the rhythm in Figure-3 is indeed a long AV Wenckebach cycle — is forthcoming from the finding of regular P waves (ie, the YELLOW arrow non-conducted P wave is right on time!) — and from the last "Footprint of Wenckebach" listed above = The pause containing the dropped beat is less than twice the shortest R-R interval.
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• P.S.: Returning back to Figure-1 for assessment of the rest of the 12-lead ECG — there is a leftward axis (but still more positive in lead II — so not leftward enough to qualify as LAHB) — there is LAA (deep negative component to the P wave in lead V1) — delayed transition, with persistence of S waves through to lead V6 — and nonspecific ST-T wave abnormalities that do not appear to be acute.

Figure-3: Laddergram illustration of today's rhythm — showing that today's rhythm is a long Wenckebach cycle.

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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".

 

ADDENDUM (9/30/2023):

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 —