ECG Blog #454 — Look for the "Break" ...

I was sent this ECG recording — and asked for my interpretation of the rhythm in Figure-1. I had little clinical information.

QUESTIONS:

• How was I able to guess the probable correct answer in less than 5 seconds?
• How was I then able to prove that my guess was correct?

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

How did I Know in Less than 5 Seconds?

With experience — it takes only seconds to systematically consider the 5 KEY Parameters for interpretation of any arrhythmia by the Ps, Qs, 3R Approach (See ECG Blog #185):

• While realizing that we only have a single lead to look at in Figure-1 (and that there are times when a single-lead rhythm strip may falsely suggest that the QRS is narrow — whereas in reality, other leads clearly show QRS widening) — the QRS complex in this lead V1 rhythm strip truly appears to be narrow. So — I instantly suspected that today’s rhythm was supraventricular!
• The overall rhythm in Figure-1 looks to be Regular (at a Rate of ~85/minute) — with the exception of a “break” in the rhythm between beats #8-to-#9.
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• PEARL #1: The BEST Clue to the etiology of a complex arrhythmia tends to be found somewhere within or around any pause that you may find in the rhythm. The RED arrow in Figure-2 shows that whereas P waves were not at all obvious elsewhere in Figure-1 — because of the slight "pause" in the rhythm that we seen between beats #8-to-9 — we are able to clearly identify a P wave in front of beat #9.
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• PEARL #2: The fact that we clearly see a P wave in front of beat #9 (ie, the RED arrow in Figure-2) — should immediately raise the question as to whether this P wave might be conducting (ie, Related to neighboring beat #9) — albeit this P wave would be conducting with a very long PR interval (ie, of 0.48 second).
• We can "test" this hypothesis — by looking for other "ECG evidence" of "hidden" P waves, keeping in mind that if other P waves were to be found, that they might also be conducting with a very long PR interval.
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• PEARL #3: Looking for "hidden" P waves is tremendously facilitated (and expedited) by using calipers. The reality is that clinicians (including cardiologists) who do not use calipers will typically not be able to interpret complex rhythms such as this one in today's case.
• PEARL #4: The BEST way to find "hidden" P waves — is to determine what a "normal" QRS and ST-T wave look like — and then to carefully look at all QRS complexes and ST-T waves on the tracing. In today's rhythm — I found it easiest to identify hidden P waves by comparing the shape of the "normal" ST-T wave of beat #9 — with the subtle-but-definite deformities present near the beginning of the ST-T waves of beat #10 and beat #11 (ie, the 5th and 6th PINK arrows in Figure-2).
• PEARL #5: Knowing that the 5th and 6th PINK arrows are almost certain to be consecutive "hidden P waves" — allows us to postulate what the P-P interval might be IF there was an underlying regular atrial rhythm (ie, the P-P interval would be the distance between these 5th and 6th PINK arrows).
• PEARL #6: It is common for 2nd-degree and 3rd-degree AV block rhythm to manifest slight irregularity (that is called ventriculophasic sinus arrhythmia) — so this needs to be kept in mind as you set your calipers to the above postulated P-P interval — and attempt to "walk through" P waves through the entire rhythm strip (See ECG Blog #344 for more on ventriculophasic sinus arrhythmia).
• Do YOU see the slight-but-real deformity in the initial part of the QRS for beats #5,6,7,8 (under the first 4 PINK arrows in Figure-2)? This subtle deformity of the initial part of the QRS is notably not present in the QRS of beat #9.
• Do YOU notice that the P-P interval between these first 4 PINK arrows is virtually the same as the P-P interval between the 4th PINK arrow and the RED arrow? 

Figure-2: I've labeled selected atrial activity (See text).

PEARL #7: At this point — I was now able to "walk out" where underlying P waves were "hiding" but present throughout the entire rhythm strip ( = RED arrows in Figure-3). 

• The fact that each of the RED arrows in Figure-3 either occurs over some slight deformity in the QRS or ST-T wave, or at a place in which atrial activity might be entirely hidden by the QRS — supports my suspicion that there is a surprisingly regular underlying sinus rhythm in today's tracing!
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• To Emphasize: I have described my approach to today's rhythm in "slow motion". In practice — it took me less than 5 seconds to work through these 7 PEARLS.

Figure-3: RED arrows highlight the surprisingly regular underlying sinus rhythm that was are able to "walk out" using calipers set at the P-P interval.

Clinically: What Does this All Mean? 

• PEARL #8: Common things are common. Since we have established that there is an underlying surprisingly regular sinus rhythm — we know that at the least — there is a very markedly prolonged 1st-degree AV Block (PR interval ~0.48 second before beat #9).
• It is very common for 2nd-degree AV Block of the Mobitz I Type (also called AV Wenckebach) — to be associated with a prolonged PR interval for those beats that are conducting.
• Although most cases of Mobitz I 2nd-degree AV Block manifest fairly short groupings of beats with a clearly evident increasing PR interval until a beat is dropped — sometimes there may be very long Wenckebach cycles until the P wave is dropped. Seeing the very long PR interval before beat #9 in today's case immediately prompted me to look for Mobitz I.

PEARL #9: Support for Mobitz I in today's rhythm was immediately forthcoming by quick search for Wenckebach Periodicity (affectionately labeled by Marriott as "The Footprints of Wenckebach). As reviewed in ECG Blog #251 — several of "the Footprints" are present in Figure-3:

• The QRS complex in today's rhythm appears to be narrow (whereas the QRS is much more likely to be wide when the form of 2nd-degree AV block is Mobitz II ).
• There is a regular underlying atrial rhythm (which rules out blocked PACs as the cause for "dropped" beats).
• There is 1st-degree AV block of conducted beats — which is commonly associated with Mobitz I (the prolonged PR interval before beat #9).
• The pause containing the dropped beat is less than twice the shortest R-R interval (as seen here since the R-R interval between beats #8-to-9 is less than twice the R-R interval between beats #7-8).

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Laddergram Confirmation:

As noted — the above discussion outlines my thought process for strongly suspecting a very long cycle of Mobitz I, 2nd-degree AV Block at the etiology of today's rhythm.

• The BEST way to confirm today's rhythm diagnosis — is to prove my suspicion by being able to draw a laddergram that illustrates progressive increase in the PR interval until a beat is dropped. 

Consider the laddergram I have drawn in Figure-4:

• PEARL #10: Because there are so many beats in a long Wenckebach cycle until an on-time P wave is finally non-conducted — there will be minimal increase in the PR interval from one beat-to-the-next. As a result — it may be difficult to appreciate that the PR interval is increasing over the course of the rhythm strip.
• The BEST way to confirm a long cycle of AV Wenckebach is to look at the PR interval just before the pause (which measures 720 msec. in Figure-4) — and to compare this to the PR interval that is seen near the end of the pause (which is 520 msec in Figure-4).
• It can be seen that the PR interval then increases for the next couple of beats (to 600 — and then 640 msec. before beats #10 and 11 in Figure-4). But after these first few beats — the "increment" (ie, increase) in PR interval from one-beat-to-the-next becomes much less in a long Wenckebach cycle.
• In Figure-4 — the on-time YELLOW arrow P wave is not conducted — and this leads to the pause, after which the PR interval "shortens" to 520 msec. as the next long Wenckebach cycle begins.

Figure-4: My proposed laddergram for today's rhythm.

BOTTOM Line: Today's arrhythmia is challenging! That said — my hope is that review of the above 9 PEARLS — and increasing your comfort with the regular use of calipers will increase your confidence in knowing that with a modest amount of practice, you'll be able to dissect complex arrhythmias in a fraction of the time it used to take for interpretation without calipers.

• In "real life" — I do not use calipers to interpret simple arrhythmias, because there is no need to do so.
• There is not time to pull out calipers when the patient in front of you is "crashing" — and you need to administer emergency measures that instant.
• BUT — Today's patient was not hemodynamically unstable with this rhythm — and no matter how much time I might have spent pondering this tracing, I simply could not have definitively solved this rhythm without calipers. Using calipers — it literally took me less than 5 seconds to know with high probability that today's rhythm was a long cycle of AV Wenckebach (which I then proved was the correct interpretation with the above laddergram).

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Acknowledgment: My appreciation to Kianseng Ng (from Malaysia) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #185 — My Ps, Qs, 3R System for Rhythm Interpretation.
• r Rhythm Interpretation.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration).

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• ECG Blog #164 and ECG Blog #251 —Review of Mobitz I 2nd-Degree AV Block, with detailed discussion of the "Footprints" of Wenckebach.
• 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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• ECG Blog #404 — Walks you through a step-by-step approach to this AV block case (with links to a VIDEO of this case, and to Blog #344 for more details).
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• ECG Blog #352 — emphasizes that 1st-degree AV block with a very long PR interval may have hemodynamic consequences.