Is this Rhythm Puzzling to You?

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My Comment by KEN GRAUER, MD (3/30/2023):

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The ECG in Figure-1 was sent to me without the benefit of any history. I thought the rhythm illustrated a number of essential concepts for clinicians dedicated to Emergency Care.

• Do YOU know what the rhythm is?
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• IF this arrhythmia is puzzling to you — READ ON!  I illustrate how to make the diagnosis within less than 15 seconds.

Figure-1: The initial ECG in today's case. This tracing was sent to me without the benefit of any history. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Initial Thoughts on the ECG in Figure-1:

As noted — I was sent today's ECG without the benefit of any history, other than knowing that the patient presented to the ED. 

• My quick initial survey of this tracing revealed T wave inversion in lead aVL and some T waves that might be hyperacute (ie, in the inferior leads and in V1,V2,V3). That said, although this ECG is suspicious — I thought it was not at this point diagnostic of acute OMI. 
• Clinically — Since this ECG is not at this point diagnostic — whether or not an acute cardiac event was in progress would appropriately be delayed until some history, a repeat ECG, and troponins could be obtained. In the meantime — I focused my attention on the "eye-catching" arrhythmia in the long lead II rhythm strip.

Time-Efficient Assessment of Today's Rhythm:

Once you ensure that the patient is hemodynamically stable — I favor the systematic Ps, Qs, 3R Approach for rhythm assessment (Are there P waves? — Is the QRS wide or narrow? — and the Rate and Regularity of the rhythm? — and whether P waves are Related to neighboring QRS complexes?):

• To Emphasize: It does not matter in what sequence you assess the Ps, Qs & 3Rs. I favor starting with whichever of these 5 parameters are easiest to assess.
• The QRS is narrow in all 12 leads of today's tracing. This tells us that the rhythm is supraventricular.
• P waves are present! (RED arrows in Figure-2 highlight those P waves that we can easily identify).
• The ventricular rhythm is not Regular! That said — there is a "pattern" to the rhythm in Figure-2 — in that there is group beating in the form of a "bigeminal" rhythm, with groups of shorter-then-longer R-R intervals (horizontal BLUE lines in Figure-2 facilitating recognition of groups with the shorter R-R interval).
• As to the atrial and ventricular Rates — these vary in Figure-2, because of the irregularity of the rhythm, but neither the atrial nor ventricular rates appear to be excessively fast.
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• The last parameter in my systematic Ps, Qs, 3R Approach — is the 3rd "R" — which asks the question of whether atrial activity is Related to neighboring QRS complexes?
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• KEY Question: Look in Figure-2 at the RED-arrow P waves in front of each QRS complex that ends a longer R-R interval (ie, Look at the PR intervals for the RED-arrow P waves in front of beats #1, 3, 5, 7 and 9). Although greatly prolonged (to a PR interval ~0.46 second) — Aren't each of these PR intervals the same? 
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• ANSWER: The fact that the PR interval in front of each of the odd-numbered beats in Figure-2 is the same tells us that these P waves are being conducted to the ventricles! (albeit with marked 1st-degree AV block).

Figure-2: I've labeled with RED arrows those P waves we can definitely identify. In addition — there is group beating (with horizontal BLUE lines facilitating recognition of groups with the shorter R-R intervals).

Is the Atrial Rhythm Regular? 

• In Figure-2 — I highlighted with RED arrows those P waves we could be certain about. Rather than the "grouped beating" of P waves suggested by these RED arrows — Wouldn't it be much more logical for the underlying atrial rhythm to be regular? — with additional "on-time" P waves being hidden within the QRS complex of beats #2, 4, 6, 8 and 10 (PINK arrows in Figure-3)?
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• Technical NOTE (Beyond-the-Core): It is not possible to "prove" that the PINK arrows in Figure-3 truly represent P waves because: i) There is an underlying sinus arrhythmia — with slight variation in the P-P interval of the RED arrows in Figure-2; and, ii) There is slight variation in QRS morphology for every-other-beat in Figure-2 — which makes it impossible to tell if this change in the QRS morphology of all even-numbered beats is purely the result of aberrant conduction — or — whether there might also be some hidden deformation of the QRS of beats #2,4,6,8,10 by hidden P waves.
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• PEARL #1: Common things are common! It is far more likely for the atrial rhythm in Figure-3 to be regular (or at least almost regular — with slight variation due to sinus arrhythmia) — than for there to be the rare rhythm of atrial parasystole with 3:2 exit block. Atrial bigeminy (ie, every-other-P wave being a PAC) is unlikely in Figure-3 — because P wave morphology under each of the RED arrows in Figure-3 looks so similar (whereas P wave morphology usually is noticeably different with PACs).

Figure-3: I've labeled with PINK arrows the likely location of hidden "on-time" P waves. Isn't it likely that the underlying atrial rhythm is regular?

Putting It All Together:

By the Ps, Qs, 3R Approach — we have determined the following for the rhythm in Figure-3:

• The rhythm is supraventricular (all QRS complexes are narrow — albeit with slight variation in QRS morphology every-other-beat due to some aberrant conduction).
• There is group beating (alternating shorter-then-longer R-R intervals).
• The atrial rhythm is essentially regular (with slight variation in the P-P interval due to some sinus arrhythmia). That said — there are more P waves than QRS complexes — so at least some of these on-time P waves are not being conducted.
• That said — the PR intervals for the RED-arrow P waves in front of each odd-numbered beat (ie, in front of beats #1,3,5,7,9) are equal! This proves that at least these beats are conducting.

PEARL #2: The above characteristics overwhelmingly point to AV Wenckebach (ie, 2nd-degree AV block, Mobitz Type I ) as the etiology of today's rhythm!

• To Emphasize: Although my above description may seem to be in "slow motion" — I literally knew within seconds that today's rhythm was almost certain to represent some form of AV Wenckebach because: i) There is group beating (which the "trained eye" should instantly recognize); ii) The atrial rhythm is almost regular ( = sinus arrhythmia, which with use of calipers is literally established within seconds); and, iii) The 1st beat in each group is conducting (witness the repeating equal PR interval in front of beats #1,3,5,7,9); and, iv) There is 1st-degree AV block (It is very common for conducted beats with AV Wenckebach to manifest 1st-degree block).  

Looking CLOSER at the Rhythm:

The reason today's rhythm is challenging — is that the 1st conducted beat in each grouping has a very long PR interval. To facilitate recognition of each 3:2 Wenckebach cycle — I have chosen a different color for each P wave (Figure-4).

• As already stated — RED-arrow P waves represent conduction of the 1st beat in each group with a long PR interval ( = 0.46 second). 
• The YELLOW-arrow P waves in each group are not conducted. (This makes sense — because these YELLOW P waves are simply not in a position where conduction is possible).
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• By the process of elimination — the BLUE-arrow P waves must therefore be conducting. That this is the case is supported by the fact that all PR intervals from a BLUE arrow until beats #2,4,6,8 and 10 are equal (albeit very long = 0.64 second). So — this is AV Wenckebach with 3:2 AV conduction (as the PR interval within each group increases from 0.46 second — to 0.64 second — until the YELLOW-arrow P waves are non-conducted).

Figure-4: Colored arrows facilitate recognition of PR interval prolongation (from RED arrow P waves that conduct with a PR interval = 0.46 second — to 0.64 second for BLUE arrow P waves) — until non-conduction of the YELLOW arrow P waves.

LADDERGRAM Illustration:

• Doesn't the laddergram in Figure-5 clarify the mechanism of today's rhythm?

Figure-5: Laddergram illustration of today's rhythm.

Final POINTS:

• It is extremely common to see slight variation in the P-P interval in association with either 2nd- or 3rd-degree AV block. This is known as a "ventriculophasic" sinus arrhythmia — with the theory being that the P-P interval may vary because of improved coronary perfusion, depending on whether mechanical contraction (signaled by electrical activity from a QRS) is "sandwiched" between 2 P waves.
• Mobitz I 2nd-degree AV Block (ie, AV Wenckebach) — is very commonly associated with acute inferior infarction. IF you return for a moment to the 12-lead ECG shown in Figure-1 — I was concerned when I first saw this tracing that the T wave inversion in lead aVL — and the hypervoluminous T waves in the inferior leads and in V1,V2,V3 might be markers of a recent inferior OMI? While I did not think the initial ECG in Figure-1 was diagnostic — I felt additional information (ie, some history, repeat ECGs, troponin) was clearly indicated to clarify the clinical situation. Unfortunately — we are not privilege to the clinical outcome in today's case.

And 3 Advanced Concepts ...

• As alluded to earlier — the 2nd beat in each group is slightly wider and slightly different in morphology from the 1st beat in each grouping. This is most probably the result of aberrant conduction by the Ashman phenomenon (ie, The slightly longer R-R interval between beats #2-3; 4-5; 6-7; and 8-9, may prolong the relative refractory period of the next beat = of beats #3,5,7,9 — which results in aberrant conduction of beats #4,6,8,10).
• Another reason today's rhythm is challenging to interpret — is that the PR interval "increment" (ie, The amount that the PR interval is increased from 1 beat to the next within a Wenckebach cycle) — is much more than usual (ie, The PR interval increases from 0.46-to-0.64 second). While this might simply be the result of an atypical Wenckebach — it might alternatively reflect dual AV nodal pathways, each with its own degree of impaired conduction.
• Finally — The fact that the PR interval for conducted beats in today’s tracing is very long may of itself be of clinical significance. This is because once the PR interval extends beyond ~0.30 second — the delay in ventricular contraction that occurs may result in the atria contracting against closed AV valves, with reduction in cardiac output. Thus, although Mobitz I is often a well tolerated rhythm — that may not necessarily be the case in today's tracing (especially if it did turn out that this patient had a recent inferior infarction).

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For Readers Wanting More ...

For readers in search of additional practice of concepts from today's case — I refer you to My Comment in the following posts in Dr. Smith's ECG Blog:

• The May 16, 2020 post.
• The January 26, 2023 post.
• The August 17, 2020 post.
• The March 14, 2022 post.

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