ECG Blog #347 — Why Non-Conducted P Waves?

The lead III rhythm strip shown in Figure-1 — was obtained from an older woman following a syncopal episode. Her 12-lead ECG showed QRS widening in a nonspecific IVCD (IntraVentricular Conduction Defect) pattern — but did not suggest acute coronary occlusion. The patient was hemodynamically stable at the time the rhythm strip in Figure-1 was recorded.

CHALLENGE Questions:

Which one (or more) of the following choices is (are) correct?

• A normal sinus rhythm is present.
• There is AFib (Atrial Fibrillation). 
• AV dissociation is present.
• 3rd-Degree AV Block is present.
• Some type of Wenckebach conduction is present.

Figure-1: Long lead III rhythm strip — obtained from an older woman with syncope. How would YOU interpret the rhythm?

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• NOTE #1: As always — I favor the Ps, Qs & 3R Approach for interpretation of the cardiac rhythm (See ECG Blog #185). Assessment of these 5 parameters provides us with answers to the above Challenge Questions — and greatly narrows the possibilities for the rhythm diagnosis.
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• NOTE #2: Today’s tracing is slightly angled (slanted) — and as a result, measurements are slightly “off”. That said — this did not affect my overall interpretation of today's rhythm.

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Is AFib Present? — OR — Is there Sinus Rhythm?

P waves are definitely present in Figure-1 — which rules out the possibility of AFib. It’s clear that more than a single P wave is present within some of the R-R intervals — so a simple “sinus rhythm” is not present.

Is AV Dissociation Present?

AV dissociation is defined as the absence of any relationshipship between P waves and QRS complexes. This can be “complete” AV dissociation (in which case none of the P waves in the entire rhythm strip are related to any of the QRS complexes — as occurs with complete AV block) — or AV dissociation can be transient (that is, "intermittent" — or only occurring for one or more beats, but not for the entire tracing).

• Although at first glance it might seem as if there is AV dissociation in today's rhythm — a LOOK at Figure-2 reveals that this is not the case. WHY do I say this? 
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• HINT: What do the colored lines in Figure-2 suggest?

Figure-2: Why do the colored lines in this figure tells us that AV dissociation is not present?

Why AV Dissociation is Not Present:

Although the rhythm is not regular in Figure-2 — there is a pattern to this rhythm (ie, there is a "regular irregularity" to the rhythm). Thus, there is "group" beating (ie, alternating shorter-then-longer R-R intervals).

• Note that each of the shorter R-R intervals in Figure-2 (BLUE lines) — are of approximately equal length.
• Each of the longer R-R intervals (RED lines) — are also of similar (if not the same) length. This is not by chance!

PEARL #1: As emphasized in ECG Blog #186 — Whenever you see group beating in a rhythm — Consider the possibility of some type of Wenckebach conduction!

• The type of Wenckebach conduction that most providers are familiar with — is 2nd-degree AV Block of the Mobitz I Type — in which the PR interval progressively increases until a beat is dropped. Another name for Mobitz I = AV Wenckebach!
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• It's important to appreciate that there are many other examples of Wenckebach conduction! These include SA Wenckebach — AFib, AFlutter or ATach with Wenckebach conduction (in which there is group beating with Wenckebach periodicity) — junctional or ventricular rhythms with retrograde Wenckebach or with Wenckebach exit block — and many others.
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• PEARL #2: Although the specific mechanism for many of these types of Wenckebach conduction is complex (and beyond the scope of today's ECG Blog) — the "Take-Home" Point from today's post is to Consider some type of Wenckebach conduction whenever you see "group" beating. 
• Wenckebach conduction will not always be present in such cases (ie, you can see group beating with other rhythms — such as atrial bigeminy or trigeminy) — but prompt recognition of group beating when it does occur will facilitate rapid identification of Wenckebach rhythms! 

Continuing with Figure-2:

Did YOU Notice the repetitive pattern of PR intervals in Figure-2?

• Note that each of the shorter PR intervals in Figure-2 (YELLOW lines) — are of approximately equal length.
• Each of the longer PR intervals (GREEN lines) — are also of similar length. This is also not by chance!

BOTTOM Line for Figure-2:

AV dissociation is not present in Figure-2 — because all P waves are related in some way to neighboring QRS complexes. We know this — because the length of the 2 different PR intervals that we see in this rhythm (highlighted by the YELLOW and GREEN lines) are constantly repeated! This means that there must be some type of conduction!

• Since there is some type of conduction in Figure-2 — this rules out the possibility of 3rd-degree (complete) AV block! You can't have complete AV block IF there is some conduction.
• Instead — the fact that we see group beating in Figure-2 (in the form of alternating shorter-then-longer R-R intervals) — means that some form of Wenckebach conduction is probably present!

Is the Atrial Rhythm Regular?

Take another LOOK at today's rhythm (ie, See Figure-3).

• Is the atrial rhythm regular?

Figure-3: Take another LOOK at today's rhythm (which I've reproduced from Figure-1). Is the atrial rhythm regular? How can we tell?

PEARL #3: Wenckebach conduction is very commonly seen with both ATach (Atrial Tachycardia) and AFlutter (Atrial Flutter). Therefore — IF you see a fast and regular atrial rhythm in association with group beating — the underlying mechanism will usually involve some type of Wenckebach conduction.

• As I so often emphasize — Using calipers speeds up your interpretation and makes it EASY to determine if the underlying atrial rhythm is (or is not) regular.
• Find a place in the rhythm you are looking at where you can definitely see 2 P waves in a row (ie, Any of the longer R-R intervals in Figure-3 will do!). Set you calipers precisely to the distance between these 2 consecutive P waves — and it then becomes EASY to "walk out" P waves through the entire rhythm strip (ie, RED arrows in Figure-4).

PEARL #4: Once you've established that the atrial rhythm is regular — Determining the atrial rate provides an important clue to the type of atrial rhythm.

• As explained in ECG Blog #210 — the Every-Other-Beat Method for determining the ventricular rate — works equally well for determining the rate of fast atrial rhythms.

Figure 4: RED arrows highlight that the atrial rhythm in today's tracing is regular! Application of the Every-Other-Beat Method facilitates determining the rate of the atrial rhythm. Note that it takes just under 5 large boxes to record 2 P waves (YELLOW numbers in this Figure). Therefore — HALF the atrial rate is a little faster than 300/5 ~60-65/minute — which means that the actual atrial rate in today's tracing is ~2X this rate or ~125/minute. This is consistent with an atrial tachycardia (The atrial rate is not fast enough to be AFlutter). 

What We Do Know at This Point!

Although we have not yet determined the specific mechanism of today's rhythm — We have answered all 5 of the Challenge Questions:

• There is neither AFib nor simple sinus rhythm. The presence of regular P waves rules out AFib. The rapid atrial rate (ie, at ~125/minute) with failure to conduct all P waves rules out a simple sinus rhythm. Instead — there is atrial tachycardia.
• Group beating with 2 PR intervals that regularly repeat (as highlighted by the YELLOW and GREEN lines in Figure-2) — rules out 3rd-degree AV block and AV dissociation. Instead — these findings strongly suggest that some type of Wenckebach conduction is occurring.
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• PEARL #5: To Emphasize — the presence of Wenckebach conduction in association with either ATach or AFlutter does not necessarily indicate a pathological form of AV block! Instead — this may simply result from the rapid atrial rate (in which case it is possible that normal 1:1 AV conduction may resume once the rapid atrial rhythm resolves). Sometimes only clinical correlation and the passage of a little time will tell IF Wenckebach conduction that occurs in association with ATach or AFutter is (or is not) pathologic.
• That said — the patient in Today's Case is an older woman who presented with the rhythm in Figure-1 following a syncopal episode. In addition — QRS widening from a nonspecific IVCD was present on her 12-lead tracing. Therefore — any form of AV conduction disturbance has to be considered pathologic until proven otherwise!
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• KEY "Take-Home" Message from Today's Case: The specific mechanism for today's rhythm is complex. Full understanding by primary providers is not essential for appropriate management. Instead — it would be fine to stop your interpretation after establishing that today's rhythm shows atrial tachycardia with some form of Wenckebach conduction in this older patient with syncope who clearly needs further evaluation (and who may ultimately need a pacemaker). 

 

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Beyond-the-Core: The Specific Rhythm

Looking closer at Figure-4, in which we highlight regularly-occurring P waves throughout the entire tracing in today's rhythm — Aren't there P waves that we know are not being conducted to the ventricles?

• My answer to this question is proposed in Figure-5 — in which I highlight P waves that seem incapable of being conducted all the way to the ventricles (WHITE arrows in Figure-5).
• This leaves us with alternate R-R intervals in which we see 2 consecutive RED-arrow P waves. It's hard to imagine that both of these consecutive RED-arrow P waves (that occur between beats #2-3; 4-5; 6-7 and 8-9) — could be conducting to the ventricles. HOW then can we explain this?

Figure 5: WHITE-arrow P waves seem incapable of being conducted to the ventricles. But HOW to explain what is going on with alternate R-R intervals in which there are 2 consecutive RED-arrow P waves?

Dual-Level AV Block:

In ECG Blog #259 — I discussed the concept of Dual-Level AV Block (See the ADDENDUM below for an Audio Pearl review of this concept). In brief — Wenckebach conduction may occur at more than a single level as atrial impulses exit out of the AV Node. This concept is easiest to illustrate by means of a laddergram (See Figure-6).

• The atrial rhythm in the laddergram is illustrated by the regular vertical RED lines in the Atrial Tier.
• The horizontal BLACK dotted line schematically divides the AV Nodal Tier into 2 levels, each conducting impulses with its own degree of AV Block.
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• WHITE-arrow P waves do not make it through the upper AV Nodal level. Note that there is alternating 2:1 and 3:2 Wenckebach conduction in this upper AV Nodal level.
• PINK-arrow P waves make it through the upper AV Nodal level — but do not make it through the lower AV Nodal level.
• RED-arrow P waves make it through both AV Nodal levels — and are conducted to the ventricles. Note that there is 3:2 AV Wenckebach conduction through this lower AV Nodal level. 

Figure-6: My proposed laddergram for the mechanism of today's rhythm (See text).

CASE Conclusion:

The laddergram in Figure-6 makes sense — because all P waves and all QRS complexes in this tracing are accounted for. The underlying rhythm is atrial tachycardia with dual-level Wenckebach conduction out of the AV node, manifesting alternating 2:1 and 3:2 conduction at the upper AV Nodal level — and 3:2 AV conduction at the lower level.

• Clinically (as emphasized in the above Take-Home Message) — given the presentation of this patient with syncope, full evaluation of this older woman is indicated. Pending results — she may ultimately need permanent pacing (Unfortunately — I do not have specific follow-up on this case).
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• P.S.: For those wanting direction for how to derive a laddergram when there is dual-level AV Block — Please check out the Step-by-Step Laddergram illustration for a similar case in ECG Blog #259.

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Acknowledgment: My appreciation to Nizar Jiris (from Kfar Yasif, Israel) for allowing me to use this case and these tracings.

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Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Reviews the Ps, Qs & 3Rs Approach to systematic rhythm interpretation.
• ECG Blog #210 — Reviews the Every-Other-Beat Method for determining the rate of a fast regular rhythm.
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• ECG Blog #188 — Reviews the essentials for reading (and/or drawing) Laddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts.
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• ECG Blog #186 — Highlights the importance of Group Beating — and reviews when to suspect the Mobitz I form of 2nd-Degree AV Block ( = AV Wenckebach).
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• ECG Blog #251 — Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach (Please check out the Audio Pearl in this blog post that focuses on these concepts).
• ECG Blog #164 — Reviews a case of typical Mobitz I 2nd-Degree AV Block (with detailed discussion of the "Footprints" of Wenckebach). 
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• ECG Blog #259 — Reviews the concept of Dual-Level AV Block.
• The October 25, 2021 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) reviews my approach to another case of a Dual-Level Wenckebach block. 
• ECG Blog #226 — Works through a complex Case Study (including an 11:00 minute ECG Video Pearl that walks you through step-by-step in the construction of a laddergram with Wenckebach conduction and dual-level block within the AV node).
• ECG Blog #243 — Reviews a case of AFlutter with Dual-Level Wenckebach out of the AV Node.

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ADDENDUM (11/28/2022): 

• For review on the concept of "Dual-Level" Wenckebach.

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ECG Media PEARL #71 (5:45 minutes Audio) — Reviews the phenomenon of Dual-Level Wenckebach out of the AV Node (HOW to recognize this phenomenon — and how to distinguish it from Mobitz II).

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ADDENDUM (11/29/2021): I received a comment today on this tracing from David Richley, who is well known to most ECG enthusiasts who frequent any of the many ECG internet forums. Dave always offers the most astute commentary on complex Arrhythmia interpretation. NOTE: What follows below goes beyond-the-core! — But — It illustrates the important concept that complex arrhythmias may have more than a single potentially plausible interpretation.

• Dave writes the following: Hi Ken. I enjoyed your latest ECG blog (as always) — but while I agree that there is dual-level Wenckebach AV block — I could not explain why 2:1 block should alternate with 3:2 block in the upper AV node. What could be the physiological explanation for this? 
• I’ve come up with a modification of your hypothesis: I think there could be 5:4 Wenckebach AV block in the upper AV node — then 2:1 block of those impulses that reach the lower AV node, as illustrated in this laddergram. What do you think? — Dave —

Figure-7: Proposed laddergram submitted by David Richley.

MY Reply to Dave:

• Dave — I think both your laddergram and my laddergram are plausible. I always find it challenging with dual-level AV blocks — to try and figure out whether it is the upper or lower AV nodal level that has a higher degree of block. (Going back to some of the original articles on dual-level AV blocks — they describe even more than 2 levels of block within the AV node — so it can get even more complicated that what we see in today's rhythm).
• As per my Figure-6 — those beats with shorter PR intervals (ie, beats #1,3,5,7,9) — are preceded by longer R-R intervals — so I thought that might allow more time to recover, therefore allowing better (faster) conduction through the lower AV nodal level. I fully acknowledge that I may be wrong ...
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• BOTTOM LINE: The KEY point is in your initial sentence, in which you state how we both agree that there is dual-level AV block. The geometric relationships (with repeating PR and R-R intervals) are not by chance — do not represent complete AV block — but rather manifest some variation of AV Wenckebach conduction. On this we both agree! THANKS again for your always superb insights Dave!