And now for something completely different (a very difficult electrophysiology case)

Written by Willy Frick

"Two episodes of syncope in the past month. Only speaks French. No chest pain or shortness of breath."

Figure 1: ECG

That's the history I had when I reviewed this ECG. Have a look and see how many diagnostic statements and electrophysiologic phenomena you can name before reading further.

This patient is uninsured and was seen in an indigent clinic, so unfortunately extensive diagnostic testing is not available. I am told the patient was referred to electrophysiology, but I do not have any definitive follow up. What follows is my personal interpretation, but I discussed this ECG with some friends, and others favor different interpretations specifically vis-à-vis QRS morphology.

Starting with the atrial rhythm, the undulating baseline is immediately noticeable. It looks believable for flutter, but there are two potential problems.

The first potential problem is that there is relative sparing of lead I which raises the possibility of artifact from the left leg electrode (lead I is the only lead that does not include the left leg electrode). However, two lines of reasoning support true flutter. First, the rate is about 240 bpm which is just about at the lower limit of a reasonable flutter rate. And it is VERY regular as shown in the image below. Second, the architecture of conducted QRS complexes maps very nicely onto a ladder diagram discussed further below.

Figure 2: Highly atrial activity

The second possible problem is that the morphology does not look quite right for typical atrial flutter. Typical flutter is a counterclockwise circuit around the tricuspid annulus, and classically it produces flutter waves that are "upright" in V1 and "upside down" in the inferior leads. This is a subjective assessment, and in some cases you can convince yourself that the waves go both directions -- it is a circuit after all. In this case, I think V1 convincingly shows predominantly negative flutter waves.

Figure 3: Negative flutter waves in V1

The most likely explanation here is "reverse" typical flutter, i.e. a clockwise circuit around the tricuspid annulus. Here is an excellent review on this topic. Here is another ECG from a separate patient that showed reverse typical flutter, confirmed on EP study.

Figure 4: Clockwise flutter

Moving on to the ventricular rhythm in our case, it is helpful to look back at the lead II rhythm strip which I have reproduced here with labeled R waves.

Figure 5: Lead II rhythm strip

If you stare long enough, you may notice that there are a few different "groups" of R waves. R1 and R2 belong together and then there's a break. R3 and R4 belong together and then there's a break. Ditto R5-6 and R7-8. Then R9-11 stand alone but occur at regularly spaced intervals. When flutter produces repeating patterns of grouped beating, but does not fit into an easily explainable n:1 conduction pattern, it is very likely due to a multilevel block.

What is multilevel block, you ask? It has been discussed on this blog before. My conceptual framework, based on this seminal paper by Kosowski et al. is a total of three tiers of possible block in the AV node. The top level can produce 2:1 block, the middle level can produce Wenckebach periodicity, and the bottom level can again produce 2:1 conduction. For most patients most of the time, each level is conducting 1:1. When you see atrial flutter with 2:1 block, then one of the levels is "active" and the other two levels are conducting 1:1. When you see 4:1, this is likely two sequential 2:1 blocks, with the remaining level conducting 1:1. (Remember that 2:1 can be thought of as the shortest possible Wenckebach cycle.)

Figure 6: Theoretical framework

In order to figure out the arrangement, you have to find a segment of a repeating pattern, then count up all the atrial activations and all the ventricular activations to determine the ratio. It is then usually a matter of brute combinatorial force. The commonest arrangement is 2:1 on top and Wenckebach on bottom (with the implicit third level conducting 1:1). For example look at this ECG sent to me by Basit Masoodi.

Figure 7: Unrelated example

Inspection reveals a 6:2 ratio of atrial:ventricular activations. How would we get this using the framework above? How do we turn 6 into 2? Well, if you start with 2:1, then 6 atrial impulses is immediately cut down to 3. Then you simply add a Wenckebach cycle of 3:2 and the aggregate is 6:2. This is shown below. (Note that the atrial activation is drawn as a zig zag to represent reentry from flutter.)

Figure 8: Ladder diagram for Figure 7

Here's another example:

Figure 9: Another unrelated example

If we count all activations, we get atrial:ventricular ratio 5:2. Immediately, we know the top 2:1 level cannot be active. Otherwise, we'd end up with 5 ÷ 2 = 2.5 impulses surviving beyond the first level, and we must always have an integer number. So instead, it must be Wenckebach as the first level (since the top 2:1 level is inactive), which is 5:4. Then to get from 4 to 2, we have 2:1 on bottom. This is shown below. (My ladder diagramming has gotten better since I drew this, please excuse stylistic differences.)

Figure 10: Ladder diagram for figure 9 

Now, returning to the ECG in this post, we do not see a consistent integer ratio of atrial:ventricular activations across the entire rhythm strip.

Figure 5: Lead II rhythm strip

However, if we just focus on the first part of the ECG (R1-8), we can make out 6:2. This reminds us of the ECG in Figure 7 and suggests 2:1 on top with 3:2 on bottom. Then we just have to explain R9-11. In fact, they exhibit very regular 4:1 conduction! Which is another way of saying 2:1 followed by 2:1. This is shown below.

Figure 11: Multilevel block with 2:1 on top and variable conduction on bottom

Note that Dr. Grauer and I independently came up with the exact same ladder diagram for this! There is one remaining finding on the ECG in need of explanation, and that's the changing QRS morphology. This requires all 12 leads for analysis.

Figure 1: ECG

The commonest QRS morphology is the one seen in R1, 3, 5, 7, 9, 10, 11. Looking at the limb leads, it has left anterior fascicular block. R2 by contrast has RBBB (and LPFB) morphology. This is due to Ashman's phenomenon, where in irregular rhythms, a long R-R cycle (between the R wave that occurred before this ECG started and R1) followed by a short R-R cycle (between R1 and 2) results in rate related aberrancy, classically RBBB.

How do we explain the axis change in R4 and R6? Dr. Grauer and I have different interpretations, but here is mine: I believe R4 and R6 exhibit "supernormal conduction" through the left anterior fascicle. A complete description of supernormal conduction is beyond the scope of this post. In brief, there is a brief window of time near the conclusion of the T wave where the next QRS complex has a chance to have better-than-expected conduction. In this case the patient's baseline is LAFB, but R4 and R6 no longer look like LAFB and instead just look like normal conduction. Supernormal conduction is almost like the opposite of Ashman's phenomenon.

I do not have an elegant explanation for why R8 is different from R4 and R6. My best guess is that it is not quite "as supernormal," and may have intact but delayed conduction through the LAF. I have represented this on my final ladder diagram below.

Figure 12: Final ladder diagram

So, in summary:

• Reverse typical atrial flutter
• Multilevel block, 2:1 on top and variable Wenckebach on the bottom (3:2 then 2:1)
• Left anterior fascicular block as the "baseline" QRS conduction
• Rate-related aberrancy resulting in bifascicular block (RBBB + LPFB) for R2
• Supernormal conduction through the left anterior fascicle for R4 and 6
• Partially supernormal conduction through the left anterior fascicle for R8

Sadly, no follow up is available for this patient. Ideally he would undergo flutter ablation which is curative for most patients. Could his syncope have been related to higher level block at some point? It is certainly possible.

  

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MY Comment, by KEN GRAUER, MD (5/24/2025):

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I found the initial tracing in today's case that Dr. Frick shared with me to be fascinating — and, illustrative of a number of important principles related to sophisticated, challenging arrhythmia problem solving.

• To Emphasize: The concepts presented by Dr. Frick and myself are advanced. These concepts involve laddergram explanations. That said — I emphasize in My Comment below that drawing laddergrams is not a necessary requirement for becoming skilled in time-efficient emergency care arrhythmia management.
• It took me many years to become skilled in drawing laddergrams. But learning to read laddergrams is EASY (and even if you've never seen a laddergram before reading this Blog post — you'll be able to understand how to interpret the information that a laddergram provides after you finish reviewing My Comment).

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NOTE: For those with an interest in becoming skilled in drawing laddergrams:

• My Comment at the bottom of the page in the May 16, 2020 post in Dr. Smith's ECG Blog — reviews in step-by-step fashion my approach to drawing the laddergram to another complex AV block case.
• For readers wanting additional guidance and instruction (with LINKS to more than 120 examples of explained laddergrams — and access to FREE download of my Power Point Laddergram Stencil) — CLICK HERE —

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My Approach to Today's CASE:

As noted above by Dr. Frick in his superb discussion — he and I came to similar conclusions about the etiology of today's rhythm. However — Our approach was somewhat different.

First — My Non-Laddergram Approach:

• For clarity in Figure-1 — I've extracted the long lead II rhythm strip from today's tracing. As per Dr. Frick — the underlying rhythm is AFluter (Atrial Flutter).
• I always start with the simple step of numbering the beats. In addition to being the only consistent way to guarantee that all involved clinicians can know with certainty which beat is being referred to — Don't the numbers suggest a pattern to the rhythm?
• 
  
• HINT: The Answer to this question is revealed below in Figure-2.

Figure-1: The long lead II in today's tracing.

ANSWER:

As suggested by the color-coding of the double arrows in Figure-2 — there is group beating!

• As per Dr. Frick — the underlying rhythm is AFlutter. And, the reasons we know there is some form of conduction with this AFlutter are: i) The overall R-R interval is not regular (whereas IF the rhythm with AFlutter was complete AV block — then the ventricular escape rhythm should be quite regular); and, ii) The PR interval in front of each of the beats ending a longer R-R interval is EQUAL (ie, All of the YELLOW double arrows in Figure-2 are precisely equal in duration — whereas IF the rhythm was complete AV block, this interval would continually change). 
• NOTE #1: The rhythm in Figure-2 starts with 4 pairs of beats — in which there are alternating shorter then longer R-R intervals (The duration of the double BLUE arrows are all equal — and duration of the longer double RED arrows are also equal). With practice — your "eye" should be able to recognize this repetitive pattern of beats within seconds.
• KEY Point: Once I recognized that the underlying rhythm in Figure-2 is AFlutter — and that there is group beating with a constant PR interval in front of each of the beats ending a longer R-R interval — I knew that this rhythm was AFlutter with dual-level AV Wenckebach Block out of the AV node. No laddergram was needed for me to arrive at this conclusion within less than 20 seconds.

Additional Points regarding Figure-2: 

• Using calipers instantly makes you both smarter and faster! To emphasize that I never pulled out my calipers when the patient in front of me was "crashing". That said — most of the time in the management of a case, there will be a "moment" when you will have time to use calipers. The clinician who never uses calipers — is a clinician who will fail to correctly diagnose a significant number of more complex rhythm disorders. 
• NOTE #2: The pattern of group beating in Figure-2 changes for the last 2 beats in the tracing, in which the "groups" no longer have 2 beats in them. That said — there still is a pattern, in that the duration of the PINK double arrows is the same. To emphasize that the patterns of Wenckebach-type blocks are not always perfect, such that this change for beats #10 and 11 did not alter my immediate impression of AFlutter with Wenckebach block out of the AV Node.

Figure-2: I've color-coded PR and R-R intervals in today's tracing.

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Next — My Laddergram Approach:

• For clarity in Figure-1 — I've extracted the long lead II rhythm strip from today's tracing. As per Dr. Frick — the underlying rhythm is AFluter (Atrial Flutter).

Figure-3: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial Tier. GREEN arrows show the peaking of P waves (in this case, flutter waves) as my reference point for drawing in atrial activity. Because conduction through the atria is generally rapid — I drew in near-vertical lines in the Atrial Tier.
=  =  =  =  =
NOTE: Because I suspected dual-level AV block within the AV Node — I have divided the AV Nodal Tier into 2 parts (by drawing in a horizontal BLACK dotted line).

Figure-4: Laddergram STEP-2. I next fill in the Ventricular Tier.
 GREEN arrows show the onset of each QRS complex as my reference point for each of the QRS complexes in this tracing. 
=  =  =  =  =
KEY Point: The "EASY part" for constructing most laddergrams consists of these first 2 STEPS (that are shown in Figures-3 and -4). Now the challenge begins — for trying to "solve" the laddergram by figuring out which of the P waves (in this case, flutter waves) in the Atrial Tier are being conducted to the ventricles.

Figure-5: While realizing that many complex tracings may have more than a single potential laddergram explanation — I find postulating 2:1 AV conduction in the upper AV Nodal level most often works for me when contemplating AFlutter with dual-level Wenckebach conduction.
NOTE: Rather than spending valuable time drawing in conduction of all flutter impulses in the upper AV Nodal level — I decided to test a small segment of the repetitive pattern (the BLUE lines that I've added in the upper AV Nodal Tier).
= = = = =
KEY Point: Because of "concealed conduction" — flutter waves that are very close to a QRS complex are usually not able to conduct to the ventricles. Awareness of this phenomenon provides an important clue for postulating which of the flutter waves are the most likely to be conducting. 

Figure-6: Following through with the 2:1 conduction through the upper AV Nodal level that I postulated in Figure-5 — the BLUE-lined paths that I added to the lower AV Nodal level in this Figure seem to suggest the most logical conduction passageway to produce beats #1,2 and beats #3,4.

Figure-7: Since the pattern for beats #5,6 and #7,8 — is the same as for beats #1,2 and #3,4 — I simply duplicated the conduction paths that I drew in Figure-6. (I find Power Point is ideal for its preciseness and speed for duplication of ladder elements that repeat).
= = = = =
NOTE #1: As per Dr. Frick's above discussion — aberrant vs supernormal conduction are potential explanations for the variable QRS morphology of beats #2,4,6 and 8. But the pattern of group beating in terms of alternating shorter-then-longer R-R intervals remains the same for these four 2-beat groups.
= = = = =
NOTE #2: The reason that I did not fill in a path through the AV Node for beats #9 and 10 — is that the pattern of group beating changes.

Figure-8: I've added what appears to be the most logical BLUE-lined path for beats #10 and 11.

Figure-9: This conduction path appears to be correct — as it allows continuation of my postulated 2:1 AV block in the upper AV Nodal level.

Figure-10: This leaves 2:1 AV block in the lower AV Nodal level as the logical BLUE-line path after beat #9.

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FINAL Laddergram:

For clarity in Figure-11 — I've color-coded selected flutter waves to facilitate recognizing the pattern of conduction in today's tracing:

• RED arrows — highlight which flutter waves conduct the 1st QRS complex in the 2-beat groups, and the only QRS complex in the 1-beat groups at the end of the tracing.
• BLUE arrows — highlight which flutter waves take slightly longer to get through the lower AV Nodal level in the 2-beat groups.
• YELLOW arrows — highlight which fluter waves are blocked in the lower AV Nodal level.
• NOTE — The repetitive pattern of these color-coded arrows highlights consistency of group beating in today's tracing.

Figure-11: Final Laddergram. Color-coded flutter waves highlight the consistency of group beating.

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"Take-Home" Points: 

To remember: It is very common to see group beating with AFlutter — and — there will often be dual-level AV block of these rapid flutter waves out of the AV Node.

• Even without having to draw a laddergram — we can quickly recognize dual-level Wenckebach conduction out of the AV Node by seeing group beating that occurs in association with consecutive non-conducted flutter waves (as is evident in Figure-11) — especially when there are identical PR intervals before many of the beats.
• The "good news" — is that these consecutive non-conducted flutter waves do not necessarily represent a fixed (pathologic) form of AV block! Instead — normal 1:1 AV conduction will sometimes resume once AFlutter is converted to sinus rhythm.