Tuesday, November 2, 2021

ECG Blog #259 (71) — What is Dual-Level Block?

How would you interpret the 3-lead rhythm strip shown in Figure-1? No clinical information is available.

  • Is there AV block? If so — What kind?
  • Or are there PACs (Premature Atrial Contractions)? 
Figure-1: How would you interpret this 3-lead rhythm strip? (See text).



NOTE #1: This is a complex tracing. In recognition of the diverse ECG interpretation experience of my audience — I am presenting the answer to this tracing in two formats:

  • FIRST: The "Quick" Answer — that explains the basic mechanism of this rhythm.
  • SECOND: The "Detailed" Answer — that I visually explain in the ADDENDUM by use of step-by-step laddergrams. I'll emphasize that although drawing laddergrams can be challenging — I bet that any reader of this ECG Blog will be able to better understand the mechanism operative for today's arrhythmia by reviewing the user-friendly color-coded laddergrams in Figures 4-thru-14 below. 


NOTE #2: Some readers may prefer at this point to listen to the 5:45-minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-71).


Today's 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).



My THOUGHTS for Arriving at the "Quick" Answer:

As usual — I approach interpretation of today's rhythm in systematic fashion, by addressing the Ps, Qs & 3Rs (See ECG Blog #185). NOTE — It is not essential to perform this assessment in order (ie, I'll often look at QRS width or regularity of the rhythm before assessing atrial activity — depending on whatever is easiest to assess for the tracing at hand). For the rhythm in Figure-1 — My sequential thought process was as follows:

  • The QRS complex is narrow in each of the 3 simultaneously-recorded leads — so the rhythm appears to be supraventricular.
  • The rhythm in Figure-1 is not Regular. Therefore, the Rate will vary. That said — there is a pattern to the rhythm, in the form of alternating longer and shorter R-R intervals (ie, group beating is present).

PEARL #1: To prove that the rhythm in Figure-1 truly represents group beating — Measure all R-R intervals! This is easiest to do with calipers. Doing so will establish that each of the "longer" R-R intervals are of virtually the same duration (ie, ~1280 msec.) — and — each of the "shorter" R-R intervals are also virtually the same (ie, ~820 msec.). Finding that one or more of the R-R intervals in the tracing repeat confirms the presence of group beating.

  • NOTE: Despite marked baseline artifact between beats #8-9 (which prevents assessment of atrial activity) — We can still tell that the R-R interval between beats #8-9 is the same as that for the other longer R-R intervals in this tracing. 
  • For more on the concept of Group Beating — See ECG Blog #186.


Continuing with my Sequential Approach to Today's Rhythm:

  • Regular P waves are present throughout the tracing (RED arrows in Figure-2).
Figure-2: For clarity — I focus on a single lead for the rest of today's discussion. RED arrows highlight regular sinus P waves (ie, upright in lead II) throughout this tracing.

PEARL #2: Because many of the P waves in today's tracing are hidden within the ST-T wave — it might not initially be apparent that the atrial rhythm is regular.

  • Once again — calipers facilitate this determination. Find a place in Figure-2 where you can definitely see at least 2 P waves in a row. The first 3 RED arrows at the beginning of the tracing highlight such a place.
  • Set your calipers to precisely the P-P interval for these consecutive P waves. Doing so confirms the presence of the 4th P wave in Figure-2 — which occurs right on time, and which produces extra peaking of the T wave of beat #2 (4th RED arrow in Figure-2). 
  • The RED arrows I have placed through the rest of Figure-2 confirm that regular on-time P waves occur throughout this tracing.


This leaves us with assessment of the final parameter in the Ps, Qs & 3R Approach — which is to determine IF any (and if so, how many) P waves are Related to neighboring QRS complexes.



PEARL #3: The easiest way to assess this 5th parameter, regarding whether any of the P waves in Figure-2 are related to neighboring QRS complexes — is to begin by looking in front of the beat that ends each of the longer R-R intervals (ie, Looking in front of beats #1, 3, 5 and 7 in Figure-2)

  • QUESTION: Is there a P wave in front of beats #1, 3, 5 and 7? 
  • If so — Is the PR interval in front of each of these QRS complexes constant? 
  • KEY Point — IF you can find places in the rhythm where the same PR interval repeats — Then you have established that there probably is at least some conduction of these atrial impulses. The more places you find in which an identical PR interval repeats — the more certain it becomes that there is conduction of at least some atrial impulses.


With regard to Figure-2:

  • The PR intervals before beats #1, 3, 5 and 7 are all = 0.40 second. The finding that each of these PR intervals is of the same duration — strongly suggests that these beats (each of which terminates a relative "pause" in the rhythm) — are being conducted, albeit with a long 1st-degree AV block.



  • Are there any other PR intervals in Figure-2 that repeat?




  • The PR intervals before beats #2, 4, 6 and 8 in Figure-2 also repeat! These PR intervals are much shorter than the PR intervals before beats #1, 3, 5 and 7 — so I was not at all certain which P wave was conducting which QRS complex — BUT — the fact that there are 2 sets of repeating PR intervals in Figure-2 is not by chance!



Putting IAll Together: The "Quick" Answer to the complex rhythm in Figure-2 is based on the ECG findings we have just described in our systematic approach:

  • There is group beating (ie, a bigeminal rhythm, with alternating shorter and longer R-R intervals).
  • The QRS is narrow — so the rhythm is supraventricular.
  • The atrial rhythm is regular (ie, There are regular sinus P waves — as shown by the equally-spaced RED arrows in Figure-2).
  • There are 2 sets of repetitive PR intervals (ie, the PR intervals before beats #1,3,5 and 7 are all the same — and — the PR intervals before beats #2,4,6 and 8 are all the same). This is not due to chance — and it tells you that there is conduction of these impulses.

Additional Observations:

  • There are more P waves (RED arrows in Figure-2) than QRS complexes — therefore not all of the P waves are being conducted. Since the atrial rhythm is regular — but not all of these regularly-occurring sinus P waves are being conducted — some form of 2nd-Degree ABlock is present. 
  • The finding of group beating, in association with some form of 2nd-degree AV block — strongly suggests some type of Wenckebach conduction.
  • The clinical severity of this 2nd-degree AV block is difficult to ascertain without clinical information on the patient — but the fact that the QRS complex is narrow (and that the rhythm is therefore supraventricular) — with conduction of many beats — and an overall ventricular rate that is not bradycardic (ie, about 60/minute) — all suggest that the patient's prognosis might not be bad, and that a pacemaker may not necessarily be needed (More on clinical implications of this rhythm below, in our detailed answer that appears in the Addendum).



PEARL #4: An additional finding in support of Wenckebach conduction — is that Wenckebach periodicity is present. As discussed in today's Audio Pearl — the 3 most easily recognized features of Wenckebach Periodicity are: i) Group beating; ii) A regular (or almost regular) atrial rhythm; andiii) The finding that the pause that contains the dropped beat is less than twice the shortest R-R interval.

  • The reason for iii) — is that the greatest increment in the PR interval with AV Wenckebach tends to be between the 1st and 2nd beats in any group. The reason awareness of this feature of Wenckebach periodicity is clinically important — is that when the pause containing the dropped beat is more than twice the shortest R-R interval — it is likely that "something else" other than Wenckebach block is responsible for the rhythm disturbance (ie, blocked PACs, sinus pauses from Sick Sinus Syndrome).
  • In Figure-2 — Note that the shorter R-R intervals measure 820 msec. — and the longer R-R intervals measure 1280 msec. — so that the longer R-R intervals are clearly less than twice the shorter R-R intervals. Therefore — this is consistent with "Wenckebach periodicity".


The LADDERGRAM: To illustrate the mechanism for the "Quick" Answer to today's case — I've drawn a laddergram (in Figure-3). The major premise for this proposed laddergram — is that there is a dual-level of AV block within the AV Node (dotted BLACK horizontal line within the AV Nodal Tier).

  • To Emphasize: While drawing laddergrams can be complicated — understanding a laddergram that has been drawn for you is surprisingly EASY (User-friendly review of this concept was presented in ECG Blog #188).
  • It should be apparent from the laddergram in Figure-3, that I propose the presence of Wenckebach block at 2 levels within the AV node (ie, with 4:3 AV conduction out of the upper AV Nodal level — and 3:2 AV conduction out of the lower AV Nodal level).
  • Clinically — Significance of the conduction disturbance in Figure-3 will depend on the history and clinical circumstances. Despite non-conduction of 2 P waves in a row (just before and just after the 2nd beat in each group) — this is not necessarily a "high-grade" block, and a pacemaker will not necessarily be needed!
  • NOTE: For step-by-step illustration of HOW I derived the laddergram in Figure-3 — See the ADDENDUM below.


Figure-3: My proposed laddergram for the rhythm in today's case.


Acknowledgment: My appreciation Victor Svensson (from Växjö, Sweden) and Peter Hammarlund (from Helsingborg, Sweden) for making me aware of this case and allowing me to use this tracing.



Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs & 3Rs Approach to systematic rhythm interpretation.
  • 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 #186 — Highlights the importance of Group Beating — and reviews when to suspect the Mobitz I form of 2nd-Degree AV Block ( = AV Wenckebach).
  • ECG Blog #188 — Reviews the essentials for reading (and/or drawingLaddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts.
  • ECG Blog #243 — Reviews a case of AFlutter with Dual-Level Wenckebach out of the AV Node.
  • 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 #251  Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach (Please check out Audio Pearl #65 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)


ADDENDUM (11/2/2021): For those wanting to see the stepwise approach that I used to arrive at the laddergram shown in Figure-3I've added Figures #4-thru-14.

  • CLICK on Figure-4 — which will magnify this Figure for you. Then advance one-by-one through the next 10 Figures in the magnified mode to see my stepwise approach for deriving the laddergram that I showed earlier in Figure-3.



Figure-4: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial TierBLUE arrows show the onset of P waves as my reference point for drawing in atrial activity.


Figure-5: Laddergram STEP-2. I next fill in the Ventricular TierBLUE arrows show the onset of each QRS complex as my reference point for each of the 9 supraventricular beats in this tracing.



Figure-6: The "easy part" for constructing most laddergrams consists of these first 2 STEPS (that are shown in Figure-5 and Figure-6). Since conduction of sinus P waves and conduction of narrow QRS complexes is usually quite rapid — near-vertical lines are used, which start from the onset of all P waves — and from the onset of all supraventricular (ie, narrow-QRS complex) beats. Since all beats in Figure-6 are narrow and manifest very similar QRS morphology — we don't have to worry about any ventricular beats. Now the challenge begins! — with the task of trying to figure out which of the P waves in the Atrial Tier are being conducted to the ventricles.


Figure-7: Laddergram STEP-3. An important observation to make is that there are more P waves than QRS complexes in this tracing (ie, There are 17 vertical lines in the Atrial Tier — but only 9 lines in the Ventricular Tier). This means that 8 of the 17 P waves are not conducted — and we have to figure which ones they are. It would seem that RED arrow P waves are being conducted — since beats #1,3,5 and 7 are all preceded by a P wave with the same PR interval (of 0.40 second). It would also seem that WHITE arrow P waves are not being conducted — since these P waves all occur within the ST segment (therefore likely to occur during the refractory period — as well as being far removed from the next QRS complex). This leaves us with having to decide which of the 2 GREEN arrow P waves within each group is being conducted.



Figure-8: I felt it highly unlikely that the P waves immediately before beats #2,4,6 and 8 were conducting — because it simply would not make sense for the 2nd beat in each group to have a PR interval so much shorter than the PR interval for the 1st beat in each group. Therefore — WHITE arrow P waves in Figure-8 represent those P waves that I thought unlikely to be conducting. This meant that the only P waves left (ie, the GREEN arrow P waves) would have to be conducting in order to make this laddergram "work". NOTE: If my theory is correct — this would mean that 2 consecutive P waves in each 2-beat group are not conducting ( = the WHITE arrow P waves). The only way I know to make the laddergram work when there is Wenckebach conduction and 2 consecutive non-conducting P waves — is for there to be a dual-level of Ablock within the AV Node (BLACK dotted line within the AV Nodal Tier — as is fully explained in today's Audio Pearl).


Figure-9: Laddergram STEP-4. Now that the "thought process" is complete for determining which beats in this tracing are likely to be conducted — all that remains in order to "prove" my proposed theory, is to see IF I am able to construct a reasonable laddergram for explaining events occurring within the AV Nodal Tier. To do this, I begin connecting those P waves in the Atrial Tier that I feel are most likely to be conducted to the 9 beats in the Ventricular Tier (BLUE lines passing through the 2 levels within the AV Nodal Tier).


Figure-10: Continuation of STEP-4. On the theory that the 2nd P wave in each 2-beat group is also conducting — I've added at a slightly greater degree of inclination (to represent slower conduction) a slanted BLUE line passing through the upper AV Nodal level.



Figure-11: Continuation of STEP-4. IF this 2nd P wave in each group is to be conducting — it would have to do so at the inclination shown by the slanted BLUE line that I drew passing through the lower AV Nodal level. IF this is correct — then all QRS complexes will now be "accounted for".



Figure-12: Laddergram STEP-5. Since according to Figure-11, all QRS complexes are now "accounted for" — this means that the 2 remaining P waves in each group (ie, the 2 WHITE arrow P waves in each group) do not make it through to the ventricles. Continuing on the assumption of Wenckebach conduction — I've added slanted BLUE lines passing through  the upper and lower levels within the AV Node at an inclination that reflects progressively delayed conduction (à la Wenckebach).



Figure-13: Completion of STEP-5. The only possible option for the last remaining P wave in each group is shown by adding a terminal "butt" that shows blocked conduction within the upper AV Nodal level.



Figure-14: My completed laddergram. For clarity — I've colored the 4 P waves in each group. Note that the pattern seen here repeats in each of the 2-beat groups in this tracing.
— The RED arrow P waves represent conduction with a prolonged PR interval of 0.40 second.
— The GREEN arrow P waves are also conducted, albeit with even greater prolongation of the PR interval (to just over 0.70 second).
— The GRAY arrow P waves make it through the upper AV Nodal level — but not through the lower level.
— The WHITE arrow P waves do not even make it through the upper AV Nodal level. 

 The laddergram makes sense. All P waves and all QRS complexes in this tracing are accounted for. There is 
2nd-Degree ABlock with dual-level Wenckebach conduction out of the AV Node, manifesting 4:3 conduction at the upper AV Nodal level — and 3:2 AV conduction at the lower level.

Final PEARL: — Significance of the conduction disturbance in Figure-14 will depend on the history and clinical circumstances. Despite non-conduction of 2 P waves in a row (Neither the GRAY, nor the WHITE P waves in Figure-14 are conducted) this rhythm does not necessarily represent a "high-grade" block, and a pacemaker will not necessarily be needed! Instead, there is Wenckebach conduction out of 2 levels within the AV Node — and AV Wenckebach is far less likely than Mobitz II to require permanent pacing.

ADDENDUM #2 (11/4/2021):
As I've often emphasized — there may be more than a single potential explanation for a complex arrhythmia. I view this as a positive — since it expands our horizons in the "world" of ECG interpretation. In this way we learn from each other.
  • David Richley's expertise in arrhythmia interpretation is well known to many of you who follow any of the many ECG internet forums. I thought it worthwhile to add Dave's thoughts on another possible explanation for the ECG in today's case:

Dave wrote the following:
Ken — When I first saw the most recent ECG you posted, I immediately thought of 2 AV nodal pathways – one fast, one slow. Then I looked at your ingenious explanation and thought that this was probably much more plausible than my own suggestion. However — I’ve attached a laddergram to show what I originally thought (Figure-15): 
  • 4:1 block in a fast pathway — and 4:1 block also in a slow pathway, with the timing such that every other impulse blocks simultaneously in both pathways. 
  • I don’t think this is necessarily correct — but it does illustrate something that we have both long known — that complex arrhythmic ECG patterns can often have more than one theoretically possible explanation.

Figure-15: Laddergram submitted by David Richley — with another theory for the mechanism in today's case.


  1. In pearl 4, last point- the word "not" is wrongly added and reverses the meaning of the statement" the longer R-R intervals are clearly less than twice the shorter R-R intervals".

    1. THANK YOU Subhasish for picking up on this TYPO. I have now fixed it. I appreciate your "eagle eye"! — :)