Tuesday, April 20, 2021

ECG Blog #216 (ECG MP-33) — Is AV Block Complete?

PLEASE NOTE: Before I begin — there are lessons to be learned from this tracing. Although I immediately recognized the basic conduction disturbance when this tracing was first sent my way — it was only several months later (as I was preparing this ECG Blog post) — that I realized my initial interpretation was not correct.

  • We all live and learn daily!
  • This is a challenging tracing! Are YOU up for the challenge? (NOTE: Even if you don’t completely get the interpretation of this tracing — There is still LOTS to be learned.).




The long lead II rhythm strip in Figure-1 was obtained from a 65-year-old man, who came in for a “routine” check-up. A slow and irregular heart rhythm had been heard on auscultation — which prompted an ECG. The 12-lead tracing showed an incomplete RBBB — but no acute changes.


QUESTION: How would you interpret the rhythm shown in Figure-1?

  • Is complete (ie, 3rd-degree) AV block present?


Figure-1: Long lead II rhythm strip, obtained from an asymptomatic 65-year-old man. Is there complete AV block?



NOTE #1: Some readers may prefer at this point to listen to the 7:00 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-31).


Today’s ECG Media PEARL #33 (7:00 minutes Audio) — Explores what to do when you are not certain of the Rhythm Diagnosis. How often does this really matter?




MY Thoughts on Figure-1:

There is slight angulation of this tracing — therefore some distortion. That said — I believe the quality of this tracing is still adequate for accurate rhythm assessment. By the “Ps, Qs & 3R Approach” that I favor (Reviewed in ECG Blog #185):

  • P waves are present (To be discussed in detail below!).
  • The QRS complex is of borderline duration (ie, about 0.10 second). This is consistent with the incomplete RBBB that was seen on the 12-lead ECG. KEY Point: The rhythm seen in Figure-1 is supraventricular!


Regarding the 3 Rs:

  • Regularity of the Rhythm? — The R-R interval in Figure-1 is not Regular! (NOTEThis is a very important observation — that the R-R interval is not regular!).
  • Rate? — The rate of the ventricular rhythm is slow. Since the R-R interval varies between 7-to-8 large boxes in duration — the ventricular rate is in the low 40s.
  • Related? (ie, Are any P waves related to neighboring QRS complexes?) — This is another KEY Question. Do YOU think any of the P waves in Figure-1 are related to neighboring QRS complexes?


QUESTION: Is the atrial rhythm in Figure-1 regular?

  • My answer appears in Figure-2.

Figure-2: I’ve added RED arrows to Figure-1 to highlight sinus P waves (See text).

ANSWER: I believe that a regular atrial rhythm at ~100/minute is present in Figure-2 (RED arrows).

  • Although tiny in amplitude, upright sinus P waves are regularly seen in this lead II rhythm strip — except for the 2 places where I added RED question marks.
  • Given consistent equal-spacing for the arrows that I’ve drawn — it seems logical to intuit that regularly-occurring P waves continue throughout the entire rhythm strip (and that 2 P waves are almost certainly present and hidden within the QRS complex in the 2 places where I’ve put question marks).
  • NOTE: Were this not the case — We’d have to postulate a highly unusual form of SA block. Common things are common — and — Life is far simpler IF we presume that regular sinus P waves continue throughout the entire tracing (even though we do not see clear indication for 2 of the 18 P waves that I believe are present in Figure-2).

PEARL #1: Using calipers is essential for assessment of complex arrhythmias — especially those involving challenging AV block rhythms. 

  • I am continually amazed at how helpful the simple step of labeling P waves has been to me as an aid for facilitating visualization of even complex P-to-QRS relationships (which is why I added the RED arrows in Figure-2).


PEARL #2: With rare exceptions, one KEY to recognizing 2nd- or 3rd-degree AV Block — is that the atrial rhythm should be regular (or at least almost regular)

  • Awareness that the atrial rhythm should be at least fairly regular when there is 2nd- or 3rd-degree AV Block — is important for ruling out entities such as sinus pauses, sinus arrest, SA block, and blocked PACs as a potential cause of the rhythm disturbance.
  • Therefore — Since the atrial rhythm in Figure-2 is regular — and, since it is obvious that many of the P waves highlighted by the RED arrows are not being conducted to the ventricles (despite having adequate opportunity to be conducted) — some form of 2ndor 3rd-degree AV Block is present.

KEY QUESTION: Is the degree of AV Block in Figure-2 complete?

  • HINT #1: How does the fact that the R-R interval is not regular help to answer this question?
  • HINT #2: Take another LOOK at all of the P waves in Figure-2. Do any of the PR intervals repeat? (ie, Do YOU see the same PR interval in more than 1 place in this rhythm strip?).

PEARL #3: One KEY to recognizing 3rd-degree (ie, complete) AV Block — is that most of the time, the ventricular rhythm should be regular (or at least almost regular). The reason this is true in most cases of complete AV block — is that escape rhythms arising from the AV node, the His or the ventricles tend to be regular.

  • As a result — one of the most helpful clues that one or more beats in an AV block rhythm are being conducted — is IF one or more of the QRS complexes occur earlier-than-expected.
  • It should be apparent in Figure-2 that beats #4 and 8 both occur earlier-than-expected (IF this is not apparent to you — please measure with calipers the R-R intervals between beats #3-4 and #7-8 — and compare these to all of the other R-R intervals in this tracing). The reason beats #4 and 8 occur earlier-than-expected — is that both of these beats are almost certainly being conducted.

Do ANY of the PR Intervals in Figure-2 Repeat?

  • My answer appears in Figure-3.

Figure-3: I have color-coded my theory for which of the P waves in Figure-2 are conducting (See text).

MY Explanation of Figure-3: Unfortunately — there is some slanting of this rhythm strip recording, and this distorts certain measurements (especially toward the end of the tracing). That said — I believe accurate interpretation is still possible.

  • PEARL #4: One of the most helpful clues that certain P waves are conducting — is the finding of one or more identical PR intervals elsewhere on the tracing. While possible by chance for a given PR interval to be repeated elsewhere on the tracing — when you find multiple PR intervals that repeat — this becomes very unlikely to be the result of chance — and it strongly suggests that there is conduction!
  • It is clear that many of the P waves in Figure-3 are not conducting. I chose BLACK arrows to represent those P waves that I believe are not conducting.
  • The 2 RED arrows in Figure-3 highlight P waves I believe are conducting with normal and equal PR intervals.
  • Although extremely long — the 2 YELLOW arrows in Figure-3 highlight P waves that have an identical PR interval. Therefore — I thought these YELLOW P waves were conducting.
  • Even longer, but nevertheless equal — are the PR intervals associated with the 2 BLUE arrow P waves. The reason I strongly suspect that these 2 BLUE arrow P waves are conducting — is that the R-R intervals that contain these 2 BLUE P waves are equal and shorter than all other R-R intervals on this tracing! (As per Pearl #3 — beats #4 and 8 occur earlier-than-expected — and therefore these beats are likely to be conducted).
  • NOTE: Although the P waves immediately preceding beats #4 and 8 appear to manifest similar PR intervals — these PR intervals are quite short — so I thought it less likely that these P waves were conducting (which is why I used BLACK arrows to highlight these P waves).
  • At this point — I was not sure about whether the P wave highlighted by the WHITE arrow was or was not conducting.

Putting IAll Together: At this point — I did not have a definitive diagnosis for the cardiac rhythm in Figure-3. Nevertheless — I had established the following:

  • The QRS was narrow.
  • The atrial rhythm was regular. Some P waves were conducting — but other P waves were not conducting (despite having "adequate opportunity" to conduct — as suggested by their occurrence at many different points within the R-R interval). Therefore — some type of AV block was present.
  • The fact that: i) The ventricular rhythm was not regular; and, ii) Several PR intervals repeated — told us that the rhythm was not 3rd-degree AV block. Since some form of AV block other than 3rd-degree was present — the rhythm in Figure-3 had to represent some type of 2nd-degree AV block.
  • Since the overall ventricular rate was in the low 40s — the effect of the conduction defect was clearly significant. So, despite the fact that this 65-year-old man was asymptomatic at the time he was seen — unless some reversible cause of his conduction defect could be identified and corrected, it seemed probable that a permanent pacemaker would be needed in the near future. Clinical correlation would help to sort this out.

The Laddergram: This case provides an excellent example of how I was uncertain about the specific mechanism of this arrhythmia until I was able to work things out in a laddergram.

  • The reason I was uncertain about a precise etiology — is that the mechanism of this arrhythmia is unusual and complex. I believe there is a dual-level block occurring within the AV Node (represented by the horizontal BLACK line within the AV Nodal Tier in Figure-4).
  • I reviewed basics for reading and drawing laddergrams in ECG Blog #188. That said — I admittedly had to try a number of different possible mechanisms before arriving at a proposed mechanism that seemed to work (Figure-4). This wasn’t a simple case.
  • KEY POINT: Learning to to read laddergrams is not difficult (guarantee anyone reading this blog post can quickly gain confidence and competence in reading laddergrams after Review of the material in ECG Blog #188). However — it does take time and application to learn how to draw laddergrams. That said — You do not have to know how to draw laddergrams to be able to competently manage this case! (as demonstrated by my analysis above under, “Putting It All Together”).

Figure-4: My proposed laddergram for the rhythm in today’s case (See text).


Explanation of My Laddergram:

  • The atrial rhythm is regular at ~100/minute (regularly-spaced arrows in Figure-4).
  • There is a dual-level block occurring within the AV Node.
  • Every-other-P wave makes it through the upper level of the AV Node (ie, there is 2:1 Wenckebach block out of this upper level within the AV Node).
  • At this point — it’s easiest to follow the path of the P wave highlighted by the 1st RED arrow. This 1st RED arrow P wave is conducted to the ventricles with a normal PR interval ( = beat #2).
  • The 1st YELLOW arrow highlights the next P wave that was able to make it through the upper level of the AV Node. This 1st YELLOW arrow P wave also makes it through the 2nd level within the AV Node — albeit requiring additional time to do so (resulting in the increased PR interval that precedes beat #3).
  • The P waves highlighted by the 1st BLUE arrow — and then the WHITE arrow also make it through both levels of the AV Node, albeit with a progressively increasing PR interval (to produce beats #4 and 5) — until finally, the P wave highlighted by the BLACK arrow occurring at the end of the 5th QRS complex fails to make it out of the lower AV Nodal level. This results in a 5:4 Wenckebach cycle out of the lower level of the AV Node (as seen by the progressively increasing PR interval that precedes beats #2, 3, 4, and 5).
  • This dual level Wenckebach block then repeats for the last 3 beats in Figure-4. Note how the pattern of RED, YELLOW and BLUE P waves repeats for these last few beats in the tracing.
  • PEARL #5: I am often asked, “How long can the PR interval be and still conduct?” The P wave highlighted by the WHITE arrow in Figure-4 illustrates that it is possible for the PR interval to be longer than 1.0 second and still conduct.

BOTTOM LINE for Today’s Case: There are lessons-to-be-learned from today’s case for all interpreters, regardless of their level of experience. Even though I did not initially interpret this tracing correctly — the points that I highlight under, “Putting It All Together” form the message I hoped to convey by presenting this challenging case.



Acknowledgment: My appreciation to Shomi Ganguly and Prabal Chakrabarty (from Bangladesh) for the case and this tracing.

  • My THANKS on this case also extend to David Richley (Scarborough, UK) — who is well known to many of my readers, for his always astute insights on complex arrhythmia interpretation. My thought process and Dave’s were similar on this tracing — both regarding our initial misdiagnosis, as well as on our revised (hopefully correct) laddergram interpretation.




Relevant ECG Blog Posts to Today's Case: 

  • ECG Blog #185 — Reviews my Systematic Approach to Rhythm Interpretation using the “Ps, Qs & 3R Approach”.
  • 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 #186 — Reviews some basics on AV Blocks, and WHEN to suspect Mobitz I ( = AV Wenckebach) 2nd-degree AV Block. 
  • ECG Blog 189 — More on determining What TYPE of AV Block is present?


  1. Then QRS 4 and 8 are not conducted after the P-wave just in front of the QRS bur after the previous P-wave with long PR interval.
    You see the same mechanisme in atrial flutter with alternating 2:1 and 4:1 AV-conduction.

    1. You are correct — that is, the P waves just in front of beats #4 and 8 are NOT conducted. Instead, it is the 2 light BLUE P waves (in the laddergram) in front of beats #4 and 8 that are conducted to the ventricles. What IS similar to the mechanism with alternating 2:1 and 4:1 AV conduction with AFlutter — is that it will usually turn out that the P wave closest to the QRS is not the P wave that is being conducted to the ventricles. The reason for this, is that with rapid atrial rates (as in AFlutter) — “concealed conduction” from so many atrial impulses arriving at the AV node often prevents that P waves closest to the QRS from conducting — :)

  2. I am lost for words. This is simply an outstanding dissection of such a complex rhythm.