Saturday, August 5, 2023

ECG Blog #389 — A Quote from Sherlock Holmes

The long lead II rhythm strip shown in Figure-1 — was obtained from a previously healthy 30-year old woman, who presented with new abdominal pain. Her vital signs were stable — and she did not have an acute abdomen.

  • Is there complete AV block?  
  •     If not — How would YOU interpret this rhythm?

  • What is unusual about this conduction disturbance? 
  •     (HINT: I had not seen this phenomenon before! )

  • HOW would you treat this arrhythmia?

NOTE: What follows below is my “thought process” for interpreting today’s rhythm — with multiple PEARLS sprinkled in along the way.
  • To EMPHASIZE: In “real time” — I interpreted this rhythm in less time than it takes to read my intentionally long discussion below.
  • P.S.: This is a difficult tracing! It highlights many important and useful concepts in arrhythmia interpretation — but Be Forewarned that the complete solution is advanced!

Figure-1: The initial tracing in today's case. How would YOU interpret this rhythm?

MY Approach to the Rhythm in Figure-1:
As per ECG Blog #185 — I favor the Ps, Qs, 3R Approach for interpretation of the cardiac rhythm — beginning with whichever of these 5 KEY Parameters is easiest to assess for the tracing in front of me:
  • At least in the single lead II rhythm strip seen in Figure-1 — The QRS complex appears to be narrow.
  • P waves are present (See PEARL #1 below).
  • The rhythm in today's tracing is not Regular. The Rate of the rhythm varies because of this irregularity — but the overall ventricular rate is probably between 90-100/minute.

  • NOTE: We can not yet address the last of the 5 KEY Parameters — which is to determine if P waves are (or are notrelated to neighboring QRS complexes.

PEARL #1: After considering the 5 KEY Parameters — the EASIEST next step for determining the mechanism of a complex rhythm — is to label the P waves. You'll note that I also number the beats — since this instantly allows everyone involved to ensure we are all talking about the same part of the tracing.
  • Using calipers is the fastest (and most accurate) way to determine if there is an underlying regular rhythm. Simply pick 2 consecutive P waves, the location of which you are certain about (ie, For example — we clearly see 2 consecutive P waves within the R-R interval between beats #9-to-10 — or between beats #11-to-12)
  • Set your calipers to one of these P-to-P intervals — and then see IF you can "walk out" regular (or at least fairly regular) P waves throughout the rest of the tracing. The RED arrows in Figure-2 show that you can!

Figure-2: I've added RED arrows to Figure-1 — that show there is an underlying regular atrial rhythm!

Are Any P Waves in Figure-2 Conducting?
Take another LOOK at Figure-2. Now that all P waves are labeled — Isn’t it much easier to appreciate that the underlying atrial rhythm is regular? 

  • The Question that now arises is — Are any of the P waves that we see in Figure-2 being conducted to the ventricles? 
  •      IF so — Which one(s)? 

NOTE: There are lots of P waves in Figure-2 — almost all of which seem to manifest different PR intervals with respect to their neighboring QRS complexes.
  • Does this rhythm represent 3rd-degree AV block?

PEARL #2: By definition — 3rd-degree ( = complete) AV block means that no atrial impulses (P waves) are being conducted to the ventricles. Therefore — IF we can demonstrate that one or more P waves are being conducted to the ventricles — then we have proven that the rhythm is not 3rd-degree AV block.
  • The PEARL is to — Look for one or more short pauses in the rhythm! Much of the time — it will be easiest to determine the mechanism of a complex arrhythmia IF there is a "break" in the rhythm, even if this slow-down is only momentary.

  • Looking at Figure-2 — it should now be easy to see that there are short pauses in today's rhythm (ie, Aren't the R-R intervals between beats #9-10 and between #11-12 clearly longer than the other R-R intervals in this tracing?).

  • Focusing on what happens near the end of these 2 brief pauses — Doesn't it look as if one or both of the P waves in front of beats #10 and #12 might be conducting? Now, it is true that the PR intervals before beats #10 and #12 are not the same — so we have not yet "proven" that these P waves are conducting — but, the most logical assumption is that the reason for the obvious lengthening of the R-R interval between beats #9-10 and between #11-12 — is that one (or both) of the P waves that now appear before beats #10 and #12 with reasonable PR intervals are being conducted to the ventricles!

  • To EMPHASIZE: Today's rhythm is indeed challenging! This is not a simple arrhythmia. That said — by illustrating my step-by-step approach, it will hopefully become apparent how even less experienced interpreters can narrow their differential diagnosis by applying basic principles within the grasp of any emergency provider. That said — I fully acknowledge that at this point in my assessment, I was not yet sure what the rhythm was.

QUESTION: Is there any AV block in Figure-2?

ANSWER = PEARL #3: As all emergency providers are aware — there are 3 degrees of AV block. HOW then do we determine if one of these 3 degrees of AV block is present?
  • For there to be AV block — the atrial rhythm should be regular (or at least almost regular, if there is an underlying sinus arrhythmia). If P wave morphology is changing — or if the P-P interval is clearly irregular — then you are probably not dealing with a "pure" form of AV block. (Of course there are always exceptions — such as cases in which there is AV block plus "other things", such as PACs, PVCs, etc. — but these exceptions go beyond the basic concepts I am presenting here).

  • The simplest form of AV block is 1st-degree AV block — in which all P waves are conducted to the ventricles. It's just that they take longer to do so (ie, The PR interval is clearly more than 0.20 second). This form of AV block is usually easy to recognize — because each QRS complex is preceded by 1 P wave with a fixed PR interval that is greater than 0.20 second. This is clearly not what we see in Figure-2.

  • For there to be 2nd- or 3rd-degree AV block: i) The atrial rhythm should be regular (or at least almost regular, if there is an underlying sinus arrhythmia); andii) One or more of the on-time P waves are not conducted to the ventricles.
  • KEY Point: The reason I emphasize that P waves must be "on time" for there to be a "pure" AV block — is that this rules out the possibility of mistaking blocked or conducting PACs for AV block.

  • The PEARL is that IF there is a regular atrial rhythm (ie, all P waves are "on time" — as the RED arrows in Figure-2 show) — then IF there are more P waves than QRS complexesat least some of these on-time P waves are not being conducted to the ventricles. This is the situation in Figure-2. The fact that we see 2 P waves (RED arrows) within the R-R interval between beats #9-10 and between #11-12 — means that there are at least 2 on-time P waves that are not conducted — which means that there is at least 2nd-degree AV block in today's tracing!

PEARL #4: Most of the time, IF the degree of AV block is complete (3rd-degree) — then the ventricular rhythm should be regular (or at least fairly regular). This is because escape rhythms arising from the AV node, the His or ventricles are usually fairly regular rhythms. Exceptions may occur (ie, during cardiopulmonary resuscitation) — but even then, there will usually be a recognizable pattern of ventricular regularity. 
  • "Armed" with PEARL #4 — knew within seconds that today's rhythm was not complete AV block — simply because the above-noted pauses indicate that the ventricular rhythm in Figure-2 is definitely not regular!

PEARL #5: While fully acknowledging the complexity of today's arrhythmia — the above deductions (See PEARLS #3 and #4) — tell us that some form of 2nd-degree AV block is present.
  • There are 3 Types of 2nd-degree AV block. These are: i) Mobitz I ( = AV Wenckebach); ii) Mobitz II; and, iii) 2nd-degree AV block with 2:1 AV conduction.

  • By the process of elimination (as reviewed in ECG Blog #236) — Of these 3 types of 2nd-degree AV block, today's tracing is virtually certain to be a manifestation of some form of Mobitz I = AV Wenckebach because: i) The QRS complex is narrow; ii) The PR interval is continually varying, whereas it should be constant with Mobitz II; and, iii) Mobitz I is so much more common than Mobitz II.


KEY "Take-Home" Point: 
IF this is as far as you got in your intepretation (ie, That today's rhythm in this previously healthy 30-year old woman with new abdominal pain — represents some form of AV Wenckebach, in which the QRS is narrow and the overall ventricular rate is more than adequate) this is more than enough for appropriate initial management of this patient!
  • Much of the time — 2nd-degree AV block of the Mobitz I Type (AV Wenckebach), in which the ventricular rate is more than adequate — is a relatively benign conduction disorder. This is especially true when the patient is a previously healthy younger adult who presents with a potential cause of increased vagal tone (as is the case for this 30yo woman with new abdominal pain).

  • PEARL #6: As per ECG Blog #61 — the entity known as "Vagotonic" Block may produce a variety of AV conduction disturbances with extremely atypical features. Among potential precipitating causes of vagotonic block — are GI symptoms of nausea, vomiting and abdominal pain (ie, a vagotonic component seems likely in today's case).

  • BOTTOM Line: The chances are great that the conduction disturbance seen in today's rhythm will resolve once this patient's abdominal pain is successfully treated. 


Laddergram Illustration:
Academically — I was fascinated by today's rhythm. But at this point, since I could not yet explain all ECG features — I needed to draw a laddergram. I'll emphasize that I am almost always able to quickly and accurately recognize the likely mechanism of most complex rhythms without need for a laddergram. Today's tracing is an exception, in that I needed to see if with assistance of a laddergram, I could work out a plausible mechanism to explain this unusual rhythm. 
  • Sequential legends over the next 5 Figures illustrate my thought process as I derived this laddergram. (See ECG Blog #188 for review on how to read and/or draw Laddergrams).
  • To EMPHASIZE — This was not an easy laddergram to draw. I had to test out numerous assumptions, with my final draft suggesting a highly unusual variation of Wenckebach conduction. That said — my hope is that even readers with limited experience with laddergrams will be able to follow the drawing of my reasoning in the final figure. Stay with me!

Figure-3: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (RED lines drawn directly below the onset of each of the P waves — as shown by the large BLUE arrows). Note that these RED lines in the Atrial Tier are nearly vertical — since conduction of sinus P waves through the atria is rapid. Note also that the P-P interval between successive P waves (vertical RED lines) is equal!

Figure-4: The most challenging part of most laddergrams is construction of the AV Nodal Tier — so I typically save that for last. Therefore, after drawing in all P waves into the Atrial Tier — It's easiest to next add indication of all narrow (ie, conducting) QRS complexes into the Ventricular Tier. The large BLUE arrows show that I use the onset of each QRS as my landmark. Note that the RED lines in the Ventricular Tier are also nearly vertical — since conduction of these narrow QRS complexes through the ventricles is rapid.

Figure-5: It's time to begin "solving" what we can in the laddergram. I do this by connecting P waves in the Atrial Tier that might logically be conducting to narrow QRS complexes in the Ventricular Tier.
—  —  —  —
Because of the complexity of today's rhythm — I had to make assumptions that would need to be "tested out". As per PEARL #2, I started by looking at the 1st brief pause ( = the short pause between beats #9-10). I thought it most logical that P wave "k" was not conducting, because it occurs so close to the QRS of beat #9 (ie, almost certain to fall within the absolute refractory period). IF this was the case — then it would be logical for P wave "l" to be conducting to the ventricles (albeit with a long PR interval) to produce beat #10. IF this was so — then P wave "m" was probably also conducting with a slightly longer PR interval to produce beat #11 — which looks to be followed by a non-conducted P wave "n" (thereby producing a 3:2 AV Wenckebach sequence for beats #10 and 11!).
—  —  —  —
Next — I thought it logical that P wave "o" probably begins the next Wenckebach cycle (that results in progressive BLUE line slanting that represents the increasing PR intervals seen before beats #13 and 14).

PEARL #7: I find the saying, "Birds of a Feather Flock Together" — to be an extremely helpful reminder of an important concept in arrhythmia interpretation. 
  • The meaning of this English proverb, "Birds of a Feather ..." — is that people of similar type, interest or character tend to mutually associate.

  • The relationship to cardiac arrhythmias of the saying, "Birds of a Feather" — is that IF we see other clear evidence on an arrhythmia of a frequent-occurring phenomenon — then additional less evident findings in that same patient probably reflect the same phenomenon.

  • Application to Today's Tracing: We know from deductions made thus far — that today's tracing manifests 2nd-degree AV block — with some form of AV Wenckebach. We have just shown in Figure-5 — that the last half of today's tracing manifests 1 cycle of 3:2 AV Wenckebach (beats #10 and 11) — and a longer AV Wenckebach cycle for which we don't get to see the dropped beat, because the rhythm strip ends with beat #14. Since the atrial rhythm is regular — by the "Birds of a Feather" concept, I suspected that other unexplained findings probably also represent some form of AV Wenckebach. I believe we see this in Figure-6.

Figure-6: It might seem at first glance that none of the P waves during the 1st half of this tracing are being conducted. But — "armed" with the knowledge that AV Wenckebach is present over the last half of today's tracing — I was able to draw in BLUE lines that manifest progressively increasing angulation for the first 5 beats in this tracing (ie, postulating that P waves a - b - c - d and e — are all being conducted in an AV Wenckebach sequence with extremely long PR intervals).
—  —  —  —
IF we then assume that P wave "g" begins a new cycle, and is conducted with a normal PR interval — this would result in a 5:4 AV Wenckebach cycle to produce beats #6,7,8 and 9 — with non-conduction of P wave "k" that ends this Wenckebach cycle.
—  —  —  —
—  —  —  —
The ONLY problem: What about P wave " f " ???
—  —  —  —

PEARL #8: My final PEARL in today's case features a quote from Sir Arthur Conan Doyle's renowned mystery series about master detective Sherlock Holmes: "When you have eliminated the impossible — whatever remains, however improbable — must be the truth!"
  • As emphasized above (and in ECG Blog #164) — "typical" AV Wenckebach cycles end with an on-time non-conducted P wave. Two examples of typical AV Wenckebach do occur in today's tracing (ie, the 5:4 AV Wenckebach cycle from beats #6-thru-9 — and the 3:2 cycle between beats #10-11).
  • In previous Blog posts, I've shown examples of atypical AV Wenckebach rhythms — which manifest potential variations on the typical Wenckebach "theme", including PR intervals that do not always sequentially prolong from one beat to the next — dual AV nodal pathways, each with their own conduction properties — and Wenckebach cycles ending with Echo beats, conducted or blocked PACs.

  • Today's tracing manifests yet another variation on the typical "Wenckebach theme". Today's variation is so uncommon — that I had not previously seen (or at least recognized) this phenomenon.

Applying PEARL #8 to Today's Rhythm:
  • In Figure-7 — I complete today's laddergram with the only resolution that seems possible (ie, As per Sherlock Holmes: "Once you eliminate the impossible — whatever remains, however improbable — must be the truth! ).

  • None of the common "other ways" to end a Wenckebach cycle (ie, Echo beats; blocked or conducted PACs) are possible in Figure-7 — because all of the P waves in today's tracing look alike and are on time!
  • This leaves as the only possible disposition of P wave f being that this P wave is not conducted to the ventricles (as per the BLUE butt end that I have drawn within the AV Nodal Tier of Figure-7).
  • Somehow, it must be that despite the lack of any pause after non-conducted P wave f — that P wave g is conducted to the ventricles, thereby beginning the next Wenckebach sequence with beat #6 (ie, via "supernormal" conduction? — vs via some alteration resulting from autonomic stimulation from the patient's abdominal pain?).

  • BOTTOM Line in Figure-7: By the "Birds of a Feather" concept — since everything else in Figure-7 is consistent with AV Wenckebach conduction — beats #1-thru-5 almost certainly also represent AV Wenckebach conduction, albeit with the highly unusual absence of any pause after the last non-conducted (but on-time) P wave in the sequence.

Figure-7: My proposed completion of today's laddergram (See text).
—  —  —  —
P.S.: Other proposed laddergrams explaining the mechanism of today's rhythm will be welcomed!



Acknowledgment: My appreciation to Firdhaus Jaya (from Malaysia) for the case and this tracing. 


Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
  • ECG Blog #185 — Review of the Ps, Qs, 3R Approach for systematic rhythm interpretation.

  • ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 80 laddergram cases — many with step-by-step sequential illustration).



ADDENDUM (8/5/2023): 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 way beyond-the-core! It adds an additional "twist" to what already makes for a tremendously challenging tracing. That said — I completely agree with all that Dave says! 
  • As per Dave — because of the lack of a repetitive PR interval at the end of the 2 brief pauses — there is no way that I can prove which is the non-conducted P wave that ends the 1st AV Wenckebach cycle.

  • To EMPHASIZE: As Dave acknowledges — his comment does not change the concepts I bring forth in my analysis.



BELOW is the COMMENT I Received Today from David Richley:

Hi Ken — I was fascinated by the Wenckebach example you just posted in your ECG Blog. I agree 100% with the mechanism you propose — but I just wonder how we can be sure which is the first P wave that is blocked?

  • You postulate that it is P wave ‘f’ that is blocked — but could it be P wave ‘h’ or ‘i’ as shown in these laddergrams, or even another? 
  • I’ve tried analysing the PR intervals and the approximate RR intervals (shown in my laddergrams) — but I don’t think I can find any way of working out which is the first blocked P wave. 
  • The 2 P waves that we can be certain do conduct = ‘l’ and ‘o’ — do so with different PR intervals — so I don’t think they help us to decide which of the prior P waves occurs immediately after a blocked one. 
  • I wonder if there is a vital clue I’m missing. I know that this is a trivial detail, only of academic interest, but I do find it interesting! Dave

Figure-8: I've added the 2 laddergrams that David Richley drew, which he suggests may be equally likely as my Figure-7 laddergram to reflect the true mechanism of today's rhythm (As per Dave — it could be P waves h, k and n — or — P waves i, k and n that are the non-conducted P waves in the first 3 AV Wenckebach cycles).
—  —  —  —
For Comparison: I added my proposed laddergram from Figure-7 on the bottom of this figure. I picked P waves f, k, and n as the non-conducted P waves.
—  —  —  —
BOTTOM Line: Dave and I agree on almost everything about today's tracing. We both acknowledge the uncertainty about which P wave is not conducted to end the 1st AV Wenckebach cycle. 

FINAL "Take-Home" Point: As I discuss in PEARL #6 above — the entity of vagotonic block is notorious for the unpredictable alterations it may produce in PR, P-P and R-R intervals — and I suggest that profound vagal influence from this patient's new abdominal pain is the most likely reason for the atypical features of today's arrhythmia.


    1. Why not isorhythmic AV dissociation

      1. GOOD question! My answer is that I would not expect to see the abrupt brief pauses that we see here between beats #9-10 and between #11-12.

        If you have not seen Blog #195 ( ) — it may be of interest of you, as it reviews some key concepts in isorhythmic AV dissociation (including an Audio Pearl on this subject. THANKS again for your comment — :)

    2. Why is "f" not conducted, and not "e"?

      1. EXCELLENT Question you ask! Please note — I just added an ADDENDUM to this post an hour before I saw your comment. David Richley raised similar questions as to HOW we can know which P waves are or are not conducting — and I agree completely with both you and Dave that we can NOT know this with certainty from this single tracing.

        That said — Given the PR interval of "d" that I DO think is conducting with a long PR interval — I suspect (but I can NOT prove) that "e" is conducting — which therefore provides one of the RARE times when you may see a P wave conduct not to its "neighboring" QRS, but to the next QRS after that ...

        Please check out my ADDENDUM above. I believe my Final "Take-Home" Point is the most important — namely to be aware of the unusual entity of vagotonic block, that is notorious for producing unpredictable patterns of Wenckebach conduction (and you may find Blog #61 that shows examples of vagotonic block interesting and insightful as more follow-up to your comment! — ) — :)

    3. Dear Ken . Incredible case that illustrates those helpful rules in uncommon cases like this. That's the reason why I like interpreting ECG. Cause it's a way of train your clinic brain or maintaining it in the best way. many THANKS for sharing your detailed working out.
      I only approached to say mobitz1 av 2 degree. With some blocked P but not all...