ECG Blog #534 — What is or is not Conducting?

I was sent this ECG with the question, "What is or is not conducting?" Unfortunately — no clinical information was available. 

QUESTIONS:

• What is the rhythm?
• There are multiple interesting ECG findings on this tracing with regard both to the rhythm, as well as to the 12-lead ECG. How many of these findings can you identify?
• Do you need to draw a laddergram in order to interpret this tracing?
• Is there complete AV block?

Figure-1: Today's ECG that was sent to me. (To improve visualization — I've digitized the original ECG using PMcardio).

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NOTE: This is an extremely challenging ECG. Regardless of how far you got with your interpretation — there is much to learn for providers of all experience levels.

• Confession: My initial impression for the rhythm was wrong.
• PEARL #1: I can figure out 90-95% of complex rhythms within seconds without the need to draw a laddergram. That said — it's important to appreciate that there will always be some rhythms for which even arrhythmia specialists may not be able to determine a precise etiology without aid of a laddergram. Today's case is one of those rhythms.
• That said — You do not need a laddergram in today's case in order to make a time-efficient diagnosis of the essentials needed for appropriate initial management. As a result — I divide my discussion into 2 Parts: i) Detailed discussion of multiple interesting findings in today's ECG (including my proposed laddergram for the etiology of the rhythm); — and, ii) The steps I used to expedite time-efficient assessment sufficient for appropriate clinical decision-making.

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Part 1: Details of the many Interesting Findings:

As always, I like to start with assessment of the rhythm — for which I favor the Ps, Q's, 3R Approach for optimal time-efficient rhythm interpretation (See ECG Blog #185 — for review of the Ps,Qs,3Rs).

• NOTE: It does not matter in what sequence we assess the Ps, Qs and 3Rs. As a result — I do not always look first for P waves. Instead — I often start with whichever of the 5 KEY parameters is easiest to assess.
• Focusing on the long lead II rhythm strip at the bottom of the tracing — We should be able to appreciate that the rhythm is not Regular. But because of the relatively small difference between R-R intervals — it could be easy to mistake this tracing for a regular rhythm. Instead, there is a regular irregularity to the rhythm ( = group beating in the form of alternating longer-then-shorter R-R intervals).

PEARL #2: Use of calipers is essential for interpretation of complex rhythms such as this one. The simple fact is that with minimal practice — Use of calipers greatly speeds up and increases the accuracy of your interpretation.

• In Figure-1 — Longer R-R intervals ( = R-R intervals between beats #1-2; 3-4; 5-6; and 7-8) — alternate with shorter R-R intervals ( = R-R intervals between beats #2-3; 4-5; and 6-7).
• PEARL #3: Practically speaking — this finding of alternating longer-then-shorter R-R intervals is too consistent in Figure-1 to be due to chance. This means there is "group" beating — which should always suggest the possibility of some form of Wenckebach conduction (ie, There are other causes of group beating not due to Wenckebach, such as atrial bigeminy with either blocked or conducted PACs. That said — it is helpful clinically to always consider Wenckebach conduction whenever you realize that there is a repetitive pattern of beats).

Continuing with the Ps, Qs and 3Rs ... 

• The QRS in Figure-1 is intermittently wide. Depending on which lead(s) you used to assess QRS width — it could be EASY to overlook the fact that some QRS complexes are wide, while others are not wide.
• PEARL #4: 12 leads are better than one! Appreciation that some QRS complexes are wide while others are not is best seen in lead V1 — in which beat #5 (which corresponds to the 1st beat seen in lead V1) is wide with  the appearance of RBBB conduction. On the other hand — beat #6 ( = the 2nd beat in lead V1) is narrow! (See Figure-2).
• Armed with the knowledge that beat #5 in Figure-2 is wide, but beat #6 is not wide — We can see that in the long lead II rhythm strip, a terminal S wave is present at the end of every-other-QRS complex (ie, a terminal wide S wave is seen at the end of the QRS of each odd-numbered beat = beats #1,3,5 and 7).
• PEARL #5: The fact that the QRS of each of the even-numbered beats is narrow — suggests that the longer preceding R-R interval before beats #2,4,6,8 allowed enough additional time for recovery of right bundle branch conduction (ie, that the reason for intermittent QRS widening is the result of some form of rate-related RBBB block).

Figure-2: I've added BLUE arrows to highlight that every-other beat is wide (as per the wide terminal S wave in beats #1,3,5,7).

P waves are present!

I've highlighted with RED arrows in Figure-3 that an underlying regular atrial rhythm is present.

• You may or may not have initially seen P waves in all of the places where I've added RED arrows — because some of the P waves are partially hidden within the end of the QRS or within peaked T waves.
• PEARL #6: Use of calipers allows us to very quickly verify where all of the P waves lie. For example — we definitely see P waves under the 3rd and 4th RED arrows — and if we set our calipers to the P-P interval between these 3rd and 4th red arrows, we can "walk out" where the partially hidden P waves lie throughout the rest of the tracing.
• PEARL #7: I find the simple steps of numbering the beats and labeling the P waves (with arrows) — tremendously facilitates the next step in our assessment of the rhythm, which is to determine if P waves are Related to neighboring QRS complexes?
• NOTE (Beyond-the-Core): If you carefully measured all P-P intervals in Figure-3 — You may have noted slight variation in the atrial rate. Technically, this is the result of a slight ventriculophasic sinus arrhythmia — which is a common phenomenon in 2nd- and 3rd-degree AV blocks. That said, for practical purposes — We can say that the underlying atrial rhythm is essentially regular.

Figure-3: I've added RED arrows to highlight the regular atrial rhythm.

Are P Waves Related to Neighboring QRS Complexes?

The KEY step for determining if some form of AV block is present — is to determine if at least some P waves are Related to neighboring QRS complex?

• To do this — I survey the entire rhythm strip, looking to see if any PR intervals repeat? Once again — calipers greatly facilitate (and expedite) this step, since calipers enable us to very quickly tell if PR intervals are or are not varying in duration.
• In Figure-4 — I've added PINK arrows to highlight identical (albeit prolonged) PR intervals that are seen in front of beats #1,3,5,7.
• PEARL #8: The fact that at least some of the PR intervals in today's tracing repeat tells us that at least some beats are being conducted to the ventricles!

Figure-4: Pink arrows highlight P waves with identical (albeit prolonged) PR intervals that repetitively occur throughout the long lead II rhythm strip (ie, before beats #1,3,5,7).

Are Any More P Waves Conducting?

At this point — we know from Figure-4 that beats #1,3,5,7 are all being conducted with a long PR interval.

• As we continue to look at the PR intervals preceding the remaining beats — it should be apparent that the shorter PR intervals highlighted by RED arrows in Figure-5 are also all identical (ie, all equal to 0.16 second). 
• This tells us that each of these RED arrow P waves are also conducting to the ventricles.

Figure-5: RED arrows highlight P waves identical PR intervals

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Part 2: Putting It All Together ...

I've synthesized in Figure-6 what we've worked out thus far regarding today's ECG.

• Today's rhythm manifests an overall slow ventricular rhythm that is not regular. Instead — there is group beating in the form of a regularly irregular rhythm with alternating shorter-then-longer R-R intervals.
• There is an underlying regular atrial rhythm ( = the colored arrows in Figure-6).
• Many of the P waves in Figure-6 are conducting — albeit with 2 different PR intervals (highlighted by RED and PINK arrow P waves).
• But there clearly are more P waves than QRS complexes — with the result that lots of P waves are not conducting (ie, the YELLOW arrow P waves in Figure-6 are not conducting).
• In addition, there are 4 places in the long lead rhythm strip where consecutive P waves are not conducting (ie, neither of the consecutive YELLOW arrow P waves that are seen between beats #1-2; 3-4; 5-6; and 7-8 are conducting).

Impression:

• Some form of AV block is present — because we have a regular atrial rhythm, but not all of the on-time P waves are being conducted.
• We can immediately recognize that the rhythm is not complete AV block because: i) The ventricular rhythm is clearly not regular (whereas with complete AV block — there is usually a regular [or at least, almost regular] ventricular escape rhythm); — and, ii) The fact that the RED and PINK arrow P waves are conducting means that "complete" AV block can not be present because there is some conduction.
• Since AV block is not "complete" — the rhythm must represent some form of 2nd-degree AV block. And since there are consecutive on-time P waves that fail to conduct — today's rhythm represents a high-grade form of 2nd-degree AV block.

PEARL #9: Despite the length of my discussion up to this point — the clinically important conclusion (ie, that today's rhythm represents some form of high-grade 2nd-degree AV block with resultant bradycardia) — can be reached quickly, simply by noting the following:

• The ventricular rhythm is slow and irregular in the form of group beating.
• Regular P waves are present.
• Some PR intervals repeat — which means that at least some P waves are conducted. But many "on time" P waves fail to conduct, including several instances of consecutive "on time" P waves that fail to conduct.
• Therefore — some form of high-grade 2nd-degree AV block is present. Especially in view of the bradycardia — pacing may be needed if the rhythm persists. In the meantime — We need to find out the history. In addition — We need to carefully review the rest of the ECG to see if recent or ongoing ischemia/infarction is a likely cause of this AV block?

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What Type of 2nd-Degree AV Block is Present?

I've reviewed the 2nd-degree AV blocks in many posts on this ECG Blog (See ECG Blog #62 and ECG Blog #465 — with Video Review of these concepts in the Addendum of Blog #465). In brief — the 2nd-degree AV blocks are divided into the Mobitz I and Mobitz II forms. Reasons why today's AV block is almost certain to represent some form of Mobitz I ( = AV Wenckebach) include the following:

• i) Mobitz I is much more common than Mobitz II; 
• ii) As was seen in Figure-2 — the QRS complex of conducting beats is intermittently narrow — whereas the QRS is almost always consistently wide with the more severe Mobitz II form of 2nd-degree AV block; 
• iii) There is group beating, and as noted earlier in PEARL #3 — the presence of group beating in association with a regular atrial rhythm in which some beats are conducted but others are not — is often the result of AV Wenckebach ( = Mobitz I).
• iv) Mobitz I is very commonly associated with inferior and/or posterior MI — so another point in support of Mobitz I would be IF the 12-lead ECG was suggestive of recent or ongoing inferior and/or posterior MI. (In contrast — Mobitz II is more often associated with anterior MI).

Take another LOOK at today's 12-lead ECG in Figure-6.

• Is there evidence of recent or ongoing infarction?

Figure-6: Today's ECG. Is there evidence of recent or ongoing infarction?

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Is there Evidence of Recent Infarction?

As we established earlier — both the PINK and RED arrow P waves in the long lead II rhythm strip of Figure-6 are being conducted to the ventricles, albeit with different PR intervals.

• This means that beat #5 in lead V1 of Figure-6 is being conducted to the ventricles with RBBB (Right Bundle Branch Block) — whereas the longer preceding R-R interval before beat #6 allows enough extra time for recovery of normal conduction.
• Although some ST-T wave depression will normally be seen in anterior leads when there is RBBB conduction — the amount of ST-T wave depression in lead V2 (if not also in leads V1 and V3) appears to be disproportionately increased. This suggests recent posterior OMI as a likely cause of the Mobitz I conduction disturbance.

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My Proposed Laddergram for Today's Rhythm:

As I alluded to in my introduction to today's case — today's rhythm was complex enough such that I needed to draw a laddergram in order to precisely determine the etiology of this conduction disturbance.

• To emphasize (as stated in PEARL #9) — a laddergram is not needed to quickly derive the clinically important conclusion that bradycardia with high-grade Mobitz I 2nd-degree AV block is present, most probably as a result of recent posterior MI.
• That said — I find laddergram illustration of complex arrhythmia mechanisms insightful and helpful in understanding the physiologic process.
• For readers with an interest in learning how to draw laddergrams — I review my approach with over 100 illustrated cases in ECG Blog #188.
• To emphasize that although it takes time and practice to become comfortable drawing laddergrams — with minimal instruction, it becomes EASY to understand even complex laddergrams that are already drawn for you. This should become evident with explanation of my proposed laddergram for today's case.

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Therefore — I conclude today's case with laddergram derivation of the group beating pattern that we recognize in today's tracing.

Figure-7: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial Tier. Here — GREEN arrows show the onset of P waves as my reference point for drawing in atrial activity. Because conduction through the atria is generally rapid — I draw near-vertical lines in the Atrial Tier to represent this.

Figure-8: Laddergram STEP-2. I next fill in the Ventricular Tier. BLUE 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-7 and -8). Now the challenge begins — for trying to "solve" the laddergram by figuring out which of the P waves in the Atrial Tier are being conducted to the ventricles.

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NOTE (Beyond-the-Core): Because consecutive non-conducted P waves are seen in several places in today's tracing — I suspected that there may be dual-level AV block within the AV Node (I explain and illustrate this advanced concept in ECG Blog #463). It is for this reason that I divide the AV Nodal Tier into 2 parts (by drawing in a horizontal BLACK dotted line in Figure-9).

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Figure-9: Laddergram STEP-3. Whle realizing that many complex tracings may have more than a single potential laddergram explanation — I begin my derivation of conduction through the AV Nodal Tier by drawing in the path of those P waves that I know are conducting. In today's case — this will consist of the RED and PINK arrow P waves.

Figure-10: As was shown in Figure-5 — the constant normal PR interval (of ~0.16 second) suggests that RED arrow P waves in today's tracing are normally conducted to the ventricles. As a result — these RED arrow P waves are the first P waves that I connect to neighboring QRS complexes within the Ventricular Tier (ie, to beats #2,4,6,8).

Figure-11: As was shown in Figure-4 — the constant but much longer PR interval of PINK arrow P waves suggests that beats #1,3,5,7 are also conducted to the ventricles — but much more slowly than conduction of the RED arrow P waves. I illustrate this by an increase in angulation as these impulses pass through both parts in the AV Nodal Tier (BLUE lines). As is often the case — conduction of impulses through the dual parts of the AV Nodal Tier is slower through the lower part.

Figure-12: As we look at the laddergram in Figure-11 — We see that each of the 8 conducted beats in today's tracing has already been assigned to a P wave that conducts to the ventricles. This tells us that none of the remaining P waves (ie, none of the YELLOW arrow P waves in Figure-12) make it through the AV Nodal Tier to the ventricles. Therefore — the remaining STEP in developing my proposed laddergram is to postulate a logical mechanism by which conduction might be halted within the AV Nodal Tier.
= = = = = = =
It seems logical to postulate that the single YELLOW arrow P waves that occur within the shorter R-R intervals (ie, between beats #2-3; 4-5; 6-7) do not make it through the upper AV Nodal level (BLUE butt-ends that I've drawn in this upper AV Nodal level).

 

Figure-13: As we look at the laddergram in Figure-12 — We see that a repetitive pattern remains for the 2 unattached YELLOW arrow P waves within each of the longer R-R intervals. So — If I can figure out a logical mechanism by which conduction might be halted within one of these longer R-R intervals — the same mechanism will probably apply to the 3 other longer R-R intervals.
= = = = = = =
NOTE: Sometimes I simply need to "try out" several possibilities by "trial and error" — until I find one that works. I illustrate this process in Figure-13 — in which I postulate 3:2 AV conduction through the upper AV Nodal level — and then 2:1 AV conduction through the lower AV Nodal level (BLUE lines within the AV Nodal Tier).

Figure-14: Since the BLUE lines that I drew within the AV Nodal Tier in Figure-13 appear to be the most logical solution for the path of conduction for the 2 YELLOW arrow P waves within beats #1-2 — I repeat this sequence as the likely conduction path occurring within the remaining 3 longer R-R intervals.

Figure-15: My completed laddergram. Today's rhythm is most consistent with high-grade 2nd-degree AV block, with dual-level Wenckebach conduction out of the AV Node. 

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Final Thoughts:

As I speculate in my explanation of Figure-6 — I suspect recent posterior OMI as the cause of today's conduction disturbance.

• Alternatively — it's possible that the disproportionately increased anterior lead T wave inversion in Figure-6 might represent an LAD "culprit" in a patient with multi-vessel disease, now with some evidence of spontaneous reperfusion. 
• In either case, prompt cath to define the anatomy, with PCI reperfusion if a "culprit" artery is identified — would seem the preferred management approach. 
• The "good news" is that if PCI is successful in reperfusing a "culprit" artery — then the high-grade AV block with resultant bradycardia (as well as the intermittent rate-related RBBB) might both resolve. 

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Acknowledgment: My appreciation to Omar Hassan Seddik (from Mansoura City, Egypt) for submission of today's case with these tracings.

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