ECG Blog #449 — Isorhythmic AV Dissociation?

The ECG in Figure-1 — was obtained from a 45-year old man with diabetes, who was being treated for septic shock.

QUESTIONS:

• How would YOU interpret the ECG in Figure-1?
• What is the rhythm?
• Why does QRS morphology in the long lead II rhythm strip change every-other-beat?

Figure-1: The initial ECG in today's case.

MY Thoughts on the ECG in Figure-1:

This is a challenging ECG to interpret because of: i) the changing QRS morphology; and, ii) the difficulty in seeing all of the P waves. That said — I was able to determine the rhythm within seconds because of the following observations:

• All beats on this ECG are supraventricular! The "beauty" of having a 12-lead tracing with a simultaneously-recorded long lead II rhythm strip — is that this allows us to view each of the 12 beats on this tracing in 3 other simultaneously-recorded leads. Doing so confirms that even though there are 2 distinct QRS morphologies in the long lead II rhythm strip (especially obvious in lead III) — the QRS is narrow in all 12 leads. Therefore — the rhythm is supraventricular!
• There is group beating! (seen in Figure-1 in the form of a bigeminal rhythm, in which the same shorter — then the same longer R-R interval alternates throughout the tracing).
• Regular P waves are present throughout the entire tracing. This KEY observation can be verified within seconds — simply by using calipers (See Figure-2).

Figure-2: I've added RED arrows to the initial ECG — showing that regular P waves are present throughout the entire tracing.

PEARL #1: The clinician who does not use calipers to interpret complex arrhythmias like today's tracing — will invariably take more time for their interpretation, only to discover that they will never be certain about the regularity of atrial activity.

• To optimally assess atrial activity — I simply set my calipers to the P-P interval between any 2 consecutive P waves that I can clearly identify. For example, in Figure-2 — 2 consecutive P wave deflections are clearly seen to occur at the same place within each of the longer R-R intervals (so that I chose to set my calipers to the P-P interval between the 3rd and 4th RED arrows in Figure-2).
• Once I set my calipers to this P-P interval — I was able to easily "walk out" regular P waves throughout the entire long lead II rhythm strip.
• PEARL #2: The reason today's tracing is so challenging — is that every 3rd P wave is so well hidden within the T waves of every odd-numbered beat. This is where awareness of simultaneously-recorded leads may prove invaluable! Although we do not see every 3rd P wave in the long lead II rhythm strip (because these P waves are hidden within the T waves of beats #1,3,5,7,9 and 11) — we do see an "on-time" tiny negative deflection right after the 1st T wave in lead V1 (2nd RED arrow in lead V1). This confirms that P waves are hidden at a similar point in the T wave of all odd-numbered beats. Therefore — there is an underlying regular atrial rhythm, with sinus P waves at ~110/minute.

PEARL #3: Today's rhythm does not represent complete AV block (and it does not represent isorhythmic AV dissociation). We can quickly determine this by focusing our attention on the PR interval just before the QRS complex of each beat that ends each of the longer R-R intervals (ie, the PR interval before beats #3,5,7,9 and 11).

• Labeling P waves (as we have done with RED arrows in Figure-2) — greatly facilitates this process — and expedites us being able to tell that although the PR interval before beats #1,3,5,7,9 and 11 is slightly prolonged (ie, ~0.22 second) — this PR interval in front of all odd-numbered beats remains constant! Therefore — there clearly is at least some conduction.
• PEARL #4: Despite the finding of a regular atrial rhythm — there are more P waves than QRS complexes (ie, there are 18 RED arrow P waves in Figure-2 — but only 12 QRS complexes). This means that some form of 2nd-degree AV block must be present — since not all P waves are being conducted to the ventricles.

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PEARL #5: At this point in my interpretation — I was virtually certain that today's rhythm represented some form of AV Wenckebach (ie, 2nd-degree AV block of the Mobitz I Type) — because of the information the above-described 5 Observations told me:

• Observation-1: The QRS complex for all beats in all 12 leads of Figure-2 is narrow. Therefore — today's rhythm is supraventricular.
• Observation-2: There is group beating (in the form of alternating longer-then-shorter R-R intervals). The presence of "group beating" should always raise the possibility of Wenckebach conduction — IF certain other features are also present.
• Observation-3: The underlying atrial rhythm is regular (RED arrows in Figure-2). Knowing there is a regular sinus rhythm rules out the non-Wenckebach causes of group beating, such as atrial bigeminy.
• Observation-4: At least some beats are being conducted to the ventricles (because the PR interval is constant before all of the beats that end longer R-R intervals). As per PEARL #3 — since at least some P waves are being conducted to the ventricles — this rules out 3rd-degree (complete) AV block!
• Observation-5: There are more P waves than QRS complexes. This means that some of the regularly-occurring sinus P waves are not being conducted (which means some form of 2nd-degree AV block must be present).

MY Impression of ECG #1: The above 5 Observations confer ECG features that characterize "the Footprints of Wenckebach!" — which is why within seconds of seeing today's ECG, I was virtually certain there was some form of 2nd-degree AV Wenckebach (See ECG Blog #164 — ECG Blog #55 — ECG Blog #347 — and ECG Blog #154).

• That said — I had not yet demonstrated cycles with progressive increase in the PR interval until an on-time sinus P wave is dropped (as should be seen with typical AV Wenckebach).

For Practical Purposes: I would be happy IF you stopped at this point, knowing that the above 5 observations make it virtually certain that today's rhythm represents 2nd-degree AV block of the Mobitz I Type ( = AV Wenckebach).

• Beyond-the-Core: Take a LOOK at Figure-3 — in which I have used 3 colors to label a certain P wave pattern in this tracing that repeats itself over the 12 beats in the long lead II rhythm strip. What does each color signify?

Figure-3: I've used 3 colors to label all P waves in today's rhythm. What does each color signify?

The Colors in Figure-3:

It's easiest to break down what is happening in Figure-3 — by starting with the RED arrow P waves.

• As noted above in Observation-4 — the RED arrow P waves in Figure-3 all manifest the same 0.22 second PR interval. Therefore — beats #1,3,5,7,9 and 11 are all conducted with 1st-degree AV block.
• IF the rhythm in Figure-3 is AV Wenckebach — then either the PINK or the YELLOW arrow P waves must not be conducted. Doesn't it seem more logical for the YELLOW arrow P waves to be non-conducted? (which would mean that the PINK arrow P waves would be conducting beats #2,4,6,8,10 and 12 with a very long PR interval of ~0.38 second).

PEARL #6: The last 2 features to explain regarding today's ECG are: i) Why QRS morphology changes slightly with every-other-beat; and, ii) What the rest of today's 12-lead ECG shows.

• Since we know that all beats in today's tracing are supraventricular (Observation-1 in PEARL #5) — and since all QRS complexes are conducted — the reason for slight change in QRS morphology every-other-beat must be the result of some aberrant conduction. As explained in ECG Blog #211 — whether a beat does or does not conduct with some aberration depends on the interplay between coupling intervals and the preceding R-R interval. Although I do not see a specific form of conduction delay in the slightly wider QRS complexes (which are the odd-numbered beats) — the differing R-R intervals most probably accounts for the aberrant conduction.
• Mobitz I 2nd-degree AV block is commonly seen in association with acute or recent inferior and/or posterior infarction. That said — I thought ST-T wave appearance in the rest of the 12-lead ECG showed nonspecific (nondiagnostic) abnormalities. I did not see evidence on this tracing for recent or acute MI (but it is always important to look for ECG signs of recent inferior and/or posterior MI whenever you encounter Mobitz I 2nd-degree AV block).

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Laddergram Illustration:

The BEST way to prove that today's rhythm is 2nd-degree AV block of the Mobitz I (AV Wenckebach) Type — is to construct a laddergram, which I illustrate step-by-step in Figures-4 thru -10.

Figure-4: I find the easiest 1st step in drawing a laddergram is to complete the Atrial Tier, that shows atrial activity.

Figure-5: I next fill in the Ventricular Tier — which corresponds to the timing of those ventricular beats I am sure about. Since we know in today's tracing that all 12 beats are supraventricular — I drew the arrow for each of these QRS complexes in the Ventricular Tier facing downward (representing normal conduction of supraventricular impulses through the ventricles).

Figure-6: We are now ready to begin solving the laddergram. I do this by connecting those P waves from the Atrial Tier — to those QRS complexes that I am certain each of these P waves is conducting to (BLUE lines that I've drawn within the AV Nodal Tier).

Figure-7: It seems logical that the next P waves to conduct to the ventricles are those that are highlighted by BLUE arrows in this Figure-7. It should now be easy to see that conduction of this 2nd P wave in each group takes a bit longer to be conducted, which is the principal characteristic of Wenckebach conduction (BLUE lines that I've drawn within the AV Nodal Tier).

Figure-8: By the process of elimination — this means that the remaining BLUE arrow P wave is not conducted (ie, there are no unconnected QRS complexes left). This therefore "completes" the laddergram — by the "butt end" that I've added to the remaining BLUE arrow P waves.

Figure-9: For clarity — I now labeled each of the P waves in the long lead II rhythm strip with the same colors that I used above in Figure-3.

Figure-10: Most of the time when I draw a laddergram — I use the same color for all lines in the illustration, as shown here.

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Acknowledgment: My appreciation to Vansh Verma (from New Delhi, India) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #185 — My Ps, Qs, 3R System for Rhythm.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 100 laddergram cases — many with step-by-step sequential illustration).

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.

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• ECG Blog #192 — The 3 Causes of AV Dissociation. 
• ECG Blog #191 — AV Dissociation vs Complete AV Block.
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• ECG Blog #389 — ECG Blog #373 — for review of some cases that illustrate "AV block problem-solving".
• ECG Blog #236 — for an ECG Video Pearl on the 3 Types of 2nd-degree AV block.
• ECG Blog #344 — thoroughly reviews the Types of 2nd-degree AV block (Mobitz I vs Mobitz II vs 2:1 AV Block).
• 
  
• ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.
• ECG Blog #164 — Step-by-Step laddergram of Mobitz I.
• 
  
• ECG Blog #195 — reviews Isorhythmic AV Dissociation.

 

 

ADDENDUM (10/2/2024) — Based on Question by Akash

For those readers who enjoy the challenge of drawing laddergrams — I've decided to publish the Question sent to me from Akash (in the Comments below).

• As I've often emphasized — there may on occasion be more than a single possible explanation for the mechanism of a given complex arrhythmia!
• IF you are able to "draw" your theory for a potentially plausible mechanism — then consider yourself correct in proposing an alternative mechanism (in which case — the only way to verify which proposed mechanism is correct for the case at hand would be by EP study).
• The reason I thought it worthwhile to publish the excellent question that Akash asks — is that it illustrates the problem-solving process for complex rhythms, in which I have to "play" with a few potentially plausible laddergram solutions until I am able to come up with one that works.

This is the question by Akash (that I copied from the Comments below):

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Hello Dr. Grauer!
Long time lurker (and learner) here, posting for the first time.

• Could this be a 2:1 AV block?
• The first P wave (labelled with a RED arrow in Figure 3) is conducted (albeit with a slightly prolonged PR interval).
• The next P wave (the one hidden within the T wave = PINK arrow) is not conducted.
• The QRS complex labelled beat #2 is a junctional escape beat.
• The third P wave (YELLOW arrow) fails to conduct — because it finds the distal conduction system refractory (because of the junctional escape beat).

Is this a reasonable explanation of this rhythm? I’m sure I’m missing something, but am unsure what that is. Please throw light, Dr. Grauer!
Warm regards — Akash

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MY Reply to Akash:

Your question Akash is an excellent one — and I actually thought of the solution you propose when first working through this case.

• The problem however — is that if the 2nd P wave (first YELLOW arrow in Figure-11) was non-conducted because of 2:1 AV block and if beat #2 was a junctional escape beat — this would mean that you are proposing an accelerated junctional escape rate (because the R-R interval before beat #2 that you are proposing is junctional = 3.7 large boxes — which corresponds to an accelerated junctional escape rate of ~81/minute).
• And — this would mean that unless inhibited by a sinus-conducted beat — the next junctional escape beat would occur 3.7 large boxes later = where I placed the BLUE circle. But since the next sinus P wave is set to occur where the 3rd RED arrow occurs — we can see that this next junctional beat would prevent the 3rd sinus P wave from conducting .... (You'd have to propose "takeover" of the rhythm by an accelerated junctional rhythm — which I would not expect given the constant and reasonable PR interval before beats #1,3,5,7,9,11).
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• P.S.: Accelerated junctional rhythms and junctional tachycardia can occur — but they are relatively uncommon in adults unless there is some underlying cause (ie, ischemia, shock, electrolyte disorders, post-cardiac surgery, etc.).

BOTTOM Line: I thought it unlikely that there was 2:1 block with a "usurping" accelerated junctional rhythm. I also thought the laddergram that I derived in Figure-10 looked perfectly plausible for 2nd-degree AV Block, Mobitz Type I ( = AV Wenckebach).

Figure-11: Unsuccessful proposed laddergram (See above).