ECG Blog #395 — No Simple Explanation ...

The tracing in Figure-1 was sent to me for my interpretation. As per the title of today's Blog post — There is no simple explanation for all that is happening in this challenging tracing. Are YOU up for the challenge?

• I found that the most challenging part of today's tracing is determining the etiology of beat "X". As I explain below — I think it unlikely that beat "X" represents a single, slightly accelerated junctional beat.
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• Can YOU come up with a more likely explanation? 

Figure-1: How would YOU interpret this challenging tracing? 

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NOTE:  Although determining the etiology of beat "X" involves a number of advanced concepts — today's case also reviews KEY points in the assessment of any arrhythmia — so, regardless of your level of experience interpreting arrhythmias — I hope you'll stick with me!

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HOW to Begin?

Regular followers of my ECG Blog know I routinely emphasize a series of user-friendly points to facilitate the interpretation of any rhythm. These include the following:

• IF there is more than a single feature in a given arrhythmia (ie, When the tracing is not a simple sinus rhythm) — then LOOK to see if there is an underlying rhythm?
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• IF there are easier elements of the rhythm to interpret, as well as more difficult elements — Start with the EASIER elements (and save the more difficult elements for last). You'll often be pleasantly surprised that explanation of the more difficult elements becomes evident once you've interpreted some of the less difficult elements.
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• Number the beats. This way you and your colleagues can be certain you are all talking about the same part(s) of the tracing. (This also saves YOU time — because you have a numbered reference point to quickly locate that part of the tracing you were looking at).
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• Label the obvious P waves.

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Take another LOOK at today's tracing in Figure-2 — in which I have numbered the beats and labeled the obvious sinus P waves with RED arrows.

• Is there an underlying rhythm in Figure-2?
• What is the "easiest" part of today's tracing to interpret?

Figure-2: I have numbered the beats and labeled (with RED arrows) the obvious sinus P waves. 

The Underlying Rhythm:

The simple step of labeling those P waves that are obvious in Figure-2 — allows us to quickly recognize that the underlying rhythm in today's tracing is sinus.

• That the underlying rhythm is sinus — is established by the RED arrows in Figure-2 that appear before beats #1,2; 4,5; 8; 10,11; and 13 — with each of these P waves manifesting a similar shape and the same normal PR interval in front of the QRS complex that follows each of these P waves.
• The 1 RED-arrow P wave in Figure-2 that is not followed by a QRS complex — is the P wave distinguished by the extra little "hump" that peaks through the ST segment of beat #7 (ie, This is the 5th RED arrow in Figure-2). 
• We know that this extra "hump" in the ST segment of beat #7 is a sinus P wave — because the distance from this little "hump" until the next sinus P wave — is virtually the same as the distance between the P waves that precede beats #1-2; beats #4-5; and beats #10-11.

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Helpful Reminder: Using calipers greatly facilitates (and speeds up) the interpretation of any complex arrhythmia! In particular — use of calipers will make it much easier to answer this next Question:

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QUESTION:

• As you take another LOOK at Figure-2 — Do you see any indication of additional P waves?

ANSWER:

• In Figure-3 — I've added BLUE arrows to today's rhythm, to show how using calipers facilitates visualization of an underlying regular atrial rhythm (ie, on-time P-wave-induced deflections that deform the ST-T waves of beats #3,6,9 and 12 are seen under each of the BLUE arrows).

Figure-3: I've added BLUE arrows to today's rhythm — to show that the underlying atrial rhythm continues throughout the entire rhythm.

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NOTE: The P-P interval between each of the colored arrows is not precisely regular. As is often the case — there may be slight sinus arrhythmia. That said — it should be apparent that the "theme" of the P waves in Figure-3 is that of an underlying almost regular sinus rhythm.

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What About the Wide Beats in Figure-3?

At this point — it's worthwhile considering those parts of today's tracing that we've established.

• The underlying rhythm in Figure-3 is sinus (ie, Beats #1,2; 4,5; 8; 10,11; and 13 are all clearly sinus-conducted).
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• Beats #3,6,9 and 12 are wide, very different in morphology from sinus-conducted beats — and not preceded by P waves. This defines beats #3,6,9 and 12 as PVCs (Premature Ventricular Contractions) — until proven otherwise.
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• PEARL #1: PVCs may or may not conduct retrograde back to the atria. When PVCs do conduct retrograde — they will depolarize the atria, thereby resetting the SA node. As a result — there will be a delay before the next sinus P wave appears.
• In contrast, when PVCs do not conduct retrograde — the next sinus P wave will usually occur on time! This is precisely what we see with the BLUE arrows in Figure-3 — namely that on-time sinus P waves continue throughout the entire tracing!
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• PEARL #2: The fact the early-occurring wide beats in Figure-3 do not reset the SA node proves that beats #3,6,9 and 12 are PVCs! This is because if these early beats were PACs or PJCs conducted with aberrancy — these premature supraventricular beats would have reset the SA node!

Putting It All Together:

We have explained almost everything in today's rhythm — except beat #7, which is a narrow QRS complex that is not preceded by any P wave.

• To Emphasize: IF this is as far as you got — namely, that the rhythm in today's tracing is sinus with frequent PVCs — this clearly would be sufficient for appropriate clinical management of this patient.
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• BUT — If you are intellectually curious as to what I feel is the most likely explanation for beat #7 — Read on!

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The CHALLENGE in Today's Rhythm:

What follows is the advanced part of today's arrhythmia. For clarity in Figure-4 — I've labeled each of the P waves in this tracing.

• At first glance — the absence of any P wave in front of beat #7 suggests that this narrow QRS complex (with identical QRS morphology as is seen for other sinus-conducted beats) would be a junctional beat.
• However, if beat #7 represented a slightly accelerated junctional escape focus — I would have expected beats #4, 10 and 13 (which are preceded by longer R-R intervals) — to also be junctional escape beats instead of sinus-conducted.

Figure-4: I've labeled each of the P waves in today's rhythm.

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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 8 Figures illustrate my thought process as I derived this laddergram. (See ECG Blog #188 for review on how to read and/or draw Laddergrams).
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• To EMPHASIZE — This laddergram was challenging to draw, as I explain below. That said — my hope is that even readers with limited experience with laddergrams will be able to follow my rationale for deriving the most plausible explanation for beat #7.

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Figure-5: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (as per the RED lines drawn directly below the onset of each of the P waves — as shown here 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 similar, albeit not quite equal (ie,There is slight sinus arrhythmia).

Figure-6: 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-7: It's time to begin "solving" what we can in the laddergram. I start by connecting those sinus P waves in the Atrial Tier that I know are conducting to narrow QRS complexes in the Ventricular Tier (slanted BLUE lines that I've drawn connecting P waves a,b; d,e; h; j,k; and m to beats #1,2; 4,5; 8; 10,11; and 13 in the Ventricular Tier).

Figure-8: The next element in today's tracing that I am certain about — is that beats #3,6,9 and 12 are PVCs (since these beats are wide — very different in morphology from sinus-conducted beats — and not preceded by P waves). I've drawn these PVCs in BLUE, as originating from the ventricles — and conducting back toward the atria.

Figure-9: How far back into the AV Nodal Tier these 4 PVCs conduct is uncertain — but given that RED-arrow sinus P waves continue at a fairly regular rate throughout the entire rhythm strip — we can surmise that retrograde activity from these PVCs does not reach the Atrial Tier (the dotted BLUE lines suggesting retrograde conduction from these PVCs stops within the AV Nodal Tier). NOTE: I added a question mark ( ? ) within the AV Nodal Tier to indicate my uncertainty about how far down P wave "g" is able to conduct.

Figure-10: We know that P waves "c", "i" and "l" do not conduct down to the ventricles (because P waves "d", "j" and "m" are all clearly sinus-conducted). As a result — I added in BLUE butt-ends to show P waves c,i,l are all stopped within the AV Nodal Tier.
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KEY Point: I also added a BLUE butt-end for P wave "g". Since P wave "h" is clearly sinus-conducted — P wave "g" can not possibly be conducting, since there is no QRS complex after P wave "g" that could be conducted. For this reason — I added a question mark ( ? ) under P wave "f ", since this is the only P wave left to be accounted for.

Figure-11: Since P wave "f " is the only unaccounted for P wave — and beat #7 is the only unaccounted for QRS complex — P wave "f " must be conducting to produce beat #7. This could occur (as per the slanted BLUE line connecting P wave f with beat #7) — IF retrograde conduction from PVC #6 delayed forward conduction of "f".

Figure-12: The logical completion of today's laddergram is that passage of P wave "f" through the AV Nodal Tier blocks forward conduction of P wave "g" (as per the dotted BLUE line).
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KEY Question: Why is P wave "f" able to make it through the AV Nodal Tier — when P waves "c", "i" and "l" are blocked by retrograde conduction from PVCs #3,9 and 12?
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ANSWER: The fact that there is an underlying sinus arrhythmia affects the point within the Refractory Period that P waves c,f,i and l occur. Note that the longest P-P interval of 880 msec. occurs between P waves "e" and "f " — which allows a little bit more time for recovery of conduction properties, which is why P wave "f" is able to conduct to produce beat #7, albeit with a very long PR interval.

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NOTE: Learning to draw Laddergrams is challenging. I fully acknowledge that it took me many years until I felt comfortable with this skill. That said — learning to read laddergrams is not difficult! Hopefully, even readers with minimal experience in this area were able to understand my reasoning in design of the laddergram for today's complex arrhythmia.

• I review the basics for reading and drawing laddergrams in my ECG Blog #188. I also add links to more than 90 step-by-step laddergrams that I have added over the years to my ECG Blog — so that anyone wanting practice reading and/or drawing laddergrams has a wealth of material to practice with.
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• As always — I welcome questions and/or comments on today's ECG Blog post! THANK YOU for your interest! 

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Acknowledgment: My appreciation to Mayan Kain (from Tel Aviv, Israel) for the case and this tracing. 

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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.
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• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 90 laddergram cases — many with step-by-step sequential illustration).