ECG Blog #397 — An Unusually Long Cycle?

You are asked to interpret the ECG in Figure-1.

• What is the rhythm in ECG #1?

Figure-1: You are asked to interpret this tracing. What is the rhythm?

MY Thoughts on the ECG in Figure-1:

I routinely begin assessment of each 12-lead ECG I encounter — with interpretation of the rhythm. To do this — I apply the Ps, Qs, 3R Approach (See ECG Blog #185 — for review of my system).

• The long lead rhythm strip at the bottom of the 12-lead ECG in Figure-1 — shows the rhythm to be Regular at a Rate of ~90/minute — with the exception of a slight pause at the end of the tracing.
• The QRS complex is narrow in all 12 leads of this tracing — which tells us that the rhythm is supraventricular.
• P waves are present. 
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• The 5th Parameter in the Ps,Qs,3R Approach is the 3rd "R" — which is to assess whether the P waves that are present are Related to "neighboring" QRS complexes. At first glance — Doesn’t it look as if the PR interval is constant (albeit with a slightly prolonged PR interval) — for most of this tracing?

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. I do this in Figure-2.

• 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. 

Take another LOOK at today’s tracing in Figure-2.

• Do the RED arrows facilitate assessment of today’s Rhythm?

Figure-2: I've numbered the beats — and have highlighted P waves with RED arrows.

What is the Cause of the Pause in the Rhythm?

The simple step of labeling all of the obvious P waves — is surprisingly helpful in evaluating the rhythm.

• Isn’t it now more obvious that P waves remain regular throughout the entire long lead rhythm strip?
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• KEY Point: When most beats in an arrhythmia are regular, but some are not — the best CLUE to interpretation often resides in looking for a “break” in the rhythm (such as is seen between beats #13 and 14 in Figure-2).

PEARL #2:  The most common cause of a pause in an otherwise regular rhythm — is a blocked PAC (See ECG Blog #66 and ECG Blog #147). To emphasize — blocked PACs are a much more common cause of one or more relative pauses in a rhythm than AV block!

• That said — the reason we know that the cause of the pause in today’s rhythm is not a blocked PAC — is that the atrial rhythm remains regular throughout the entire tracing — which means that there is no premature P wave at the onset of the short pause between beats #13-to-14 (RED arrows in Figure-2).

This raises the question as to whether the rhythm disturbance in Figure-2 could be the result of AV Wenckebach? (with "AV Wenckebach" being a synonym for 2nd-degree AV block, Mobitz Type I).

• We typically think of AV Wenckebach as that form of 2nd-degree AV block in which the PR interval progressively lengthens until a beat is dropped — after which another cycle begins, with shortening of the PR interval (See the Video Pearl and extra links below in the ADDENDUM for more on ECG diagnosis of the AV Blocks).
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• The above said — the majority of Mobitz I, 2nd-degree AV blocks consist of groups with a limited number of beats — until a P wave is non-conducted. As a result — it will almost always be possible to see progressive lengthening of the PR interval from one beat-to-the-next. However — We do not see progressive PR interval lengthening from one beat-to-the-next in Figure-2!

So — Is Today’s Rhythm a Type of AV Wenckebach?

On occasion — Mobitz I may present as a long Wenckebach cycle. Although this is not common — it does occur. The BEST way to recognize a long Wenckebach cycle when it does occur — is to look for at least of couple of the “Footprints of Wenckebach”. As discussed in the ECG Video in today’s Addendum — these “Footprints” include:

• A regular (or at least almost regular) atrial rhythm (ie, Slight irregularity may be seen if there is an underlying sinus arrhythmia).
• The presence of Group beating.
• An increasing PR interval until a beat is dropped — with shortening of the PR interval as the next group begins.
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• The pause containing the dropped beat is less than twice the shortest R-R interval (which typically occurs because the greatest increase in PR interval — typically occurs between the 1st and 2nd beats in any group).

Additional Associated Features of AV Wenckebach:

• AV Wenckebach ( = Mobitz I) — is commonly seen in association with acute inferior MI.
• Because Mobitz I generally occurs at a higher level in the conduction system than Mobitz II — the QRS is usually narrow (whereas with Mobitz II — the QRS is usually wide).
• It is unlikely for a patient to go back-and-forth between Mobitz I and Mobitz II — so IF you see clear evidence elsewhere on the tracing that the patient has Mobitz I — then the 2:1 block you are assessing is probably also Mobitz I.
• Mobitz I is much more common than Mobitz II (In my experience more than 95% of all 2nd-degree AV blocks that I have seen are Mobitz I, and not Mobitz II).
• Prognosis tends to be much better with Mobitz I than with Mobitz II — and those cases of Mobitz I arising in association with acute inferior MI often spontaneously resolve. But the reason to be alert to recognizing those rare cases of Mobitz II — is that a pacemaker is usually needed for Mobitz II.

CASE Conclusion: Laddergram Illustration

Because of minimal (negligible) change in the PR interval from one-beat-to-the-next in today's tracing — it's impossible to appreciate gradual PR interval prolongation.

• The Laddergram that I have drawn in Figure-3 reveals the solution — with the KEY being to focus on the short pause at the end of the rhythm strip (ie, between beats #13-and-14).
• As highlighted by the BLUE arrow — the PR interval at the end of the 13-beat run — is clearly longer than the PR interval that begins the next cycle (the RED arrow before beat #14).
• GO BACK to the beginning of this tracing. Note that the 1st beat in this long Wenckebach cycle ( = beat #1) — is preceded by a PR interval that is clearly shorter than that seen before beats #2-thru-13.
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• CONCLUSION: The rhythm in today's case is 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach) — with a very long Wenckebach cycle. The on-time P wave highlighted by the YELLOW arrow is non-conducted — after which the next group begins with shortening of the PR interval that we see before beat #14.
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• Final confirmation that the rhythm in Figure-3 is indeed a long AV Wenckebach cycle — is forthcoming from the finding of regular P waves (ie, the YELLOW arrow non-conducted P wave is right on time!) — and from the last "Footprint of Wenckebach" listed above = The pause containing the dropped beat is less than twice the shortest R-R interval.
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• P.S.: Returning back to Figure-1 for assessment of the rest of the 12-lead ECG — there is a leftward axis (but still more positive in lead II — so not leftward enough to qualify as LAHB) — there is LAA (deep negative component to the P wave in lead V1) — delayed transition, with persistence of S waves through to lead V6 — and nonspecific ST-T wave abnormalities that do not appear to be acute.

Figure-3: Laddergram illustration of today's rhythm — showing that today's rhythm is a long Wenckebach cycle.

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Acknowledgment: My appreciation to Sam Ghali  (from Jacksonville, Florida — @EM_RESUS) 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 — Reviews the Ps, Qs, 3R Approach to Rhythm Interpretation.
• 
  
• 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).
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• ECG Blog #192 — The 3 Causes of AV Dissociation.
• ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.
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• ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".

 

ADDENDUM (9/30/2023):

This 15-minute ECG Video (Media PEARL #52) — Reviews the 3 Types of 2nd-Degree AV Block — plus — the hard-to-define term of "high-grade" AV block. I supplement this material with the following 2 PDF handouts.

• Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks (This is a free download).
• Section 20 (54 pages = the "long" Answer) from my ACLS-2013-Arrhythmias Expanded Version provides detailed discussion of WHAT the AV Blocks are — and what they are not! (This is a free download).

• ECG Media Pearl #4 (4:30 minutes Audio) — The AV Blocks & When to Suspect Mobitz I  — See ECG Blog #186 —

ECG Blog #396 — Why the Flat Line?

The ECG in Figure-1 — was obtained from a middle-aged man with palpitations and shortness of breath. He was hemodynamically stable at the time this tracing was recorded.

• How would YOU interpret the ECG in Figure-1?
• Is there evidence of a recent or ongoing acute MI?
• What might you do first? 

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

PEARL #1:  Since today’s patient is hemodynamically stable — there is at least a “moment of time” for you to better contemplate the situation before having to initiate management decisions.

• KEY Point: Although true that patients with longstanding, severe pulmonary disease may manifest a QRST complex in standard lead I with marked overall reduction in QRST amplitude (See ECG Blog #65 — regarding Schamroth’s Sign) — you should never normally see a completely flat line in any of the standard limb leads. IF ever you do see a completely flat line in lead I, lead II or lead III — it is almost certain that there is some type of lead misplacement. Repeat the ECG!

PEARL #2: YES — I did see what looks like a large acute or reperfused inferior MI (QS complexes and deep T wave inversion in leads II,III,aVF) — but since this patient is stable and some type of lead reversal is almost certain (See PEARL #1) — I would repeat the ECG after verifying electrode lead placement to see whatever ECG changes might be real.

• Note also that the P wave is negative in standard lead II in Figure-1 (as well as in leads III and aVF, which are the other 2 inferior leads). It is well to remember that IF the P wave in lead II is negative — then by definition — You do not have sinus rhythm (unless there is lead misplacement or dextrocardia).
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• Again, since today’s patient is hemodynamically stable — there is no urgency. Verify lead placement. — Then repeat the ECG. 

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The CASE Continues:

The treating clinician immediately recognized the likelihood of some type of lead misplacement. As a result — he immediately repeated the ECG after verifying lead placement.

• How would you interpret the repeat ECG in Figure-2?
• What KIND of lead reversal was present in ECG #1?

Figure-2: The repeat ECG in today's case — recorded as soon as the treating clinician recognized the lead misplacement. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Interpretation of ECG #2:

The repeat ECG in Figure-2 — appears to be technically appropriate. The rhythm is sinus bradycardia at a rate just over 50/minute. All intervals (PR,QRS,QTc) are normal. The frontal plane axis is normal at +70 degrees.

• Although difficult to measure (because of marked overlap of the QRS in multiple chest leads) — there appears to be greatly increased QRS amplitude, consistent with voltage for LVH.
• R wave progression is probably normal (again — difficult to assess given marked QRS overlap in multiple chest leads). That said — the R wave is unexpectedly tall already by lead V2 (~15 mm in height).
• T waves are prominently peaked in multiple leads — with 1-2 mm of gradually upsloping ST elevation in leads V1-thru-V4. There is no reciprocal ST depression.
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• KEY Point: Once all electrode leads were properly placed for ECG #2 — the negative inferior lead P waves disappeared (and the normal upright P waves in lead II of a sinus rhythm were then seen) — and the inferior lead QS waves with deep T wave inversion disappeared.
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• Impression: In this middle-aged man with palpitations and dyspnea, but no chest pain — I suspect that the T wave peaking and slight, upward-sloping chest lead ST elevation represent a repolarization variant. Especially given the unexpectedly tall R wave by lead V2, with greatly increased and overlapping chest lead QRS amplitudes — I did not think any of these findings represented an acute process. In view of the increased QRS amplitudes — I'd favor an Echo.
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• Follow-Up: Nothing acute was found on further evaluation.

What Happens with Lead Reversal?

My favorite on-line “Quick GO-TO” reference for the most common types of lead misplacement comes from LITFL ( = Life-In-The-Fast-Lane). I have used the superb web page they post in their web site on this subject for years. It’s EASY to find — Simply put in, “LITFL Lead Reversal” in the Search bar — and the link comes up instantly.

• This LITFL web page describes the 7 most common lead reversals. There are other possibilities (ie, in which there may be misplacement of multiple leads) — but these are less common and more difficult to predict.
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• By far (!) — the most common lead reversal is mix-up of the LA (Left Arm) and RA (Right Arm) electrodes. This lead reversal is usually EASY to spot — because it typically produces global negativity of the P wave, QRS and T wave in lead I — which is something that is virtually never normally seen (See ECG Blog #264 — for an example of LA-RA lead reversal).
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• In contrast — it is EASY to overlook what is probably the 2nd-most frequent form of lead misplacement, which is LA-LL reversal. This is because the ECG picture seen with LA-LL reversal does not immediately stand out as physiologically “off” (See Figure-3 — in which I summarize KEY points that help me most in recognizing lead reversals — with LINKS in my References below to over 15 Blog post examples of various forms of lead misplacement).

Figure-3: Tips for recognizing lead reversal (See text).

What about the Type of Lead Reversal in Today's Case?

The technical misadventure in today's case is unique from other Blog post examples I have published — in that there is bilateral arm-leg reversal (LA-LL plus RA-RL — with summarizing features in Figure-4).

• Note how each of the abnormal findings in ECG #1 — are corrected in ECG #2 after proper electrode lead placement.
• Note again that the KEY to facilitating immediate recognition of this technical misadventure — is seeing the flat line in lead I.

Figure-4: Comparison between limb lead appearance in ECG #1 with ECG #2 (These distinguishing features summarized from the LITFL web site that I referred to above).

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Acknowledgment: My appreciation to H.S. Cho = 조현석 (from Seoul, South Korea) 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 #264 — Review of limb lead reversal (ie, LA-RA lead reversal in a patient with an acute MI).
• ECG Blog #375 — LA-LL Lead Reversal.

OTHER Examples of Lead Reversal (from Dr. Smith's ECG Blog):

Technical errors featuring a variety of lead reversal placements remain a surprisingly common “mishap” of everyday practice. As a result — it's important to familiarize ourselves with how best to recognize the various forms of these "misadventures". For review — Check out My Comment — at the bottom of the page in the following posts on Dr. Smith's ECG Blog:

• The June 4, 2018 post (LA-LL reversal).
• The July 29, 2018 post (LA-RA reversal).
• The November 4, 2018 post (Leads V1,V2 misplacement).
• The February 11, 2020 post (LA-RA reversal).
• The March 18, 2020 post (LA-RA reversal).
• The July 28, 2020 post (RA-LL reversal).
• The August 28, 2020 post (LA-LL reversal).
• The November 19, 2020 post (LA-LL reversal).
• The November 27, 2021 post (LA-RA reversal).
• The April 17, 2022 post (Leads V1,V2 misplacement).
• The May 5, 2022 post (LA-RA reversal).
• The May 24, 2022 post (LA-LL reversal).
• The May 26, 2022 post (LA-LL reversal).
• The August 17, 2022 post (LA-RA reversal).
• The May 13, 2023 post (LA-RA reversal).
• The May 30, 2023 post (LA-RA reversal).

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.
—  —  —  — 
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?
—  —  —  —
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.
• 
  
• 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).