How would YOU interpret the rhythm in Figure-1 — if you were sent this tracing without the benefit of any history?
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| Figure-1: How would YOU interpret this rhythm? |
The "Quick" Answer:
My initial thoughts on the rhythm in Figure-1:
- The rhythm in Figure-1 is not completely regular. That said — the R-R interval for the first 5 beats looks regular. The R-R interval for the last few beats is shorter than this — but also regular. Thus, there is "group beating", in that there are 2 parts ( = 2 groups of beats) in today's tracing (ie, beats #1-thru-5 — and then beats #6-thru-8).
- The QRS complex does not look wide in this single lead rhythm strip. That said — since we only see a single lead, we have no idea if the QRS might look wider if viewed from the perspective of a different lead.
- P waves are present in Figure-1. There appears to be more P waves than QRS complexes — which means that not all P waves are conducting.
- On the other hand — IF we look in front of each of the 8 QRS complexes in this tracing — each of these 8 beats is preceded by a P wave, with what appears to be a constant PR interval. Therefore, each of these 8 beats is conducted. That said, since the P waves occurring near the mid-point of the R-R interval for the first 5 beats are not conducted — some form of AV block is present.
- PEARL #1: Of the 3 degrees of AV block — the fact that at least some P waves are conducted tells us that the rhythm is not complete AV block. And, since this is not a simple 1st-degree AV block — it must be some form of 2nd-degree AV block ( = the "Quick" Answer).
Additional "Quick" Thoughts:
There are many reasons why today's tracing is challenging to interpret. These include the following:
- From the long lead II rhythm strip that we see in Figure-1 — We do not know if the QRS complex is wide or narrow. Although it does not look as if the QRS in this rhythm strip exceeds 0.10 second in duration — it is possible that a part of the QRS may lie on the baseline, thereby leading to a false impression that the QRS is narrow when in fact it is wide. The wide terminal S wave that we see in Figure-1 suggests there may be RBBB (Right Bundle Branch Block) — although we cannot tell for certain without a 12-lead tracing.
- The T waves in this long lead II rhythm strip look to be extra "peaked". This peaking is so pronounced — that it looks like it would still be present regardless of whether additional P waves might be hidden within these T waves. These T waves are also preceded by a surprisingly flat ST segment — which raises 2 possibilities: i) That the patient may have hyperkalemia (and possibly also hypocalcemia given the flat ST segments preceding these peaked T waves); and, ii) There may be recent and/or ongoing infarction — as these may represent either hyperacute T waves or reperfusion T waves that are commonly seen after acute MI.
- The reason determining if either hyperkalemia or recent infarction have occurred is important — is that optimal management of AV block may depend on the answer (ie, Correction of hyperkalemia or reperfusion of acute coronary occlusion may be essential components of initial treatment).
- PEARL #3: A brief relevant History — is an important component for optimal management of complex rhythm disorders. A brief relevant history may also provide an important clue to etiology (ie, AV Wenckebach = Mobitz I 2nd-degree AV block — most commonly occurs in association with recent or acute inferior MI — which is why I'd want to know IF the ST-T wave appearance in this lead II rhythm strip is the result of inferior MI).
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QUESTION: Are P Waves Regular?
Most of the time with AV block — the atrial rhythm will be regular (or at least almost regular — if there is an underlying sinus arrhythmia).
- The EASIEST way to determine if the underlying atrial rhythm is regular — is to find 2 P wave deflections in a row — and then to see IF you can "walk out" this P-P interval throughout the entire rhythm strip.
- RED arrows in Figure-2 show the 2 P waves I've selected.
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| Figure-2: RED arrows highlight 2 P wave deflections in a row. |
ANSWER: P waves are Regular!
The FASTEST way to establish that the underlying atrial rhythm in Figure-2 is regular — is by use of calipers.
- RED arrows in Figure-3 highlight these regular P waves. I was able to easily "walk out" a regular P-P interval throughout this rhythm strip.
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| Figure-3: The atrial rhythm is regular. |
PEARL #4: The combination of group beating — with an underlying regular atrial rhythm in which some (but not all) P waves are conducting — should immediately suggest the possibility of AV Wenckebach (especially IF the 12-lead ECG suggests recent or acute inferior and/or infero-postero infarction).
- That said — the fact the more than a single RED arrow P wave in a row is not conducted in Figure-3 — indicates a more complex mechanism than simple Mobitz I.
PEARL #5: As noted above — today's arrhythmia appears to represent some form of 2nd-degree AV block. It's important to appreciate that we can not rule out the possibility that this rhythm represents Mobitz II 2nd-degree AV block, in which case the patient will need a pacemaker (See ECG Blog #236 — for review on the ECG diagnosis of the 2nd-degree AV Blocks).
- KEY criteria for Mobitz II are: i) That we see at least 2 consecutive on-time but non-conducted P waves; and, ii) That the PR interval remains constant in front of all conducted beats. Both of these criteria are fulfilled in Figure-3.
- Most (albeit not all) of the time — the QRS complex will be wide with Mobitz II (because the level of this AV block originates below the AV Node). And although the QRS complex does not look wide in Figure-3 — we are only looking at a single lead, and it is possible that a part of the QRS lies on the baseline (ie, giving the false impression that the QRS is narrow when in fact it is wide).
- Mobitz I is much more common than Mobitz II. In my experience — well over 95% of all the 2nd-degree AV blocks I have seen over my decades of studying this phenomena have been Mobitz I. Clinically, this is important — because pacing is much less likely to be needed with Mobitz I. In contrast — prompt pacing is essential for Mobitz II, because of the disturbing tendency for this conduction defect to suddenly progress to non-conduction of multiple consecutive beats (and even ventricular standstill).
- Seeing the 12-lead ECG on this patient would be insightful! In addition to settling the issue regarding QRS width — it should provide insight as to whether we are dealing with hyperkalemia (diffuse T wave peaking in multiple leads?) — inferior MI (which would favor Mobitz I) — or anterior MI (which would favor Mobitz II).
- Reviewing recent telemetry monitoring could provide the answer! It is because we do not see consecutively conducted QRS complexes in Figure-3 — that we are unable to determine if the PR interval is increasing before failed conduction. It is extremely unlikely for a patient to switch back-and-forth between Mobitz I and Mobitz II. Therefore — IF ongoing telemetry monitoring shows periods during which clear Mobitz I is present (ie, with progressive increase in the PR interval until a beat is dropped) — then it becomes very likely that all monitoring on this patient represents some variation of AV Wenckebach.
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The LADDERGRAM:
At this point I needed a laddergram to help me work out my theory for what might represent a plausible mechanism other than Mobitz II for today's complex arrhythmia (Figure-4).
- NOTE: I fully acknowledge that more than a single solution may be plausible for today's arrhythmia (ie, As per PEARL #5 — I can not rule out the possibility that today's arrhythmia might represent Mobitz II 2nd-degree AV block).
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| Figure-4: I needed to draw a laddergram . . . |
KEY Point: As I've commented in previous ECG Blog posts — Wenckebach conduction may occur at more than a single level within the AV Node.
- When this happens — complex conduction relationships may be seen, including non-conduction of consecutive on-time P waves (See ECG Blog #259 and ECG Blog #416 — for more on Dual-Level AV Wenckebach — which is what I suspected in today's rhythm).
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STEP-by-STEP . . .
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| Figure-5: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial Tier. BLUE 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 drew in near-vertical lines in the Atrial Tier. |
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NOTE: Because I suspected dual-level AV block within the AV Node — from this point forth (beginning with Figure-7) — I divided the AV Nodal Tier into 2 parts (by drawing in a horizontal BLACK dotted line).
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| Figure-8: BLUE lines reflect conduction of every-other-P-wave through the lower level within the AV Node, until finally reaching the ventricles. |
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| Figure-9: Since we know that the P waves that occur just before beats #5-thru-8 are all conducting (because they all manifest the same PR interval) — the P wave before beat #1 must also be conducting, because this beat is preceded by the same PR interval! |
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| Figure-10: Similarly — the P waves before beats #2,3 and 4 must also be conducting — because each of these beats is preceded by the same PR interval! This leaves us with having to postulate the path through the conduction system for the 2 unaccounted-for RED arrow P waves. |
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| Figure-11: Looking first at the R-R interval between beats #1-to-2 — I thought the most logical theory is to postulate that the 1st atrial impulse makes it through the upper but not the lower AV Nodal level. The 2nd atrial impulse makes it through the Atrial Tier, but not through any part of the AV Node. |
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| Figure-12: Once again — the "symmetry" in the relative intervals for each of the atrial impulses in the remaining 3 groups (BLUE lines within the AV Nodal Tier) — suggests a similar conduction pattern to the one I drew in Figure-11 for the atrial impulses between beats #1 and 2. |
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| Figure-13: My completed laddergram. I explain the consecutive non-conducted P waves as the result of 2nd-degree AV block — in which there is dual-level block within the AV Node for the first 5 beats (ie, with 3:2 Wenckebach conduction through the upper AV Nodal level — and 2:1 conduction through the lower AV Nodal level). In contrast — there is simple 2:1 AV block for the last few beats. Since it would be unlikely to switch back-and-forth between the Mobitz I and Mobitz II forms of 2nd-degree AV block — I thought it most likely that the conduction defect in today's case reflects 2nd-degree AV block, with AV Wenckebach occurring at 2 levels within the AV Node. |
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FINAL Points Regarding Today's Rhythm:
- P waves are upright in lead II — and the atrial rate is ~100/minute. As a result, rather than atrial tachycardia — I suspect we are seeing a slightly increased sinus rate with the above described 2nd-degree dual-level AV block.
- As to the question of whether today's rhythm does or does not represent a "high-grade" form of AV block — I find the answer semantic, in that: i) The block could be considered "high-grade" — because more than 1 on-time P wave in a row is not conducted; — BUT — ii) The block occurs at 2 levels out of the AV Node — at each of these levels being a Mobitz I type block (with Mobitz I generally not considered as a "high-grade" conduction disturbance).
- That said — the overall ventricular rate in today's rhythm is slow (dropping down to the 40s). As a result, if significant bradycardia persists (especially if accompanied by symptoms) — then pacing may be needed.
- The "good news" — is that the cause of this bradycardia and conduction block may be the result of a "fixable" condition that if identified and successfully treated, might restore a normal rate and rhythm (ie, If the peaked T waves were due to hyperkalemia — or if there could be successful reperfusion of an acute infarction).
- Successful management of today's patient will depend on determining the cause of this rhythm.
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Acknowledgment: My appreciation to Sam Collis (from England) 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 my System for Rhythm Interpretation, using the Ps, Qs, 3R Approach.
- 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 #192 — The 3 Causes of AV Dissociation.
- ECG Blog #191 — Reviews the difference between AV Dissociation vs Complete AV Block.
- ECG Blog #389 — ECG Blog #373 — and ECG Blog #344 — for review of some cases that illustrate "AV block problem-solving".
- ECG Blog #251 — Reviews the concepts of Wenckebach periodicity and the "Footprints" of Wenckebach.
- ECG Blog #164 — Reviews a case of typical Mobitz I 2nd-Degree AV Block (with detailed discussion of the "Footprints" of Wenckebach).
- ECG Blog #63 — Mobitz I, 2nd-Degree AV Block with Junctional Escape Beats.
- 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 #259 — Reviews step-by-step laddergram for a patient with Dual-Level AV Block.
- ECG Blog #243 — Reviews a case of AFlutter with Dual-Level Wenckebach out of the AV Node.
- ECG Blog #226 — Works through a complex Case Study (including an 11:00 minute ECG Video Pearl that walks you through step-by-step in the construction of a laddergram with Wenckebach conduction and dual-level block within the AV node).
- ECG Blog #416 — Case Study of acute OMI with step-by-step illustration of dual-level AV Wenckebach.
- ECG Blog #405 — ECG Video presentation that reviews the distinction between AV Dissociation vs Complete (3rd-degree) AV Block (For a LINKED Contents to this ECG Video — Click on MORE in the Description under the video on YouTube).
ADDENDUM (7/18/2024):
ECG Media PEARL #71 (5:45 minutes Audio) — Reviews the phenomenon of Dual-Level Wenckebach out of the AV Node (HOW to recognize this phenomenon — and how to distinguish it from Mobitz II).
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