Thursday, July 18, 2024

ECG Blog #439 — How Many Possibilities?

How would YOU interpret the rhythm in Figure-1 — if you were sent this tracing without the benefit of any history? 

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.

PEARL #2: No definite conclusions can be reached regarding ST-T wave appearance from a single-lead rhythm strip. For this — a 12-lead tracing is needed
  • 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)


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.

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.

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 #236for 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).

The above said — I suspect today's rhythm is not Mobitz II, but instead represents a more complex form of Wenckebach conduction.
  • 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.


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).

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).

STEP-by-STEP . . .

Figure-5: Laddergram STEP-1. It is usually easiest to begin a laddergram by filling in the Atrial TierBLUE 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.

Figure-6: 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.

Note that I drew in near-vertical lines for each of the 8 QRS beats in the Ventricular Tier. (If the 12-lead ECG were to show bundle branch block with a wide QRS complex — I would then draw more angled lines in the ventricular tier to reflect slower conduction as impulses passed through the ventricles).

 The "EASY part" for constructing most laddergrams consists of these first 2 STEPS (
that are shown in Figure-5 and Figure-6)
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.

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).

Figure-7: Of the 8 beats in today's tracing — I decided to save the first 4 beats for last, since non-conduction of consecutive P waves portended a more complex mechanism. In contrast — there appeared to be a straight-forward 2:1 block for beats #5-thru-8. I've drawn in BLUE lines reflect this 2:1 conduction through the upper level within the AV Node.

Figure-8: BLUE lines reflect conduction of every-other-P-wave through the lower level within the AV Node, until finally reaching the ventricles.

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!

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.

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.

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.

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.


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.
  • 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.


Acknowledgment: My appreciation to Sam Collis (from England) for the case and this tracing.




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 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).


Wednesday, July 10, 2024

ECG Blog #438 — A Paramedic Diagnosis ...

The ECG in Figure-1 was obtained from a previously healthy middle-aged woman — who was awakened by new-onset CP (Chest Pain) that began ~3 hours before this ECG was obtained. She was stable hemodynamically — but still with CP at the time ECG #1 was recorded.
  • The patient reports 3 shorter episodes of CP the day before.

  • Given this history — How would YOU interpret this ECG?
  • Should the cath lab be activated?

Figure-1: The initial ECG in today's case (See text).

MY Thoughts on Today’s ECG:
In view of the above history — the initial ECG in today's case is clearly of concern. The rhythm is sinus at 70-75/minute — with normal intervals (PR,QRS, QTc) — normal axis — and no chamber enlargement.
  • My "eye" was immediately attracted to leads V2 and V3 (within the RED rectangle in Figure-2). There is early transition in these chest leads (with surprisingly tall R waves in leads V1,V2 — and the R becoming taller than the S wave is deep by lead V3).
  • The ST segment in leads V2 and V3 is inappropriately straightened (as often emphasized in this ECG Blog — there should normally be slight, gently upsloping ST elevation in these leads).
  • Continuing wth neighboring leads V4, V5 and V6 — the T waves in these leads look hyperacute (BLUE arrows) — in that their ST segment takeoff is straightened, with these T waves being larger-than-expected in size with a widened base.
  • PEARL #1: By itself — the appearance of the T wave in lead V4 does not necessarily look abnormal. However, in the context of the clearly abnormal ST-T wave findings in leads V2,V3 — and the more acute-looking appearance of the T waves in leads V5 and V6 — I interpreted the T wave in lead V4 as representing the beginning of this hyperacute change.

MY Impression of ECG #1:
 Given the history of new-onset CP in this middle-aged woman — this initial prehospital ECG in Figure-2 suggests posterior OMI beginning and/or ongoing over the past day that this patient has had symptoms.
  • Given her new episode of CP that awakened her from sleep just 3 hours earlier (and which is still ongoing)the cath lab should be activated! 

  • PEARL #2: Admittedly — the ECG findings in this initial ECG are subtle. But abnormalities are present in each of the chest leads in this patient with new CP. In this clinical context — this ECG is diagnostic of acute posterior OMI. 
  • It should be appreciated that the reason for the lack of clear ST depression in leads V2,V3 — and the lack of ST elevation in leads V4,V5,V6 — may be the ongoing clinical course in this patient, that includes multiple CP episodes over the past day (ie, There could be spontaneous opening and reclosing of the "culprit" vessel over this period of time). It may be that the ECG in Figure-2 represents some pseudo-normalization of what might have be more prominent ST-T wave changes on an earlier ECG.

  • PEARL #3: In addition to posterior involvement — the hyperacute T waves in the lateral chest leads suggest there is acute postero-lateral OMI, which in the absence of obvious inferior lead changes — points to the LCx (Left Circumflex) as the most likely "culprit" artery.

  • PEARL #4: The unexpectedly tall R waves in leads V1,V2 lend support to concern that there may be completed posterior infarction (See ECG Blog #354) — as the mirror-image reflection of deepening Q waves is increasing amplitude of the R wave in anterior leads. Unexpectedly tall anterior R waves is especially likely to signal completed posterior infarction IF this finding is new compared to prior ECGs.

Figure-2: I've labeled KEY findings in the initial ECG.


The CASE Continues:
Serial ECGs were obtained by the prehospital team. Figure-3 offers a comparison between one of these repeat ECGs — and the original prehospital ECG (with ~30 minutes separating the time between the recording of these 2 tracings).
  • WHAT do you learn from the repeat ECG in Figure-3?

Figure-3: Comparison of the initial ECG — with the repeat ECG done in the field ~30 minutes after the initial ECG.


What We Learn from the Repeat ECG:
IF there was any doubt after seeing ECG #1 about the need for prompt cath lab activation — that doubt should be removed by the "dynamic" ST-T wave changes that are evident in virtually all leads in the repeat ECG.
  • PEARL #5: The BEST way to compare one serial ECG with another — is to put both tracings side-by-side — and to then go lead-by-lead, always comparing one lead area to the corresponding lead area in the 2nd ECG.
  • Instead — it is all-to-common for clinicians to ignore this important procedural point. Instead, the tendency is to try to "gain time" by looking at 1 tracing in its entirety — and then the 2nd tracing in its entirety without taking the time to directly compare each lead area with the corresponding lead area in the 2nd tracing. The reality is that unless you compare the 2 tracings by going lead-by-lead — it is all-too-easy to overlook subtle but important findings.
  • Along the way — it is also important to ensure that our lead-by-lead comparison is "comparing apples with apples, and not with oranges". By this I mean — that we want to ensure that any changes in ST-T wave morphology are not the result of a significant frontal plane axis shift or due to a change in chest lead electrode placement.

Applying this Comparison Technique
in Figure-4:
Although there is a slight frontal plane axis shift (from about +40 degrees in ECG #1 — to +60 degrees in ECG #2) — this small change is unlikely to alter our assessment of comparative limb lead ST-T wave appearance. Chest lead QRS morphology appearance is essentially the same in both tracings. Therefore — any ST-T wave changes that we note between these 2 ECGs are likely to be "real".
  • In the Limb leads of ECG #2 — there now is subtle-but-real ST elevation in each of the inferior leads (as seen by respect to the dotted RED line baseline in each of these leads)
  • ST-T waves in these inferior leads have a more acute appearance than they did in ECG #1 (ie, there is more straightening of the elevated ST segment takeoff) — and we may be seeing the beginning of small inferior lead Q waves.
  • The BLUE arrow in lead aVL — highlights ST segment straightening that was not present in ECG #1. I interpreted this as a reciprocal change to the new inferior lead ST elevation.
In Chest Leads:
  • BLUE arrows in leads V2 and V3 — highlight an increase in ST depression.
  • In leads V5 and V6 — there should be no doubt that there is a more acute appearance with straightening of the ST segment takeoff and increased ST elevation (with respect to the dotted RED lines in these leads).

MY Impression of ECG #2: In this patient with ongoing CP — the repeat ECG in Figure-4 confirms "dynamic" ST-T wave changes. 
  • Prompt cath is indicated! — in this symptomatic patient with acute ongoing infero-postero-lateral OMI.

Figure-4: I've labeled findings in the repeat ECG.

CASE Follow-Up:
  • Serial troponin values were markedly elevated.
  • Cardiac cath revealed normal left main and LAD branches — and, a small but non-obstructed RCA. The main trunk of the LCx (Left Circumflex) showed a 50% lesion in the middle of this vessel — with severe narrowing proximally in the 1st Obtuse Marginal branch of the LCx, which was felt to be the "culprit" artery. Angioplasty was successfully performed.

Acknowledgment: My appreciation to Konstantin Тихонов (from Moscow, Russia) for the case and this tracing.

Related ECG Blog Posts to Today’s Case:

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

  • ECG Blog #193 — Reviews the basics for predicting the "culprit" artery (as well as reviewing why the term "STEMI" — should be replaced by "OMI" = an acute coronary Occlusion MI).
  • CLICK HERE  for my new ECG Videos (on Rhythm interpretation — 12-lead interpretation with Case Studies for ECG diagnosis of acute OMI).
  • CLICK HERE  for my new ECG Podcasts (on ECG & Rhythm interpretation Errors — and — Errors in assessing for acute OMI).
    • Recognizing hyperacute T waves — patterns of leads — an OMI (though not a STEMI) — See My Comment at the bottom of the page in the November 8, 2020 post on Dr. Smith's ECG Blog.
    • Recognizing ECG signs of Precordial Swirl (from acute OMI of LAD Septal Perforators— See My Comment at the bottom of the page in the March 22, 2024 post on Dr. Smith's ECG Blog. 

    • ECG Blog #294 — Reviews how to tell IF the "culprit" artery has reperfused.
    • ECG Blog #230 — Reviews how to compare serial ECGs.
    • ECG Blog #115 — Shows how dramatic ST-T changes can occur in as short as an 8-minute period.
    • ECG Blog #268 — Shows an example of reperfusion T waves.
    • ECG Blog #400 — Reviews the concept of "dynamic" ST-T wave changes.

    • ECG Blog #337 — A "NSTEMI" that was really an ongoing OMI of uncertain duration (presenting with inferior lead reperfusion T waves).

    Thursday, July 4, 2024

    ECG Blog #437 — A 2-Part Answer ...

    I was sent the ECG in Figure-1 — but without the benefit of any history. 

    • How would YOU interpret this tracing?
    •   WHY do I say there is, "A 2-Part Answer?"

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

    MY Thoughts on Today's CASE: 
    As important as providing a brief, relevant history is for optimal clinical ECG interpretation — Cases like the one today often prove even more educational, because we are not given any history (and therefore need to deduce the most likely clinical setting to explain the ECG in front of us).
    • As per the title I selected — there is a 2-Part Answer to today's post, which entails: i) Determining the cardiac rhythm; and, ii) In light of this rhythm — interpreting how this 12-lead tracing is highly suggestive of the cause of this rhythm!

    Take Another LOOK at the ECG in Figure-1.
    • WHAT is the rhythm?
    • HOW does knowing what this rhythm is — help in determining the cause of this rhythm?

    Let's presume that the patient in today's case is hemodynamically stable. I favor starting with the long lead II rhythm strip — by use of the Ps, Qs, 3R Approach (See ECG Blog #185 for more on the Ps, Qs, 3Rs).
    • The ventricular rhythm in Figure-1 looks Regular — with the exception of slight irregularity seen at the end of the tracing (most notably for the R-R interval between beats #13-to-14, which is clearly longer than all other R-R intervals).
    • The overall ventricular Rate is ~100/minute, as the R-R interval for all but the last 2 beats is ~3 large boxes in duration (and 300 ÷ 3 = 100/minute).
    • The QRS complex is narrow in all 12 leads. Therefore — the rhythm is supraventricular.
    • P waves are present!

    PEARL #1: The simple steps of numbering beats and labeling those P waves that we definitely see — greatly facilitates our recognition of what is going on.
    • Starting with beat #2 in Figure-2 — RED arrows highlight each of the P waves that we definitely see. 

    • Do YOU Think that the atrial rhythm in Figure-2 remains regular throughout the entire rhythm strip?

    •   HINT: Do you see any indication that an additional on-time P wave may be occurring between beat #12 and beat #14?

    Figure-2: I have added RED arrows for P waves I definitely see.

    • PEARL #2: When the underlying atrial rhythm is regular (with possible exception of 1 or 2 P waves that are not clearly seen) — the chances are excellent that those 1 or 2 "missing" P waves may be present (and may be occurring on-time) — but are simply hidden within a QRS complex or ST-T wave. 
    • Using calipers facilitates the search for any potentially "missing" P waves that might be in hiding — because you know where to look

    Consider the following in Figure-3:
    • IF the underlying atrial rhythm in Figure-3 was to be regular — Wouldn't we expect to see another "on-time" P wave at the point marked by the YELLOW arrow?
    • Doesn't knowing where to look in Figure-3 — help us to identify that there is an extra deflection (just under the YELLOW arrow — at the very end of the QRS complex of beat #13?).

    Figure-3: I've added a YELLOW arrow at the point where I'd expect to find another on-time P wave.

    We now have the elements needed to solve today's arrhythmia:
    • Colored arrows in Figure-4 highlight that there is an underlying regular atrial rhythm.
    • The QRS complex is narrow.
    • The slight pause in the rhythm (that occurs between beats #13-to-14) — ends with a sinus-conducted P wave (ie, the PINK-arrow P wave in Figure-4). This PINK P wave produces sinus-conducted beat #14, which manifests a minimally prolonged PR interval.
    • The next P wave is on-time — and is conducted with 1st-degree AV block (ie, the PURPLE-arrow P wave in front of beat #15 which manifests a PR interval = 0.31 second).
    • Working backward — no QRS follows the YELLOW arrow P wave in Figure-4. This suggests that the RED arrow P wave that precedes beat #13 is being conducted, albeit with a very long 1st-degree AV block (ie, PR interval >0.52 second).
    • This suggests that each of the previous RED arrow P waves in Figure-4 are also conducting with very long PR intervals to produce the next QRS complex in front of them.

    • Conclusion: The rhythm in today's case is 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach).

    PEARL #3: The reason it is so challenging to recognize Mobitz I in today's tracing — is that there is a very long cycle until a beat is dropped.
    • With long Wenckebach cycles — the increment in PR interval from one beat to the next may be minimal and hard to appreciate by visual comparison. In such cases — the easiest way to verify that the PR interval is increasing, is to LOOK at the PR interval just before the pause (ie, the PR interval before beat #13 in Figure-4) and compare it to the PR interval that starts the next cycle (ie, the PR interval from the PINK arrow P wave in Figure-4 — until beat #14).
    • That today's rhythm is Mobitz I, 2nd-degree AV block — is confirmed by the presence of other "Footprints" of Wenckebach, which are: i) Regular atrial rhythm; ii) The pause containing the dropped beat is less than twice the shortest R-R interval; and, iii) Progressive increase in the PR interval, which is easy to see in Figure-4 in the next Wenckebach cycle (ie, The PR interval formed by the PURPLE arrow P wave is clearly longer than that formed by the PINK arrow P wave). For more on the "Footprints" of Wenckebach — See ECG Blog #164.

    Figure-4: The diagnosis of Mobitz I becomes apparent from the colored arrows at the end of this tracing (See text).

    How Does Knowing the Rhythm Help to Interpret the 12-Lead?
    As I often emphasize — 2nd-degree AV block, Mobitz Type I is most commonly seen in association with acute inferior and/or posterior OMI.
    • Knowing that today's rhythm is Mobitz I therefore prompts me to look especially closely for any suggestion of acute inferior and/or posterior OMI.

    In Figure-5 — I've labled the 2 leads in today's 12-lead tracing that immediately "caught my eye". 
    • Lead V3 (within the RED rectangle) — is the most remarkable lead. Normally, there should be a slight amount of gentle upsloping ST elevation in leads V2 and V3. In view of this — there is no way the ST segment straightening seen without any hint of ST elevation in lead V3 can be a "normal" finding.
    • Lead V2 (within the BLUE rectangle) — also manifests an inappropriately straightened ST segment, thereby supporting our impression that the ST-T wave in neighboring lead V3 is not normal.
    • ST-T wave changes in other leads show nonspecific ST-T wave flattening, and are non-diagnostic.

    • To Emphasize: The ST-T wave changes highlighted above are subtle. That said — Given 2nd-degree AV block, Mobitz Type I for the rhythm — the ECG appearance of the 2 leads in Figure-5 (within the colored rectangles) — have to be interpreted as suggestive of acute posterior OMI until proven otherwise!

    Figure-5: I've highlighted the 2 leads that "caught my eye".

    Today's LADDERGRAM:
    For clarity — I've drawn a laddergram in Figure-6.
    • Several of the essentials for Wenckebach periodicity are present in Figure-6. These include: i) Regular atrial rhythm; — ii) The pause that contains the dropped beat is less than twice the shortest R-R interval; — and, iii) Progressive PR interval lengthening is seen in the last 2 beats on the tracing.

    • NOTE: Isn't it virtually impossible to discern any change in the PR interval for the first 12 beats in this tracing?
    • That said, the YELLOW P wave in Figure-6 is not conducted — and — the PR interval just before the pause in this rhythm is clearly longer than the PR interval at the end of the pause. This makes it obvious that today's rhythm is Mobitz I, 2nd-degree AV block.

    Figure-6: Laddergram for today's rhythm.

    Acknowledgment: My appreciation for the anonymous submission of this case to me.

    Related ECG Blog Posts to Today’s Case:

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

    • ECG Blog #193 — Reviews the basics for predicting the "culprit" artery (as well as reviewing why the term "STEMI" — should be replaced by "OMI" = Occlusion-based MI).
    • CLICK HERE  for my new ECG Videos (on Rhythm interpretation — 12-lead interpretation with Case Studies for ECG diagnosis of acute OMI).
    • CLICK HERE  for my new ECG Podcasts (on ECG & Rhythm interpretation Errors — and — Errors in assessing for acute OMI).
      • Recognizing hyperacute T waves — patterns of leads — an OMI (though not a STEMI) — See My Comment at the bottom of the page in the November 8, 2020 post on Dr. Smith's ECG Blog.
      • Recognizing ECG signs of Precordial Swirl (from acute OMI of LAD Septal Perforators— See My Comment at the bottom of the page in the March 22, 2024 post on Dr. Smith's ECG Blog. 

      • ECG Blog #294 — Reviews how to tell IF the "culprit" artery has reperfused.
      • ECG Blog #230 — Reviews how to compare serial ECGs.
      • ECG Blog #115 — Shows how dramatic ST-T changes can occur in as short as an 8-minute period.
      • ECG Blog #268 — Shows an example of reperfusion T waves.
      • ECG Blog #400 — Reviews the concept of "dynamic" ST-T wave changes.

      • ECG Blog #337 — A "NSTEMI" that was really an ongoing OMI of uncertain duration (presenting with inferior lead reperfusion T waves).

      • ECG Blog #351 — for review of the ECG diagnosis of acute posterior OMI (with links to additional examples of posterior OMI in the references at the end of this post).