ECG Blog #437 — A 2-Part Answer ...

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

QUESTIONS:

• 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?

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ANSWER:

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. 

QUESTION:

• 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.

ANSWER:

• 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.

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

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Related ECG Blog Posts to Today’s Case:

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

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• CLICK HERE — for my 6 new ECG Videos (on Rhythm interpretation — 12-lead interpretation with Case Studies for ECG diagnosis of acute OMI).
• CLICK HERE — for my 2 new ECG Podcasts (on ECG & Rhythm interpretation Errors — and — Errors in assessing for acute OMI).

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