ECG Blog #421 — Has there been a Recent MI?

What if you were asked to interpret the ECG in Figure-1?

• How would YOU interpret the rhythm?
• Even without the benefit of any history — Has there been a recent MI?

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

MY Approach to Today’s Tracing:

As always — I favor beginning assessment with a quick look at the long lead rhythm strips at the bottom of the tracing. By the Ps, Qs, 3R Approach (which I review in ECG Blog #185):

• Lots of P waves are present — being well seen in the long lead II rhythm strip.
• The QRS complex is narrow in all 12 leads.
• The rhythm is not Regular. The ventricular Rate varies.
• The 5th parameter of the Ps,Qs,3R Approach — is the 3rd R, which recalls “Related” — or determining if P waves are (or are not) related to neighboring QRS complexes. This last parameter can best be assessed by labeling P waves in the long lead II rhythm strip.

QUESTIONS:

Take a LOOK at Figure-2 — in which RED arrows highlight those P waves that are definitely seen on this tracing.

• How would YOU describe the regularity (or lack thereof) of P waves in today's rhythm?
• Are all of the P waves originating from the SA node?

Figure-2: I have labeled the P waves that we definitely see with RED arrows. Is the underlying atrial rhythm regular?

ANSWERS:

We see in Figure-2 — that each of the RED arrow P waves manifest a similar and normal P wave morphology — and, that each of these P waves are upright in the long lead II rhythm strip. We can therefore presume these are normal sinus P waves.

• With the exception of 2 places in the rhythm strip where we do not see P waves at the point where we might logically expect them — Don’t the RED arrow P waves that we have labeled in Figure-2 otherwise look fairly regular?
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• NOTE: If measured with calipers — We know that the RED arrow P waves in Figure-2 are not precisely regular. But what we are trying to determine, is if there is an underlying sinus mechanism. This includes sinus arrhythmia — in which there is often slight variation in the P-P interval. This slight variation in sinus P wave regularity tends to be greater when a 2nd- or 3rd-degree AV block is present (called ventriculophasic sinus arrhythmia — as shown in ECG Blog #344).

PEARL #1: When many (most) P waves in a given tracing look regular (or at least fairly regular) — but one or two places exist in the rhythm in which P waves are not seen at the point where you expect them — it may be that additional P waves are “hiding”. As a result — I look especially carefully at QRS complexes and ST-T waves that may be hiding part (or all) of the missing P waves.

• Is there any indication in Figure-2 — that there may be more P waves than the ones we have labeled with RED arrows?
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•     HINT: My answer is shown in Figure-3.

Figure-3: How can we know if additional sinus P waves are “hiding” under the PINK arrows?

PEARL #2: It is much easier to recognize an ECG finding — IF you know that you should be looking for it!

• Because we know from PEARL #1 that it would be logical for the atrial rhythm to be more regular than suggested by the RED arrows in Figure-2 — we need to pay special attention to the T waves under the PINK arrows in Figure-3 (since this is where we would expect P waves to be “hiding” if the underlying atrial rhythm was sinus arrhythmia).

QUESTION:

• Is there a difference in ST-T wave morphology for those T waves under the PINK arrows in Figure-3 — compared to all of the other T waves in this long lead II rhythm strip? (ie, Compared to the T waves of beats #1,3,4,6,7,8?).

ANSWER:

The T waves under the PINK arrows in Figure-3 are larger and clearly “fatter” than all other T waves on this tracing.

• PEARL #3: Perhaps the greatest challenge in interpreting complex rhythms is distinguishing between “real differences” in morphology — vs artifact and/or the normal variation that is commonly seen in P wave, QRS and ST-T wave morphology.
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• The above said — Reasons I know P waves are “hiding” under the PINK arrows in Figure-3 are that: i) It is so much more logical for sinus P waves to be at least fairly regular throughout the tracing — rather than to see the SA node suddenly stop putting out impulses only in 1 or 2 places in today’s tracing; and, ii) There can be little doubt that the T waves under the 2 PINK arrows in Figure-3 are larger and “fatter” than all other T waves on this tracing.

Putting It All Together:

I find it much EASIER to assess a complex rhythm once all sinus P waves have been labeled. For clarity — I have done this in Figure-4.

• RED arrows highlight all sinus P waves. The slight variation in the P-P interval is consistent with an underlying sinus arrhythmia.
• Note that there are more P waves (RED arrows) — than QRS complexes in Figure-4. This means that at least some of the on-time sinus P waves are not being conducted to the ventricles — which defines today's rhythm as some form of AV block.
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• PEARL #4: Today's rhythm is not likely to be complete (3rd-degree) AV block. This is because most of the time when there is complete AV block — the escape rhythm will be regular (or at least fairly regular). 
• In my experience — the BEST clue that a QRS complex is being conducted, is when we see a beat occur earlier-than-expected. This is why in Figure-4 — we can immediately suspect that beats #2,5,8 are being conducted to the ventricles. 
• Since today's rhythm represents some form of AV block — but by PEARL #4, is unlikely to be complete AV block — this rhythm must represent some form of 2nd-degree AV block. Since the QRS complex is narrow — and since we know that the Mobitz I (AV Wenckebach) form of 2nd-degree AV block is so much more common than Mobitz II — we can immediately suspect that today's challenging rhythm represents some form of Mobitz I (For review of how to distinguish the 2nd-degree AV blocks — See ECG Blog #344).

Figure-4: RED arrows highlight all sinus P waves.

Important CONCEPTS:

To EMPHASIZE — I’ve intentionally dissected the above observations regarding today’s rhythm in “slow motion”. With experience — it should take no more than seconds to arrive at this point in our assessment.

• Today's rhythm is challenging! That said, the KEY point is — that precise determination of the specific kind of AV block is not essential for appropriate clinical management. 
• All that clinicians need do — is to recognize the following: i) That the underlying atrial rhythm is sinus arrhythmia; ii) That some form of AV block is present (because not all of the sinus P waves are being conducted); iii) But that 3rd-degree AV block is not likely (because the ventricular rhythm is not regular); and, iv) That statistically — Mobitz I 2nd-degree AV block is by far (well over 90% of the time) the most common form of 2nd-degree AV block, especially when the QRS complex is narrow, as it is in today's rhythm.

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PEARL #5 (Beyond-the-Core): As an advanced point — Figure-5 illustrates how using calipers should facilitate rapid confirmation of the KEY points described above.

• Using calipers allows us to quickly determine — that the R-R intervals between beats #2-3; 3-4; 5-6; and 6-7 are all equal! (ie, 142 msec.). However, the PR intervals before beats #4 and 7 are clearly too short to conduct! This strongly suggests that each of these beats with the same preceding R-R interval ( = beats #3,4,6,7) — is a junctional escape beat.
• The identical R-R interval of 142 msec. that precedes each of the above junctional escape beats — corresponds to an appropriate junctional escape rate of just over 40/minute (ie, 300 ÷ 7 large boxes).
• The reason today's rhythm is so challenging to interpret — is that the frequent occurrence of junctional escape beats serves to mask the typical progressive PR interval lengthening that we would otherwise see with Mobitz I 2nd-degree AV block.
• That said — seeing how much earlier-than-expected beats #2,5 and 8 occur provides additional support to our supposition that these 3 beats are almost certain to be conducted, albeit with differing prolonged PR intervals. This is why todays rhythm is so challenging to interpret!

Figure-5: Using calipers allows us to quickly determine that the R-R intervals between beats #2-3; 3-4; 5-6 and 6-7 are equal!

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What about Today's 12-Lead ECG?

Whereas precise determination of today's rhythm is not essential for appropriate clinical management — quickly recognizing that some form of Mobitz I 2nd-degree AV block appears to be present tremendously facilitates interpretation of today's 12-lead tracing (that I have reproduced in Figure-6).

• Overall — there is ST segment flattening with slight ST depression in multiple leads in today's 12-lead tracing.
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• PEARL #6: The most common clinical setting for seeing the Mobitz I form of 2nd-degree AV block is acute or recent inferior and/or posterior infarction. As a result, whenever I see some form of Mobitz I — I immediately search for any possible indication of recent inferior and/or posterior OMI.
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• PEARL #7: Normally, there is a slight amount of gently upsloping ST elevation in leads V2 and V3. When instead of this gently upsloping ST elevation, there is "shelf-like" ST segment flattening — I immediately suspect recent posterior OMI, especially if the patient presents with new or recent chest pain (See ECG Blog #367 — for review of this concept).
• Therefore — My "eye" was immediately drawn to the abnormal ST segment flattening in lead V2 (within the RED rectangle). Seeing a similar abnormal ST segment shape in neighboring leads V3 and V4 (within the light BLUE rectangles) — confirmed this as a "real" finding.
• NOTE: Although we are not provided with any history in today's case — knowing that today's rhythm appears to represent some form of Mobitz I strongly supports my supposition that the ST segment flattening with slight depression in leads V2,V3,V4 of Figure-6 should suggest recent posterior OMI until proven otherwise!
• The finding of additional ST segment flattening in multiple other leads may represent multi-vessel disease.

Figure-6: I've highlighted the KEY leads in today's 12-lead tracing with colored rectangles (See text).

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PEARL #8 (Beyond-the-Core): There will sometimes be slight variation in QRS morphology between sinus-conducted beats and junctional escape beats. At times, this slight difference in QRS morphology provides an important clue as to whether a given beat is conducted — or — represents a junctional escape beat (See ECG Blog #63).

• I've emphasized that although the precise mechanism of today's rhythm is complex — all that is needed for appropriate clinical management, is appreciation that some form of Mobitz I 2nd-degree AV block is present.
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• QUESTION: Did YOU notice the slight difference in QRS morphology between the 2 beats in simultaneously-recorded leads V1,V2 and V3 in Figure-6? This difference in QRS shape further supports my earlier conclusion that beat #5 (that occurs earlier-than-expected) is sinus-conducted with a long PR interval — whereas beat #6 (which is preceded by the 142 msec. R-R interval shown in Figure-5) is a junctional escape beat.

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The LADDERGRAM: 

I've emphasized that although the precise mechanism of today's rhythm is complex — all that is needed for appropriate clinical management, is appreciation that some form of Mobitz I 2nd-degree AV block is present.

• That said — I fully acknowledge that I needed a laddergram to "solve" (ie, explain) each of the ECG findings in today's tracing. I've labeled my proposed laddergram in Figure-7.

Figure-7: I've labeled my proposed laddergram in today's case.

Laddergram Explanation:

• Beats #2, 5 and 8 are each sinus-conducted, albeit with significantly prolonged PR intervals (since each of these beats occur much earlier-than-expected).
• Beats #3,4,6 and 7 are junctional escape beats. We arrived at this conclusion by the finding of PR intervals before beats #4 and 7 that are definitely too short to conduct — with the PR intervals before beats #3 and 6 being significantly shorter than each of the sinus-conducted beats — and, with each of these 4 beats that we presume to be junctional escape beats, being preceded by the identical R-R interval of 142 milliseconds. 
• Although we do not see far enough in front of beat #1 to know what its preceding R-R interval is — the fact that the PR interval before beat #1 is short (and of similar duration as the PR interval preceding junctional beats #3 and 6) — suggests that beat #1 is also a junctional escape beat.
• YELLOW arrows represent P waves that are not conducted because of the 2nd-degree AV block.
• We have no idea whether the BLUE arrow P waves might have conducted (perhaps with an increasing PR interval) — IF the junctional escape beats ( = beats #3,4; 6,7) would not have occurred.
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• BOTTOM Line: Today's case illustrates how challenging it can be to recognize certain forms of 2nd-degree AV block of the Mobitz I type (ie, in which there are frequent junctional escape beats). That said — the overall ventricular rate of today's rhythm is between 40-50/minute, which may be enough to maintain hemodynamic stability. 
• NOTE: We can not say there is "high-grade" AV block in today's tracing — because we never see 2 consecutive on-time sinus P waves that fail to conduct despite having adquate opportunity to do so (ie, We never see 2 consecutive YELLOW arrow P waves).
• Clinically: The "good news" regarding today's case — is that most of the time with Mobitz I, acute reperfusion (with PCI or thrombolytics) will result in improvement of the associated AV conduction disturbance.

I conclude today's case with the laddergram shown in Figure-8 — in which I've removed the coloration from Figure-7. 

Figure-8: Today's laddergram without coloration.

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Acknowledgment: My appreciation to Danilo Franco (from Italy) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #185 — My Ps,Qs,3R System for Rhythm interpretation.
• 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 #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.

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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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• 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 — for review of some cases that illustrate "AV block problem-solving".
• ECG Blog #344 — thoroughly reviews the Types of 2nd-degree AV block (Mobitz I vs Mobitz II vs 2:1 AV Block).
• ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.

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• ECG Blog #193 — Reviews the Mirror Test for recognition of acute Posterior MI. This blog post also reviews the basics for predicting the "Culprit" Artery — and use of the term, "OMI" ( = Occlusion-based MI) as an improvement from the outdated STEMI paradigm.
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• ECG Blog #367 — A must to review !!! — as this case reinforces the KEY concepts for recognizing subtle acute posterior OMI!
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• ECG Blog #294 — How to tell IF the "culprit" artery has reperfused.
• ECG Blog #260 — Reviews when a T wave is hyperacute — and the concept of "dynamic" ST-T wave changes.
• ECG Blog #230 — How to compare serial ECGs.
• ECG Blog #337 — an OMI misdiagnosed as an NSTEMI ...
• ECG Blog #285 — for another example of acute Posterior MI (with positive Mirror Test).
• ECG Blog #246 — for another example of acute Posterior MI (with positive Mirror Test).
• ECG Blog #80 — reviews prediction of the "culprit" artery (with another case to illustrate the Mirror Test for diagnosis of acute Posterior MI).
• ECG Blog #184 — illustrates the "magical" mirror-image opposite relationship with acute ischemia between lead III and lead aVL (featured in Audio Pearl #2 in this blog post).
• ECG Blog #167 — another case of the "magical" mirror-image opposite relationship between lead III and lead aVL that confirmed acute OMI.
• ECG Blog #271 — Reviews determination of the ST segment baseline (with discussion of the entity of diffuse Subendocardial Ischemia).
• ECG Blog #258 — How to "Date" an Infarction based on the initial ECG.
• The importance of the new OMI (vs the old STEMI) Paradigm — See My Comment in the July 31, 2020 post in Dr. Smith's ECG Blog.