ECG Interpretation: ECG Blog #411

How would YOU interpret the tracing in Figure-1 — that was sent to me without additional information?

QUESTIONS:

What is the rhythm?
Is there AV dissociation? If so — WHY?
Clinically — HOW would you approach interpretation of this tracing in optimal time-efficient fashion?

Figure-1: The initial ECG in today's case.

KEY Clinical Point:

If I was the medical provider charged with the care of the patient whose ECG is shown in Figure-1 — I would approach this tracing in the following sequential stages:

I’d first establish that the patient was hemodnamically stable with this ECG and this cardiac rhythm. Assuming this patient is at least momentarily stable — I’d then proceed as follows:
I’d take a quick look at the long lead rhythm strip.
To EMPHASIZE: The rhythm shown in the long lead II (below the 12-lead tracing in Figure-1) is not a simple arrhythmia — so rather than trying to come up with a precise interpretation of the rhythm — I’d spend no more than 5-to-10 seconds to ensure there is a reasonable, stable-looking heart rate (which there is in Figure-1 — as the rhythm is supraventricular [narrow QRS everywhere] — some P waves are seen — and, a pattern of repetitive group beating (groups of 3 beats), with an average heart rate in the 60-80/minute rate range — appears to be present).
I’d then spend no more than 5-to-10 seconds looking at the rest of the 12-lead ECG to ensure that there is no ongoing emergency in need of immediate care (ie, No acute MI; No hyperkalemia, etc.).

COMMENT:

While I always advocate a systematic approach to both rhythm and 12-lead ECG interpretation — optimal clinical ECG interpretation entails prioritizing those parts of the tracing in front of you that mandate your immediate attention. Later, when you have a moment of time — You can complete your systematic assessment.

With Regard to the Cardiac Rhythm: Our initial 5-to-10 second assessment of the long lead II rhythm strip should be all that is needed at this instant in time — because this limited amount of time should still allow you to establish: i) That the patient is hemodynamically stable; and, ii) That the rhythm is supraventricular — with at least some P waves, a narrow QRS everywhere — and, an overall heart rate in the 60-80/minute range.
PEARL #1 (Advanced Point): The fact that the QRS complex is narrow, with at least some P waves — and a pattern suggesting group beating — should clue us into: i) The possibility of Wenckebach conduction! — and, ii) That we need to carefully inquire about recent chest pain, paying special attention to the rest of this 12-lead ECG (looking carefully for signs of acute or recent infarction — because AV Wenckebach is common with acute inferior MI). It is much easier to identify Wenckebach conduction IF you are looking for it!

Regarding the 12-Lead ECG:

Applying the systematic approach I favor for 12-lead ECG interpretation (as detailed in ECG Blog #205):

Rate & Rhythm: As stated above — there is a regular, supraventricular rhythm with some P waves, group beating, and an acceptable overall ventricular rate between ~60-80/minute.
Intervals (PR-QRS-QTc): The PR interval varies — the QRS is narrow — the QTc looks normal (Best seen in leads V2,V3).
Axis: Normal (about +60 degrees).
Chamber Enlargement: Depending on age of the patient — voltage for LVH may be present (ie, Deepest S in V1,V2 + tallest R in V5,V6 ≥35 mm — IF the patient is ≥35 years of age).

Regarding Q-R-S-T Changes:

Q waves: Small q waves of uncertain significance are seen in the inferior leads.
R Wave Progression: Normal, with transition (where height of the R wave becomes taller than the S wave is deep) — occurs normally, here by lead V4.
ST-T wave Changes: Show some nonspecific ST-T wave flattening, and perhaps shallow T wave inversion in lead aVL. The slight amount of ST elevation that we see in lead V2 is common and normal.
Clinical IMPRESSION: The ECG in Figure-1 shows nonspecific ST-T wave abnormalities — but nothing that looks acute.

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Looking Closer at the Rhythm in Today's CASE:

As reviewed in ECG Blog #185 — I favor the Ps, Qs, 3R Approach for systematic, time-efficient interpretation of the cardiac rhythm.

PEARL #2: When looking for P waves — I find it easiest to first identify those P waves that I am certain about. I have done this with RED arrows in Figure-2.
The presence of multiple, regularly-spaced RED ( = definitely present) P waves in Figure-2 suggests: i) that there is a very good chance that the underlying atrial rhythm will be regular throughout the long lead II rhythm strip; and, ii) that the P-P interval between any 2 consecutive RED arrows will clue us in to where the remaining (partially hidden) P waves can be found.
Use of calipers can facilitate confirming that the underlying atrial rhythm in Figure-2 is in fact regular. With calipers — it takes only seconds to confirm this!
PINK arrows highlight that the on-time beginning little "hump" at the very onset of the QRS of beats #2 and #5 clearly represents 2 additional on-time P waves.
Completion of our "P wave search" can then be easily accomplished by noting the subtle-but-unmistakeable distortion under the PURPLE arrows toward the end of the T waves of beats #1 and 4 — and at the very onset of the QRS of beats #8 and #11.

Figure-2: I've labeled atrial activity in today's tracing with colored arrows (See text).

PEARL #3: Recognition that true group beating is present, in association with an underlying regular atrial rhythm — strongly suggests there is some type of 2nd-degree AV block in which there is Wenckebach conduction (ie, the presence of an underlying regular P wave rhythm is an essential component of the typical forms of AV block).

Focus on this concept facilitates recognizing the 3 true groupings in Figure-3 (within the dotted WHITE rectangles). Note the uncanny similarity in duration of the R-R interval between the first 2 beats in each group (ie, the R-R interval between beats #2-3; 5-6; and 8-9) — as well as for the R-R interval between the last 2 beats in each group (ie, the R-R interval between beats #3-4; 6-7; and #9-10) — as well as for duration of the R-R intervals of the short pauses that separate each of the groups (ie, the R-R interval between beats #1-2; 4-5; 7-8; and 10-11). This amount of symmetry for the relative duration of R-R intervals within each group of beats is not the result of chance!

Figure-3: RED arrows highlight that the atrial rhythm in the long lead II rhythm strip is regular. Note the 3 true groupings (within the dotted WHITE rectangles).

PEARL #4: We know that there is at least transient AV Dissociation in today's rhythm — because the PR interval of some on-time sinus P waves is clearly too short to be able to conduct (ie, Despite their on-time occurrence — the WHITE arrow P waves before beats #2,5,8 and 11 in Figure-4 — do not have enough time for normal conduction through the AV node).

Since beats #2,5,8 and 11 all manifest a narrow QRS complex — and are not preceded by sinus P waves with a realistic chance of conducting — this means that these 4 QRS complexes are junctional beats.
Note that each of these beats is preceded by a similar R-R interval of just under 5 large boxes — which corresponds to an ever-so-slightly accelerated junctional escape rate of ~65/minute.
That said — We have no idea if the WHITE-arrow P waves that appear at the onset of the QRS of beats #2,5,8 and 11 might possibly have conducted — IF they were given a chance to do so (ie, IF the junctional escape rate would have been slower than 65/minute).

PEARL #5: Despite the presence of at least transient AV dissociation — the rhythm in Figure-4 is not complete AV Block.

Most of the time when there is complete AV block — the escape rate (be this a junctional or ventricular escape rhythm) will be regular. The obvious irregularity of the ventricular response in Figure-4 — immediately tells us this is not complete AV block.
QRS complexes that occur earlier-than-expected are likely to be conducted beats! (ie, Beats #4,7 and 10 in Figure-4 are therefore likely to be conducted — and beats #3,6,9 and 12 may also be conducted).

PEARL #6: As per PEARL #4 — we have already established that there is transient AV dissociation in Figure-4 (because beats #2,5,8,11 are clearly not conducted). There appear to be 2 reasons for this transient AV dissociation in today's rhythm:

As reviewed in ECG Blog #192 — the 3 Causes of AV Dissociation are: i) Default; ii) Usurpation; and, iii) Some form of 2nd- or 3rd-degree AV block.
Today's tracing is interesting — in that the reason for transient AV dissociation (ie, non-conduction of the WHITE-arrow P waves in Figure-4) — appears to be a combination of the slightly accelerated junctional beats (beats #2,5,8 and 11) — and the 2nd-degree AV block that leads to the slight pauses after beats #1,4,7 and 10.

PEARL #7: Looking closer in Figure-4 at the P waves in each of the 3-beat groupings — tells us that there is 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach) because of the following findings:

As already stated — the WHITE-arrow P waves in each group do not conduct to the ventricles — because the PR interval is too short.
That said — We know that the next P wave in each grouping (ie, the light BLUE-arrow P waves) — does conduct to the ventricles, because the PR interval preceding beats #3,6,9 and 12 are all equal!
This is followed by the dark BLUE-arrow P waves that are also conducting — but with slight increase in the PR interval compared to the light BLUE PR interval. Once again — We know that the dark BLUE-arrow P waves are conducting — because the PR interval preceding beats #4,7 and 10 are all equal!
The last P wave in each grouping (ie, the YELLOW-arrow P waves) — are non-conducted — thus completing the AV Wenckebach cycle of progressive PR interval lengthening until a beat is dropped.
Additional typical features of AV Wenckebach in Figure-4 include: i) The narrow QRS (Mobitz II usually has a wide QRS — because of its origin at a lower point in the conduction system); ii) Progressive shortening of the R-R interval within groups (the R-R intervals between beats #3-4; 6-7; and 9-10 — are shorter than the R-R intervals between beats #2-3; 5-6; 8-9); and, iii) The pause containing the dropped beat is less than twice the shortest R-R interval.

Figure-4: I've colored the P waves from Figure-3 (See text).

SUMMARY:

The rhythm in today's case is sinus (ie, regularly-occurring P waves) — with 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach). Each of the 3-beat Wenckebach groups begins with a slightly accelerated junctional beat (which results in transient AV dissociation).

The overall ventricular rate in today's tracing should be sufficient at ~60-80/minute for adequate perfusion.
Although it is common for Mobitz I to be associated with acute inferior MI — the 12-lead ECG in today's case does not suggest any acute ST-T wave changes.

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LADDERGRAM:

To clarify the mechanism for today’s rhythm — I've derived a laddergram with step-by-step description of events. I begin with Figure-5.

NOTE: For review of the Basics for HOW to read (and draw) laddergrams — See ECG Blog #188).

Figure-5: The 1st step in laddergram construction — is to represent each of the P waves in the Atrial Tier. Since conduction through the atria is fast — vertical lines are used, drawn from each of the ARROWS that overlie P waves in this tracing.

Figure-6: Since all QRS complexes are narrow — each of the 12 beats in this tracing is supraventricular. Knowing this allows me to draw in conduction within the Ventricular Tier — which I represent with slightly inclined forward-directed lines with an arrow to indicate the downward direction of conduction.
NOTE: I find Power Point optimal for drawing laddergrams — as it allows ready duplication of laddergram elements and precisely vertical displacement to ensure laddergram elements appear exactly below P waves and QRS complexes in the original rhythm strip.

Figure-7: It's now time to "solve" the laddergram. I begin by a survey of all P waves (RED arrows — which are represented by the RED vertical lines in the Atrial Tier) — and of all 12 QRS complexes (vertical RED lines within the Ventricular Tier) — with the goal of determining which P waves seem most likely to conduct to the ventricles.

Figure-8: "Armed" with the clue that the PR intervals preceding beats #3,6,9 and 12 look to be both normal and equal — I felt comfortable presuming that each of these BLUE-arrow P waves are conducting (which I illustrate with the slanted BLUE lines within the AV Nodal Tier that connect these P waves to the appropriate QRS complexes.

Figure-9: I then connected the 2nd P wave in each group (highlighted by the dark BLUE arrows) — with the appropriate QRS complexes for beats #4,7 and 10 in the Ventricular Tier (slanted dark BLUE lines within the AV Nodal Tier). Note slight increase in the amount of slanting of these dark BLUE lines, reflecting slight increase in the PR interval since the light BLUE P waves conducted beats #3,6,9 and 12.

Figure-10: I next chose to represent the YELLOW arrow P waves — which clearly are not conducting because they are not followed by any QRS complexes (YELLOW butt ends drawn within the AV Nodal Tier). This left a limited number of P waves and QRS complexes unaccounted for ...

Figure-11: But the PR interval for the WHITE arrow P waves is clearly too short to conduct! (represented by WHITE butt ends drawn within the AV Nodal Tier).

Figure-12: Since neither the YELLOW nor WHITE arrow P waves are conducting — beats #2,5,8 and 11 have to be arising from the AV node! (The dotted RED lines with butt ends indicate retrograde conduction from the slightly accelerated junctional escape beats — that prevent WHITE arrow P waves from being conducted through the AV Nodal Tier).

Figure-13: My completed laddergram illustrating the mechanism for today's rhythm. There is 2nd-Degree AV Block, Mobitz Type I (AV Wenckebach) — in which slightly accelerated junctional escape beats produce 1 beat of AV dissociation until sinus conduction resumes with beats #3,6,9 and 12.

Final Advanced Point:

Did YOU notice in Figure-13 — that the QRS of each of the junctional beats ( = beats #2,5,8 and 11) is slightly taller than the QRS of the remaining 8 beats on this tracing?

The above subtle observation can sometimes be a very helpful clue that these slightly different-looking beats are not sinus-conducted — but instead arise from the AV Node.
The reason escape beats from the AV Node sometimes look slightly different than sinus-conducted beats — is that the precise site of origin within the AV Node from where these beats arise may be slightly different than the path within the AV Node through which sinus-conducted beats pass.
To Emphasize: This clue is not needed for us to know that beats #2,5,8,11 in today's tracing are junctional beats — since it is obvious that the P waves preceding beats #2,5,8,11 are too close to the QRS to be conducted. But there are times when it will not be readily apparent if a given QRS complex represents a sinus-conducted beat vs junctional escape — in which case, the observation that QRS morphology differs slight may reveal which beats are junctional escape beats (See ECG Blog #63 — for a clinical example of this phenomenon!).

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Acknowledgment: My appreciation to ไกรสร เต็ง (from Bangkok, Thailand) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

ECG Blog #185 — Reviews my System for Rhythm Interpretation, using the Ps, Qs & 3R Approach.
ECG Blog #205 — Reviews my System for 12-Lead ECG Interpretation.
ECG Blog #245 — Reviews ECG diagnosis of LVH.
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).
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 #63 — Reviews a case of 2nd-Degree AV Block, Mobitz Type I with Junctional Escape Beats.
ECG Blog #267 — Shows step-by-step Laddergrams, with derivation of a case of Mobitz I with more than a single possible explanation.
ECG Blog #404 (ECG Video) — applies the Ps,Qs,3R Approach to a case of 2nd-degree AV Block ( = Mobitz I or Mobitz II or Neither).
ECG Blog #405 (ECG Video) — applies the Ps,Qs,3R Approach to a case of AV Dissociation vs Complete AV Block (What's the difference?).

ADDENDUM (1/6/2024):

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.

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ECG Media PEARL #4 (4:30 minutes Audio): — takes a brief look at the AV Blocks — and focuses on WHEN to suspect Mobitz I.

Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks.
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!