ECG Interpretation: ECG Blog #393 — Why So Many Shapes?

The interesting rhythm shown in Figure-1 was obtained after Adenosine was given for a regular SVT (SupraVentricular Tachycardia).

How would YOU interpret this rhythm in Figure-1?
Why are there so many shapes for the QRS complex in the long lead II rhythm strip?

Figure-1: 12-lead ECG and long lead II rhythm strip obtained after Adenosine was given for a regular SVT rhythm. Why are there so many shapes for the QRS complexes?

MY Thoughts on the ECG in Figure-1:

As per the title of today's Blog post — there are numerous variations in the shape of the QRS complex over the course of the 17 beats seen in the long lead II rhythm strip. Doesn't this make it difficult to know where to start in our interpretation?

PEARL #1: The easiest initial step for determining the etiology of a complex rhythm such as that shown in Figure-1 — is to label the P waves.
You'll note that I also number the beats — since this instantly allows everyone involved to ensure we are all talking about the same part of the tracing.
Using calipers is the fastest (and most accurate) way to determine IF the underlying atrial rhythm is regular.
NOTE: There may be slight variation in the P-P interval if some sinus arrhythmia is present — but even then, it will usually be obvious IF the "theme" of the P-P interval is that of an underlying sinus rhythm.

PEARL #2: When the rhythm is complex — Look for "normal" beats.

I've labeled the P waves in today rhythm with RED arrows in Figure-2. Can YOU identify which of the fairly regular sinus P waves appear to conduct normally to the ventricles?

Figure-2: I've labeled the sinus P waves with RED arrows. Can YOU identify which P waves appear to conduct normally to the ventricles?

Which P Waves in Figure-2 are Conducted Normally?

It's sometimes easier to start by identifying which P waves are not conducted normally to (and through) the ventricles.

The PR interval in front of beat #1 in Figure-3 — is clearly too short for normal transmission through the entire ventricular conduction system.
Similarly — the PR interval in front of beats #3, 4 and 5 appears to be too short for normal conduction.
On the other hand — the PR interval before beat #6 is long enough at ~0.14 second to conduct normally.
Continuing — the PR intervals before beats #8,10,11,13,14,16 and 17 are all too short (ie, <0.11 second) for complete transmission through the entire ventricular conduction system.
KEY Point: I've placed an "N" (for "Normal" conduction) below beats #2,6,9,12 and 15 — because these beats all look the same, and are all preceded by the same normal PR interval.
The reason I do not think beat #7 is transmitted through the entire ventricular conduction system — is the different morphology of this QRS complex compared to beats #2,6,8,12 and 15. Instead — beat #7 appears to be a fusion beat, with a QRS and T wave morphology intermediate between the normally conducted sinus beat #6 — and the primarily ventricular beat #8.

Figure-3: For clarity — I focus solely on the 17 beats in the long lead II rhythm strip. RED arrows indicate sinus P waves.

PEARL #3: As discussed and illustrated in ECG Blog #128 — When there is near-simultaneous occurrence of a supraventricular and ventricular impulse, the result may be a fusion beat.

When fusion occurs, depolarization wavefronts from above and from below meet in the ventricles before either impulse is able to complete its path. The appearance of the resultant QRS complex takes on characteristics of both the supraventricular and ventricular complex.

The concept of fusion beats as it relates to today's tracing is best explained by means of a Laddergram, which I have drawn in Figure-4.

As depicted by the 5 "N's" that I have added in Figure-4 below beats #2,6,9,12 and 15 — these 5 beats appear to transmit normally through the entire ventricular conduction system.
Varying degrees of fusion are seen for the remaining 12 beats in today's tracing.
I schematically suggest in Figure-4 that the sinus wavefront and the ventricular wavefront of beat #7 meet at a lower point in the ventricular tier of the laddergram — because the PR interval is almost normal and QRS morphology looks more like the 5 normal sinus-conducted beats than like any of the other 11 fusion beats (ie, the sinus P wave has had a little more time to travel through the ventricles — and therefore more closely resembles QRS morphology of normal, sinus-conducted beats).
In contrast — I've drawn the wavefront "meeting point" for the most abnormal-looking beats ( = beats #1,3,16,17) relatively higher in the ventricular tier, because these beats (which are preceded by shorter PR intervals) travel further through the ventricles, and therefore take on more characteristics of the pure ventricular beats.

Figure-4: My proposed laddergram for today's rhythm — in which varying degrees of fusion are present in 12/17 beats in the long lead II rhythm strip (See text).

Final Point: Where are the Ventricular Beats Coming From?

Return for a moment to the 12-lead ECG in Figure-2. Note that although beats #1; 3,4,5; 8; 10,11; 13,14; and 16,17 are wider than the 5 normally conducted sinus beats — they are not quite 0.12 second in duration.

Abnormally-conducted beats #10 and 11 manifest an rsR' complex consistent with RBBB conduction in simultaneously-recorded lead V1. The wide terminal S wave of RBBB conduction is seen for abnormally-conducted beats #14,16,17 in simultaneously-recorded left-sided lead V6.
In the frontal plane — abnormally-conducted beats #1,3,4,8 manifest a QRS morphology in simultaneously-recorded inferior leads that is consistent with LAHB conduction.
A QRS morphology for abnormally-conducted ventricular beats that is only slightly wide (at 0.11 second) and consistent with RBBB/LAHB conduction — suggests that the ventricular focus producing varying degree of fusion in today's tracing is arising from the left posterior hemifascicle.

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Acknowledgment: My appreciation to Chun-Hung Chen = 陳俊宏 (from Taichung City, Taiwan) 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 the Ps, Qs, 3R Approach to Rhythm Interpretation.
ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 80 laddergram cases — many with step-by-step sequential illustration).
ECG Blog #128 and ECG Blog #129 — Review Fusion Beats.

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ADDENDUM (9/2/2023):

Adenosine was administered to today's patient for treatment of a regular SVT rhythm. Although this ultimately resulted in successful conversion to sinus rhythm — the immediate result of Adenosine administration caused the unusual rhythm discussed above (seen in Figure-1). This highlights the pros and cons for using Adenosine — which I review in Figure-5 and Figure-6 (excerpted from my ACLS-2013-ePub).

Adenosine is a wonderful drug for emergency treatment of reentry SVTs. It is also effective for a selected number of adenosine-responsive VT rhythms (primarily in younger adults who do not have underlying heart disease).
Because of its ultra-short half-life following IV administration — Adenosine is usually safe when given empirically to patients in whom the etiology of a WCT rhythm is uncertain. That said — side effects can occur, and these are not uniformly short-lived. As a result — Adenosine is probably best avoided for WCT rhythms for which the drug has little to no chance of being effective (ie, polymorphic VT; ischemic-etiology VTs).
To EMPHASIZE — Adenosine was definitely indicated for treatment of the regular SVT rhythm in today's case. The resultant rhythm that we saw in Figure-1 was transient, resolving as the short-lived effect of Adenosine wore off. My purpose in presenting today's case is 2-fold: i) I thought this interesting rhythm would present an illustrative challenge on "the many faces" of fusion beats; and, ii) To highlight potential arrhythmias that appropriate use of Adenosine may cause — with realization that in most cases, these arrhythmias will be transient (with end result that the reentrant SVT is converted to sinus rhythm).