ECG Blog #212 (ECG MP-29) — Why So Many QRS Shapes? Is this Ashman?

The 2 lead MCL-1 rhythm strips shown in Figure-1 were obtained from a middle-aged adult who was hemodynamically stable at the time these tracings were recorded. Tracing B was obtained just minutes after Tracing A.

 

QUESTION: WHICH of the following statements are TRUE regarding the multiple different QRS shapes seen in these 2 rhythm strips?

— a) It is likely that all beats on these 2 tracings are supraventricular.

— b) There are multiple PACs, and probably also some AFib.

— c) The Ashman Phenomenon is present.

— d) There is RBBB aberration.

— e) Beats #13-thru-16 probably represent a 4-beat run of NSVT (Non-Sustained Ventricular Tachycardia).

— f) Once the heart rate slows — it is likely that all QRS complexes will look the same.

 

Figure-1: Tracings A and B were obtained minutes apart from a hemodynamically stable, middle-aged adult. HOW do you interpret these rhythms? (See text).

• NOTE #1: Lead MCL-1 is a right-sided monitoring lead that provides a similar perspective as is seen in lead V1 on a 12-lead tracing. And, although ideally, we would have a complete 12-lead ECG available for rhythm assessment — sufficient information is evident on these 2 single lead rhythm strips to answer the above Question.

 

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NOTE #2: Some readers may prefer at this point to listen to the 8:00 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-29).

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Today’s ECG Media PEARL #29 (8:00 minutes Audio) — Reviews WHAT the Ashman Phenomenon is — HOW to use it clinically? — and — whether the Ashman phenomenon is accurate when the underlying rhythm is AFib?

• NOTE #3: For detailed review of the Ashman Phenomenon — with illustration of its clinical application — Please See ECG Blog #70. Use of the Ashman phenomenon with AFib is reviewed in ECG Blog #71.

 

MY THOUGHTS:

The 2 rhythm strips in Figure-1 are extremely challenging — but they provide an excellent example of deductive reasoning, in that it is only after interpreting Tracing A — that I was able to able with confidence to interpret Tracing B. Along the way — there are numerous PEARLS to discuss that are useful for optimal arrhythmia interpretation.

• I’ll begin by giving My Answers to the above QUESTION — Choices a, b, c, d, and f are correct.
• The only incorrect choice is e.
• IF you disagree with any of my answers — Please READ what follows below!

Beginning with Tracing A:

In both of the tracings shown in Figure-1 — there are easier parts and more difficult parts of each tracing to interpret.

• PEARL #1: When there are multiple components to a given rhythm strip — begin with the easier part(s) of the tracing. Most of the time — this consists of first looking for an underlying rhythm. In Tracing A — it should be clear that the initial 3 beats are sinus, because the QRS complex is narrow, and each QRS is preceded by a similar-looking P wave with a constant and normal PR interval (RED arrows in Figure-2).
• Beat #5 clearly represents another sinus-conducted beat — because this beat manifests a similar-looking and narrow QRS complex that is preceded by a P wave with a PR interval identical to that of the PR interval preceding sinus-conducted beats #1, 2, and 3.

 

Figure-2: Looking first a Tracing A — I’ve added RED arrows to highlight sinus P waves that precede beats #1, 2, 3 and 5, each with an identical PR interval (See text).

 

PEARL #2: The most convincing evidence that an early-occurring beat is a PAC (Premature Atrial Contraction) — is being able to demonstrate that this beat is preceded by a premature P wave. Since the timing of premature P waves often coincides with the preceding ST-T wave — identifying premature P waves that are partially hidden within the ST segment can be difficult, especially if some baseline artifact over the ST segment is present. 

• The challenge to the interpreter — is to distinguish between what is a “real” notch (ie, a notch produced by a partially hidden premature P wave) — vs what is more likely to represent either artifact or the slight “normal variation” of ECG waveforms that can often be seen with normal tracings.
• In Figure-2 — although there is slight variation in the shape of the ST-T waves of beats #1, 2 and 4 — the shape of these ST-T waves is smooth! 
• Although subtle — there clearly is a small notch in the T wave preceding early-occurring beat #4 (BLUE arrow in Figure-3). This small notch is a premature P wave — and its presence confirms that beat #4 is a PAC.
• Additional support that beat #4 is a PAC is forthcoming from a similar subtle notch that precedes beat #6 (YELLOW arrow in Figure-3). Given that the ST-T waves of beats #1, 2 and 4 are smooth — and that it is only the ST-T waves of early-occurring beats #4 and 6 that manifest subtle notching — we have to presume that this notching is real, and the result of partially hidden premature P waves. This proves that beats #4 and #6 are PACs.

 

Figure-3: I’ve added BLUE and YELLOW arrows to Figure-2 that highlight subtle notching in the preceding T waves, which confirms that beats #4 and 6 are PACs (See text).

 

PEARL #3: The reason the QRS complex of beat #6 in Figure-3 looks different than the QRS complex of the first 5 beats in this tracing — is that beat #6 is conducted with RBBB (Right Bundle Branch Block) Aberration.

• As discussed in detail in ECG Blog #211 — the most common form of aberrant conduction is with RBBB morphology. This is because in most patients, the right bundle branch generally has a longer refractory period than the refractory period of other conduction fascicles.
• QRS morphology of beat #6 in Figure-3 is absolutely typical for RBBB because: i) The QRS complex of beat #6 is slightly, but not excessively widened — compared to sinus-conducted beats #1, 2, 3 and 5; ii) There is a triphasic rSR’ morphology to beat #6 in this right-sided lead MCL-1, with a similar, slender initial positive r wave deflection — followed by an S wave that descends below the baseline — and finishing with a “taller-right-rabbit-ear” R' deflection; and, iii) The Ashman Phenomenon is present!

 

PEARL #4: As reviewed above in today’s Audio PEARL (MP-29) — as well as in our ECG Blog #70 — the Ashman Phenomenon is most easily summarized by saying, “The funniest-looking (most different) beat tends to follow the longest pause.”

• The longest “pause” in Figure-3 is the R-R interval between beats #4-5. This longer preceding R-R interval “sets up” conditions that predispose to aberrant conduction — because this longer R-R interval serves to prolong the subsequent absolute and relative refractory periods.
• NOTE: The coupling intervals for the P waves represented by the BLUE and YELLOW arrows in Figure-3 are virtually equal. The reason beat #6 conducts with RBBB aberration (whereas premature beat #4 with an identical PAC-coupling interval, conducts normally) — is that the longer pause before beat #5 lengthens the subsequent refractory period. This is the Ashman phenomenon!
• Recognition of the Ashman phenomenon in Figure-3 is essential for “solving” this rhythm — because thisremoves all doubt about the reason why beat #6 is wider and looks different than the 5 normal-looking beats that come before it. We now have a reason for aberrant conduction — and, we have a reason why beat #6 shows RBBB aberration but beat #4 does not.
• PEARL #5: Since we now know that beat #6 is aberrantly conducted — it is highly likely that all other beats in Tracing A and in Tracing B that look similar to beat #6 are also conducted with RBBB aberration. Therefore — beats #7, 13 and 14 in Figure-3 are also almost certain to be aberrantly conducted. NOTE: It is common for there to be slight variation in QRS morphology with aberration — so the fact that the S wave of beat #7 does not descend below the baseline does not at all dissuade me from strong belief that beats #6, 7; 13 and 14 in Figure-3 are all aberrantly conducted!

 

 

Continuing with my assessment of Tracing A:

• P waves are “lost” after beat #6 in Figure-3. Undulations in the baseline persist — but clearly defined P waves are no longer seen. Caliper measurement of R-R intervals, beginning with beat #6 — until the end of the tracing — reveals subtle-but-real variation in many (most) of the R-R intervals. Taking into account development of coarse baseline undulations that begin after beat #6 — it appears that this patient has developed AFib (Atrial Fibrillation), here with a rapid ventricular response and coarse “fib waves”.
• PEARL #6: When the ventricular response of AFib is rapid (as it is in Figure-3) — the R-R interval may lookregular, when in fact — subtle-but-real variation in the R-R interval persists.

NOTE: Obviously in practice, there will not always be time to pull out calipers when you are at the bedside with an acutely ill patient. I will emphasize that I did not need calipers to know that the R-R interval for beats #6-thru-16 in Figure-3 is not completely regular. With practice — it becomes easier to recognize this slight-but-real irregularity, even without calipers. That said, when I do have the time — I use calipers for confirmation.

• PEARL #7: As reviewed above in today’s Audio PEARL (MP-29) — and in our ECG Blog #71 — the Ashman phenomenon may be somewhat less reliable in the setting of AFib. Because the rate of atrial fibrillatory waves is so fast (often between 400-to-600 fib wave impulses per minute) — the potential for “concealed” conduction (that is, partial conduction through the atria into the AV node) is great — and this can alter conduction properties enough to alter refractory period duration in an unpredictable manner. That said — I still use the Ashman Phenomenon in AFib, and much of the time it works. But because of this potential for concealed conduction — realize that the Ashman Phenomenon is not as reliable with AFib as it tends to be during sinus rhythm.
• I suspect the reason beats #13 and 14 in Figure-3 are aberrantly conducted despite no lengthening of the preceding R-R interval — may be the result of “concealed” conduction that occurs here in association with AFib.

 

Moving on to Tracing B:

For clarity — I’ve combined Tracing A and Tracing B in Figure-4.

• PEARL #8: I am a firm believer in what I call the “Birds-of-a-Feather” Concept. This concept states that IF a certain rhythm phenomenon occurs in one part of a tracing – that it is likely to occur again in other parts of the tracing (or in subsequent tracings obtained from this same patient). We’ll see in a moment how this concept proves invaluable, as we begin to assess Tracing B.

Figure-4: For clarity — I’ve again put Tracings A and B together (See text).

 

Tracing B is remarkable for the introduction of a new QRS morphology — namely the 4 successive widened beats in the middle of this rhythm strip ( = beats #13, 14, 15, 16).

• Once again — I favor applying the concept I presented earlier in PEARL #1 — namely to start with those parts of a complex tracing that we have already figured out. Beats #1-thru-9; beat #12 — and beats #17-thru-22 in Figure-4 are clearly all supraventricular, and similar in morphology to sinus-conducted beats #1, 2, 3 and 5 in Tracing A.
• Beats #10 and 11 in Figure-4 are virtually certain to be supraventricular with RBBB aberration — because their morphology is identical to the 4 beats in Tracing A that we identified as aberrantly conducted.
• P waves are absent in Tracing B. Undulations in the baseline are seen that look very much like the “fib waves” we identified in Tracing A. And even without calipers — it should be apparent that all of these beats are irregularly irregular. Therefore, the underlying rhythm in Figure-4 continues to be AFib with a rapidventricular response.

 

All that remains — is to determine the etiology of beats #13, 14, 15 and 16 in Figure-4, which manifest a new QRS morphology (ie, predominantly negative wide beats). I’ll do this by making several observations.

• The underlying rhythm for the latter half of Tracing A, and for the beats we have thus far commented on in Tracing B is rapid AFib. By the “Birds-of-a-Feather” Concept ( = PEARL #8) — perhaps AFib might be present for the entire duration of Tracing B?
• Note that there is no “post-ectopic pause” after the 4-beat run of predominantly negative wide beats (beats #13-thru-16). Instead, the underlying pattern of ever-so-slight-but-real irregular irregularity is maintained throughout the entire rhythm strip in Tracing B.
• Although LBBB aberration is decidedly less common than RBBB or LAHB aberration — it does occur. The predominantly negative QRS complex in this right-sided lead MCL-1, with no more than thin, tiny initial r wave and very steep descent of the S wave is consistent with typical LBBB morphology.
• PEARL #9: This case reminds me of an uncommon but highly specific ECG finding cited by Dr. Barney Marriott — namely, that in the setting of rapid AFib — if ever you see one or more beats of RBBB aberration separated by a single normal-appearing beat — before onset of a 2nd QRS morphology that is potentially consistent with LBBB aberration — that this pattern is virtually diagnostic of shifting LBBB and RBBB aberration. Isn’t this precisely what we see in Tracing B? — namely, beats #10 and 11 conduct with RBBB aberration — beat #12 manifests normal QRS morphology — and then, beats #13-thru-16 are perfectly consistent with LBBB aberration.

 

Clinical Implications:

The importance of recognizing that all beats in Figure-4 are supraventricular, is that there is no need to worry about having to contemplate antiarrhythmic treatment for runs of VT — even if longer runs of the predominantly negative wide beats were to be seen.

• Treat the Primary! After the first 5 beats in Tracing A — the underlying rhythm became rapid AFib. Finding and correcting any potentially precipitating causes of new AFib — and, if needed in the process, use of AV nodal rate-slowing medications is likely to control the ventricular response (if not result in conversion to sinus rhythm) — and chances are that most (if not all) wide beats will resolve once this is done.

 

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Relevant ECG Blog Posts to Today’s Case:

• ECG Blog #70 — Reviews the basics of the Ashman Phenomenon. 
• ECG Blog #71 — Is the Ashman Phenomenon accurate in AFib? 
• ECG Blog #211 — Reviews in detail WHY aberrant conduction occurs (and why RBBB aberration is the most common form).