An interesting rhythm strip. What is this rhythm?

By Magnus Nossen, from Norway

I was shown this print out of a rhythm strip from a patient on telemetry. I was asked what's going on? There are four leads shown (II, dV2, V1 and V5). The three images beneath one and other are consecutive rhythm strips each 10 seconds long. The patient was on telemetry due to acute MI. Can you correctly describe what's going on in this interesting rhythm strip?

The tracing begins with sinus rhythm with a right bundle branch block pattern. P-waves are not easily discerned in the first beats, but they are present superimposed on the T wave of the preceding QRS. The P wave superimposed on top of the third visible T wave does not conduct (therefore, a pause follows that P-wave). 

The following QRS can clearly be seen with a preceding sinus P-wave, and the next beats shows clear second degree AV block type I (Wenckebach) with a gradual prolongation of the PR interval. The P-waves can be seen "moving" closer and closer to and encroaching on the T-wave making the T waves look more pointy.

Then after three sinus beats conducted fully over the AV node a beat with a different QRS morphology compared to both the preceding and following beats takes place. This beat is a fusion beat from the supraventricular sinus beat and an emerging AIVR. 

The AIVR then gradually takes over and the sinus P-waves again becomes more apparent as the isorhythmic AV dissociation is ongoing. Eventually sinus P waves are not visible. (buried in the QRS). 

This continues for a while until there is emergence of retrograde VA conduction (4th beat on the 2nd tracing) with negative P waves being visible in the ST segment of the AIVR complexes. Interestingly the retrograde VA conduction does not show any sign of delayed conduction. This is a good example that Wenckebach block in the antegrade direction doesn't mean there has to be a delayed impulse transmission in the retrograde direction. Eventually the AIVR stops with sinus rhythm and second degree AV block type I resuming. 

For a more comprehensive and illustrative analysis please see Dr Grauer's comment below.

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MY Comment, by KEN GRAUER, MD (10/3/2023):

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I was intrigued by today's tracings — which consisted of 3 4-lead rhythm strips, obtained sequentially from a patient being monitored on telemetry for acute MI.

• I agree completely with Dr. Nossen's interpretation of this interesting but complex rhythm. 
• I thought it would be informative to break down today's rhythm — with my goal of illustrating a series of helpful techniques applicable to time-efficient interpretation of any complex arrhythmia.

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My Approach to Today's Rhythm:

I'll begin by review of some EASY things to do — that greatly facilitate interpretation.

• Make SURE that your patient is hemodynamamically stable! If your patient is hypotensive, not alert, or otherwise severely symptomatic — Treat the patient before addressing intricacies of the rhythm. 
• Today's patient was hemodynamically stable (and not even Dr. Nossen's patient) — but being on telemetry with a recent MI justified wanting to know what this unusual rhythm was.
• 
  
• IF there is more than a single feature in a given arrhythmia — Start with EASIER elements (and save more difficult aspects of the rhythm for last). You'll often be pleasantly surprised that explanation of more difficult features often becomes evident once you've interpreted some of the less difficult features.
• 
  
• Number the beats. This way you and your colleagues can be certain you are all talking about the same part(s) of the tracing. (This also saves YOU time — because you have a numbered reference point to quickly locate that part of the tracing you were looking at).
• 
  
• Now — Label the obvious P waves.

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For clarity in Figure-1 — I've reproduced and labeled the 3 rhythm strips in today's case, which I have labeled A, B and C.

• NOTE: The 4 monitoring leads used in these 3 tracings are Leads II, dV2, V1 and V5 — where dV2 is a derived lead reconstructed using data from a 6-electrode recording system.

Take another LOOK at the 3 rhythm strips in Figure-1:

QUESTION:

• How would YOU begin to assess today's complex arrhythmia?
• Can you appreciate WHY I labeled these 3 tracings in the way that I did?

Figure-1: I've numbered the beats and have labeled today's tracing.

MY Approach:

Note that I've labeled all of the atrial activity that I can clearly see in today's tracings with colored arrows.

• The feature in today's tracings that most "caught my eye" — were the PINK arrow P waves. The KEY to interpreting complex arrhythmias often resides in "finding a break in the rhythm" — and both of the PINK arrow P waves in Figure-1 end a short pause (ie, the "break" in Rhythms A and C). Both of these PINK arrow P waves clearly identify 2 beats that we know are conducting, albeit with a long PR interval.
• 
  
• I next labeled all sinus P waves (RED arrows).

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Helpful Reminder: Using calipers greatly facilitates (and speeds up) the interpretation of any complex arrhythmia! 

• To Emphasize: I am not recommending that you stop to pull out calipers if the patient in front of you is "crashing". Always treat the patient! But IF your patient is hemodynamically stable and you have a "moment" of time (and you really want to find out what the rhythm is) — using calipers will speed up and tremendously facilitate your interpretation!

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Focusing First on Rhythm A:

• Note that the underlying atrial rhythm in A is surprisingly regular (Regularly spaced PINK and RED arrows).
• As noted above — beat #3 in A is conducting, albeit with 1st-degree AV block.
• Although we do not have access to a 12-lead ECG — QRS morphology in this conducted beat #3 appears consistent with RBBB (widened QRS with an RsR' in lead V1 — and wide terminal S wave in lateral lead V5) — and, with LAHB (rS complex in lead II, with predominant negativity in this lead).
• This RBBB/LAHB morphology persists for the next 2 beats (ie, beats #4 and 5 in A) — but QRS morphology then changes beginning with beat #6.
• 
  
• Use of calipers facilitates recognizing that regular P waves continue throughout rhythm A, producing an extra "spike" in the T waves of beats #4,5,6,7,8.
• 
  
• PROOF — that this T wave spiking is the result of hidden, on-time P waves — is forthcoming toward the end of the tracing, when these regular P waves begin to separate from the T of preceding beats #10 and 11.
• 
  
• KEY Point: The PR interval is increasing as we move from beat #3 — to beat #4 — to beat #5 in Rhythm A. The fact that the P wave rhythm in A is regular — and the PR interval just before the pause (ie, the PR interval before beat #2) — is clearly longer than the PR interval at the end of the pause (the PR interval before beat #3) — defines today's rhythm (at least for beats #1-thru-5 in A) — as 2nd-degree AV block, Mobitz Type I (ie, AV Wenckebach).
• 
  
• BUT — Note how short the PR interval becomes at the end of Rhythm A (ie, the RED-WHITE arrows before beats #11 and 12). This is no longer AV Wenckebach. Instead — it must be that the change in QRS morphology that began with beat #6 represents an independent rhythm! 
• A look at QRS morphology in left-sided lead I (which now shows a wider and virtually all negative fragmented complex) — and in right-sided lead V1 (which is no longer triphasic — but instead manifests a monophasic R wave) — tells us this is a ventricular rhythm. Given the rate of beats #7-thru-12 (of ~85/minute) — and the lack of conducting P waves — beats #7-thru-12 must represent AIVR (Accelerated IdioVentricular Rhythm).
• 
  
• PROOF — that beats #7-thru-12 in Rhythm A are ventricular in etiology ( = AIVR) — is forthcoming from the fact that beat #6 is a Fusion Beat (ie, intermediate in QRS and ST-T wave morphology between beat #5 and beat #7 — as is most easily appreciated in lead V1).
• NOTE: The reason the AIVR rhythm is able to "take over" control of the rhythm — is that the rate of this accelerated ventricular rhythm (ie, ~85/minute) — is slightly faster than the rate of the underlying sinus P waves.
• 
  
• Clinical NOTE: Although we know only that today's patient is being monitored for an acute MI — the finding of bifascicular block (RBBB/LAHB) + 1st-degree AV block + 2nd-degree AV Block (AV Wenckebach) + AIVR are all potentially consistent with this diagnosis of acute MI (with need for clinical correlation to determine if a pacemaker will be needed).
• The "good news" — is that development of AIVR in the setting of OMI is potentially a "good" sign, as it may indicate reperfusion of the "culprit" artery (For more on AIVR — See My Comment under Point #2 at the bottom of the page in the April 8, 2022 post in Dr. Smith's ECG Blog).

Focusing Next on Rhythm B:

• The same QRS morphology that we identified as consistent with AIVR beginning with beat #7 in A — continues throughout the 11 beats that we see in Rhythm B. Thus, AIVR is seen throughout Rhythm B.
• That said — beginning with beat #4 — retrograde P waves are seen (YELLOW arrows over the negative deflection in the ST segment of beats #4-thru-11).
• Note that there was no such negative deflection during AIVR in A — nor were there retrograde P waves in the ST segment of beats #1 and 2 in B (I put a Question Mark over the ST segment of beat #3 in B — because I wasn't sure if there was or was not any retrograde P wave).

Finishing with Rhythm C:

• AIVR (with retrograde P waves) — continues for the first 7 beats in Rhythm C. Then it stops. I do not know why retrograde conduction from AIVR stops at this point (since there has been no change in the RP' interval) — but presumably, the fact that there is no more retrograde conduction after beat #7 in C (ie, No more "resetting" the SA node because of retrograde atrial conduction) — allows the SA Node enough time to recover, and produce another sinus P wave (the PINK arrow before in C).
• 
  
• Recapture of the rhythm by the PINK arrow P wave before beat #8 now begins another Wenckebach cycle (Note the QRS complexes of beats #8,9,10 in C resume RBBB/LAHB morphology — and the PR interval is once again increasing as we move from beat #8 — to #9 — to #10).

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LADDERGRAM Illustration of Today's Rhythm: 

• For clarity in Figures-2, -3, and -4 — I've drawn laddergrams to illustrate my explanation of the mechanism of today's rhythm.

Figure-2: Rhythm A begins with the last 2 beats of a Wenckebach cycle ( = beats #1,2) — with the P wave that is non-conducted being hidden in the spike of the T wave of beat #2. The PINK arrow before beat #3 begins a new Wenckebach cycle.
=  =  =  =  =
NOTE: Beats #1-thru-5 are conducted with RBBB/LAHB (with the dotted line in the Ventricular Tier for these beats representing the conduction defect in these supraventricular beats).
=  =  =  =  =
Beats #7-thru-12 represent "usurpation" of the rhythm by an AIVR rhythm that is slightly faster than the underlying sinus P wave rhythm. Retrograde conduction part-way through the AV Nodal Tier prevents the slightly slower sinus P waves from conducting to the ventricles during the time AIVR has control of the rhythm.
=  =  =  =  =
The laddergram illustrates that beat #6 is a Fusion beat.
=  =  =  =  = 

Figure-3: All 11 beats in Rhythm B represent AIVR. YELLOW arrows show that retrograde conduction begins with beat #4. (Presumably — beats #1,2,3 conducted retrograde into the AV Nodal Tier — but did not conduct all the way back to the atria).

Figure-4: The first 7 beats in rhythm C again represent AIVR. For a reason unknown to me — retrograde conduction from ventricular beats (YELLOW arrows) stops with beat #7. Presumably — this allows the SA Node enough time to "recover", and therefore put out a spontaneous sinus beat (PINK arrow) after the brief pause between beats #7-8. Rhythm C concludes with another Wenckebach cycle (as the PR interval is seen to progressively increase as we move from beat #8 — to #9 — to #10).