ECG Blog #362 — Funny Rhythm in a 14-Year Old ...

A 14-year old boy was forced to do numerous squats as a “punishment”. He woke up the next day with severe pain in his legs — and presented to the ED (Emergency Department). 

• Renal function and serum electrolytes were normal — but CK was markedly elevated, leading to a diagnosis of Rhabdomyolysis. 
• Cardiac monitoring revealed a series of interesting (albeit asymptomatic) rhythms, 2 of which are shown below in Figure-1.

QUESTIONS:

• How would YOU interpret Rhythms A and B in Figure-1?
• Is there any evidence of AV block?
• What treatment is indicated?

Figure-1: Two of the lead II rhythm strips obtained on a 14-year old boy with acute rhabdomyolysis. Variations of this “theme” were seen on telemetry over the several days that this boy was in the hospital. How would YOU interpret these rhythms?

MY Approach to the Rhythm in Figure-1:

As always — I favor a systematic approach to rhythm interpretation — with use of the Ps, Qs, 3R Memory Aid (See ECG Blog #185 for more on the Ps, Qs, 3Rs). 

• One need not assess the Ps, Qs, 3Rs in any particular order — as long as you always remember to assess each of these 5 parameters. As a result — I usually start with whichever of the parameters is easiest to asses. For example, in Figure-1 — it is clear that the rhythm is not Regular (albeit parts of the rhythm are almost regular) — and — the coupling interval of the early beats in B is similar.
• The QRS complex in all beats seen in Figure-1 is narrow — therefore the rhythm is supraventricular.
• At least some P waves are present! That said — Assessment of the role that P waves play in today’s rhythm is the most challenging aspect of this arrhythmia.

QUESTIONS:

• Do YOU Think any of the P waves in Figure-1 are conducting to the ventricles?
• If so — Is conduction forward or backward (ie, retrograde)? That is — Is the electrical impulse moving from the SA node — then through the Atria, and finally on to reach the ventricles? — OR — Is conduction “backward” (ie, retrograde from the AV node, to return to the atria)?
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• HINT: What is the significance of the colors (RED and YELLOW) that I have chosen for atrial activity in Figure-2?

PEARL #1: As I’ve suggested many times in this ECG Blog — the most accurate (and time-efficient) way to answer the above questions is with use of calipers. Doing so allows you to instantly determine the regularity of forward-conducting P waves vs retrograde P waves.

• Without using calipers — I would have spent a lot of time guessing whether regular sinus P waves are (or are not) present. With calipers — it took me less than 5 seconds to establish the atrial rhythm!

Figure-2: I’ve added RED, PINK and YELLOW arrows to Figure-1 to highlight the relationship of P waves in these tracings to neighboring QRS complexes (See text).

MY Step-By-Step Approach in Figure-2:

The most challenging aspect of interpreting today’s rhythm — is figuring out what’s happening with atrial activity. Below is my sequential step-by-step approach:

• I first looked in front of at all 28 beats on both tracings in Figure-1 — to see IF there were any P waves that I felt were definitely conducting. The BEST example of a P wave that I could be certain was conducting is in Rhythm A. This is the RED arrow upright P wave that appears before beat #8 with a normal PR interval ( = a PR interval = 0.18 second).
• I saw a number of other places on these 2 rhythm strips in which it looked like upright P waves might be present — although it was difficult to be certain of this. So — IF there was indeed an underlying regular atrial rhythm — the best way to look for this would be by setting my calipers to the P-P interval between 2 deflections that I know represents 2 consecutive P waves. In Figure-2 — the 2nd RED arrow in Rhythm A definitely looks like it is highlighting a P wave that occurs just after the QRS of beat #9.
• Setting my calipers to the P-P interval between these first 2 RED arrows in Rhythm A — allowed me to “walk out” a fairly regular underlying sinus rhythm (Sinus arrhythmia) in Rhythm B! Some of the P waves under these RED arrows in Rhythm B are more easily seen than others (ie, some P waves are hidden within the ST-T waves — but something is seen with each "walk" of the calipers). Given that P wave deflections can be seen under all of the RED arrows in Rhythm B — it seems logical that an “on-time” P wave is also hiding within the QRS of beat #4 (and the PINK arrow in Rhythm B suggests that we do see a tiny deflection at the very end of this QRS complex — which almost certainly represents the final portion of the "hidden" on-time P wave).

What remains to interpret — is the nature of atrial activity under the YELLOW arrows in Rhythm A. 

• Since the P waves under these YELLOW arrows are negative in this lead II monitoring lead — these are retrograde (backward conducting) P waves. This is logical — since no P waves precede beats #2-thru-6 nor beats #10-thru-12 in Rhythm A — which defines these 8 QRS complexes as being junctional beats!

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Laddergram Illustration:

At this point — it might be easiest to explain details of the 2 rhythms in today's case by use of laddergrams. 

• NOTE: For readers less familiar with laddergrams — I’ll emphasize that it is EASY to learn how to read laddergrams. I bet that all it will take to become comfortable reading laddergrams  — is to review the explanations below for the 2 laddergrams from today's case.
• Learning to draw laddergrams takes more time and practice. For those interested — I review the basics for reading and/or drawing laddergrams in ECG Blog #188 (This blog post features links to more than 60 laddergrams that I’ve drawn for this ECG Blog over the years = more-than-enough practice to become comfortable drawing your own laddergrams!).

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Laddergrams for Today's Case:

In Figure-3 — I've drawn a laddergram to illustrate events in Rhythm A:

• It is often easiest to begin with a sinus-conducted beat. According to the laddergram in Figure-3 — there is only 1 sinus-conducted beat in this tracing, which is beat #8.
• We established above that beats #2-thru-6 and #10-thru-12 in Rhythm A are junctional beats — because no P wave precedes these beats — and the QRS complex of these beats is narrow (and looks identical to the QRS complex of sinus-conducted beat #8).
• We have also established that the negative deflections after these junctional beats represent P waves from retrograde conduction (which is to be expected with junctional beats — that virtually always conduct at least partially back toward the atria).
• RED circles within the AV Nodal Tier in Figure-3 — represent the imagined point origin of these junctional beats. Note that each of these junctional beats conducts both in the forward direction (ie, down through the ventricles) — as well as retrograde (the dotted lines that return to the atria).
• Beats #2-thru-6 conduct all the way back through the atria, to produce the negative deflections seen shortly after the QRS (ie, the YELLOW arrows that appear after beats #2-thru-6). 
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• KEY Point: Since the sinus P wave before beat #8 is able to conduct to the ventricles — it must be that junctional beat #7 does not conduct all the way back through the atria. As a result — whereas the 5 retrograde P waves (YELLOW arrows) before beat #7 do make it all the way back through the atria (thereby preventing the SA Node from putting out its next sinus impulse during this time) — the SA Node then has a moment to recover (because of the failed retrograde conduction of beat #7). After a brief pause — the recovered SA Node begins to fire again, putting out the sinus P wave that appears before beat #8 (RED arrow) — which results in sinus-conducted beat #8.
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• Advanced Subtlety: Note that I drew the dotted line representing retrograde conduction from junctional beat #7, with a slightly greater angle — to illustrate why retrograde conduction from beat #7 fails to conduct all the way back through the atria.

Continuing to explain the laddergram in Figure-3:

• Beat #8 in Rhythm A is sinus-conducted.
• There is no P wave before beat #9 — so this is another junctional beat.
• The P wave that occurs just after the QRS of beat #9 (3rd RED arrow in Rhythm A) is “on-time” — but this sinus P wave is not able to conduct to the ventricles because retrograde conduction from junctional beat #9 “blocks” it.
• There follows 3 junctional beats (beats #10,11,12) that do conduct all the way back through the atria to produce retrograde P waves (the final 3 YELLOW arrows in Rhythm A).
• Beat #13 is also junctional (because no P wave precedes it) — but the T wave of beat #13 is normal, because there is no retrograde P wave after this beat. In the same way that failure of retrograde conduction from junctional beat #7 allowed the RED arrow sinus P wave in front of beat #8 to conduct — the final sinus P wave in Rhythm A (ie, the last RED arrow in Figure-3) most probably would have conducted a sinus beat #14 had Rhythm A continued for 1 more beat.
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• Final Beat: I saved beat #1 in Rhythm A for last. Since no P wave precedes beat #1 — this is a junctional beat. I believe the RED arrow P wave that occurs just after the QRS of beat #1 — acts similar to the sinus P wave that occurs just after beat #9, in that this P wave occurs too soon after beat #1 to conduct.

Figure-3: Laddergram illustration for Rhythm A in today's case.

In Figure-4 — I've drawn a laddergram to illustrate events in Rhythm B:

• The principal difference between the mechanism of Rhythm A vs the mechanism of Rhythm B — is that there is no retrograde conduction in Rhythm B. Instead — RED arrows show that regular sinus P waves continue throughout the entire tracing.
• Most of these sinus P waves are unable to reach the ventricles — because their path is blocked by the underlying junctional rhythm. However — beats #3, 6, 10 and 14 are able to make it through the AV node, to be conducted to the ventricles. The laddergram in Figure-4 — illustrates the timing that is needed to allow this to happen.
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• Advanced Subtlety: Note that the PR interval before beat #6 is slightly longer than the PR interval before the other 3 sinus-conducted beats (ie, beats #3,10,14). The reason for this — is that the P wave before beat #6 occurs closer to the preceding junctional beat — thereby slightly delaying the passage of this P wave through the AV node (which I illustrate with a PINK line showing a slightly greater inclination within the AV Nodal Tier compared to the inclination of the 3 other sinus-conducted beats that manifest shorter PR intervals).

Figure-4: Laddergram illustration for Rhythm B in today's case.

QUESTION:

• Is there AV dissociation in today's rhythm?  And, if so — WHY?

ANSWER:

There is AV dissociation in today's tracing — because many of the on-time P waves in Rhythm B are not related to neighboring QRS complexes (ie, Many of these on-time P waves in Rhythm B are not conducted to the ventricles).

• In Figure-4 — None of the P waves that terminate in a butt-end (seen in the upper portion of the AV Nodal Tier) are conducted to the ventricles.

PEARL #1: Despite the AV dissociation in Rhythm B — there is no evidence of AV block. This is because none of these P waves that failed to conduct in Figure-4 had a chance to conduct!

• The P waves that appear just after the QRS of beats #1, 4, 8 and 12 — all occur too close to the preceding QRS (These P waves fall within the absolute refractory period).
• The P waves that appear just before the QRS of beats #7,11 and 15 — all occur with too short of a PR interval to allow conduction.

PEARL #2: As emphasized often in this ECG Blog — there are 3 Causes of AV dissociation. These are: i) 2nd- or 3rd-degree AV Block (in which one or more P waves that should conduct do not conduct); ii) AV dissociation by "Usurpation" (in which an accelerated junctional rhythm takes over the pacemaking function); and/or, iii) AV dissociation by "Default" (in which slowing = "default" of the SA nodal pacemaker allows a junctional escape pacemaker to emerge).

• We have already ruled out 2nd- or 3rd-degree AV block (Pearl #1).
• To determine if AV dissociation in Figure-4 is the result of "usurpation" or "default" — We need to determine and compare the atrial and junctional rates. The P-P interval of sinus P waves (RED arrows) in Rhythm B is just over 4 large boxes in duration, which corresponds to a sinus P wave rate of ~70/minute.
• The R-R interval of junctional beats in Rhythm B — is 3.4 large boxes in duration, which corresponds to a junctional rate of ~85/minute. Even accounting for the young age of today's patient — this represents an accelerated junctional rhythm.

Putting It All Together:

The principal arrhythmia in today's case is an accelerated junctional rhythm at ~85/minute. This results in AV dissociation by "usurpation". Since the junctional pacemaker is faster than the underlying sinus rhythm (~85/minute — compared to the underlying sinus rate of ~70/minute) — the junctional pacemaker takes over the rhythm, with the exception of occasional sinus P waves that occur at "just the right moment" that they are able to "capture" the ventricles (ie, Beat #8 in Rhythm A — and beats #3,6,10 and 14 in Rhythm B).

• As noted in the history for today's case — this 14-year old boy with acute rhabdomyolysis was asymptomatic from the cardiac rhythm. There is no evidence of AV block in today's tracing. Instead — the primary arrhythmia is a slightly accelerated junctional rhythm that probably is the result of this patient's acute medical problem.
• No specific treatment is needed for this arrhythmia — which will probably resolve as this patient's medical condition improves.

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Acknowledgment: My appreciation to Kianseng Ng and Leong Keen Wai (from Malaysia) 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 — Review of the Ps, Qs, 3R Approach for systematic rhythm interpretation.
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• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 50 laddergram cases — many with step-by-step sequential illustration).
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• ECG Blog #195 — Reviews the phenomenon known as Isorhythmic AV Dissociation.
• ECG Blog #192 — Reviews the 3 Causes of AV Dissociation (and emphasizes why AV Dissociation is not the same thing as Complete AV Block).
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• CLICK HERE — for an instructive review on Rhabdomyolosis.