An Intriguing Rhythm: Who Belongs to Whom?

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MY Comment, by KEN GRAUER, MD (6/26/2023): 

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The tracing in Figure-1 was sent to me — without the benefit of any history. How would YOU interpret this tracing?

Figure-1: The initial ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Initial Thoughts:

My attention was immediately drawn to the long lead rhythm strip in Figure-1. A lot is going on ...

• The rhythm is clearly irregular. 
• All QRS complexes in this tracing are narrow — so the rhythm is supraventricular.
• P waves are present — and there is more than 1 P wave shape.
• Some P waves are too close to the QRS to conduct. That said, on initial inspection — it's hard to determine, "Who belongs to whom?" 

My Approach:

When confronted with a challenging tracing consisting of multiple components, such as we see in Figure-1 — I favor starting with parts of the rhythm that I am fairly certain about.

• I find that the simple act of labeling atrial activity is incredibly helpful. In Figure-2 — I've added RED arrows to the long lead rhythm strip in today's initial tracing to highlight what appears to be sinus P waves.
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• Now look at each of these RED-arrow P waves. Which of these P waves do we know will be unable to conduct (because the PR interval until the next QRS complex is just too short?).
• It should be apparent that the PR interval in front of beats #3,4; 9; 14 and 15 is too short to conduct! Since the QRS complex for each of these non-sinus-conducted beats is narrow — beats #3,4; 9; 14 and 15 must be junctional beats.
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• PEARL #1: IF the R-R interval that contains an on-time P wave is shorter-than-expected — this is a strong clue that suggests the reason the R-R interval unexpectedly shortens, is that this P wave is being conducted. For example, in Figure-2 — Notice how the R-R interval clearly shortens before beats #6 and 11 (ie, from at least 780 msec. — to 710 and 720 msec., respectively). This strongly suggests that beats #7 and 11 are sinus-conducted!
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• PEARL #2: The R-R interval before each of the junctional beats in Figure-2 is between 780-to-800 msec. This corresponds to a junctional rate of ~80-85/minute — which means that there is an accelerated junctional rhythm in today's tracing, that is periodically interrupted when the rate of SA Node discharge increases enough to "take over" command of the rhythm (ie, which happens with sinus-conducted beats #6 and 11).
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• NOTE: It is unclear if beat #5 is junctional or sinus-conducted. Although I suspect beat #5 is sinus-conducted (because it manifests the same PR interval as beat #6, which we know is sinus-conducted) — this does not matter clinically, because regardless of the etiology of beat #5 — today's tracing still manifests an accelerated junctional rhythm that is periodically interrupted by sinus-conducted beats.

Figure-2: I've added RED arrows to the long lead II rhythm strip from Figure-1 to highlight sinus P waves. Which of these P waves are unable to conduct?

What About the Remaining Beats?

In Figure-3 — I've added YELLOW arrows over each of the remaining P waves. These YELLOW arrows highlight the 5 PACs (Premature Atrial Contractions) in today's tracing — which are recognized as PACs, because beats #2; 7,8; and 12,13 all occur earlier-than-expected — and — are preceded by P waves with an obviously different morphology compared to the sinus P waves under the RED arrows.

• BOTTOM Line: Even without calipers — We've been able to rapidly establish that today's rhythm consists of an underlying sinus rhythm — that is punctuated by PACs, and intermittently interrupted by an accelerated junctional rhythm.
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• PEARL #3: There is transient AV dissociation in today's tracing — because a number of the sinus P waves that we see in Figure-3 are not related to neighboring QRS complexes. As I discuss in detail in My Comment at the bottom of the page in the May 24, 2020 post in Dr. Smith's ECG Blog — AV dissociation is not the same as AV block. The fact that none of the P waves in Figure-3 that fail to conduct have a realistic chance to conduct (because the PR interval before beats #3,4; 9; and 14,15 is too short to conduct!) — suggests that there may be no AV block at all in today's tracing.
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• PEARL #4: The fact that the sinus P wave rate and the accelerated junctional rate in today's tracing is very similar, such that control of the rhythm alternates between the SA and AV nodes — suggests that today's rhythm represents isorhythmic AV dissociation (this concept also discussed more in my May 24, 2020 comment).
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• PEARL #5: Accelerated junctional rhythms are not common in adults. Most occurrences are associated with events such as sepsis, shock, recent infarction, post-operative state, electrolyte disturbance — or "sick patient". This led me to wonder what the clinical situation in today's case might be!

Figure-3: I've added YELLOW arrows to Figure-2 to highlight that morphology for the P waves that precede beats #2; 7,8; and 12,13 is different than P wave morphology highlighted by each of the RED arrows.

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Clinical Follow-Up in Today's CASE:

It turns out that the clinical history in today's case was that of a patient with overuse of illicit drugs — who was assaulted, suffering severe trauma. Seizure activity was witnessed at the scene — and head CT scan showed a small bleed.

• The patient's severe medical condition therefore provided a clinical setting potentially predisposing to the above described arrhythmia (with the accelerated junctional rhythm).

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LADDERGRAM Illustration:

A picture is worth 1,000 words. I've drawn in Figure-4 my proposed laddergram illustration for today's case. (IF interested in learning more on how to read and/or draw laddergrams — CLICK HERE).

• According to the laddergram — beats #5,6 and 11 are sinus-conducted (showing normal conduction through the Atria, A-V Nodal and Ventricular Tiers).
• Beats #2; 7,8; and 12,13 are PACs (showing different P wave morphology, as highlighted by YELLOW arrows). PACs originate from a non-sinus node site in the atria — and then conduct normally to the ventricles after passing through the AV node.
• Beats #3,4; 9,10; 14,15 represent an accelerated junctional focus (RED circles showing origin of impulse formation within the AV Nodal Tier). Retrograde conduction from these junctional beats (dotted RED lines) blocks forward conduction of the on-time sinus P waves with a short PR interval.

Figure-4: My proposed laddergram for today's rhythm.

One More Tracing in Today's CASE:

I was able to obtain an additional 12-lead ECG and rhythm strip from today's case — which is shown in Figure-5. This additional tracing (TOP panel in Figure-5) — is the original ECG that was done in the ED on today's patient. 

CHALLENGE Question:

• Why do we not see the accelerated junctional rhythm in ECG #2?

Figure-5: Compare the rhythm in the initial ECG obtained in the ED (TOP) — with the BOTTOM rhythm strip, that I showed in Figure-3. Why do we not see the accelerated junctional in ECG #2?

ANSWER to the Challenge Question:

Note that the rhythm in ECG #2 is sinus tachycardia at ~100-105/minute. Thus, the atrial rate in ECG #2 is faster than the atrial rate of sinus P waves in the BOTTOM rhythm strip.

• Although the junctional rhythm that we saw intermittently in today's tracing was accelerated (to ~80-85/minute) — this junctional rhythm never attains as rapid a rate as the sinus tachycardia seen in ECG #2. As a result — we don't see the junctional rhythm in ECG #2.
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• P.S.: I conclude today's case by commenting on the other leads in the 12-lead tracings. QRS voltage looked increased in both tracings (though depending on the age of the patient — this may not necessarily qualify as voltage for LVH). The frontal plane axis was vertical in ECG #2 (and slightly rightward in ECG #1). There are nonspecific ST-T wave changes — but no indication of an acute cardiac event.