ECG Blog #546 — Which Beat is Key?

 

I was sent this tracing ...

• The patient whose ECG is shown in Figure-1 is a man in his 60s with CKD (Chronic Kidney Disease) — who presented with "dizziness".

QUESTIONS:

It is immediately understandable why this patient may have dizziness — as the cardiac rhythm is obviously abnormal. 

• How to begin to assess this rhythm?
• HINT: Which one beat is KEY for our understanding of the etiology of the rhythm?

• And — What might be causing this rhythm?

Figure-1: I was sent this tracing — knowing only that it was from a man in his 60s with CKD. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts:

I fully acknowledge that it took me a moment to know how to proceed for assessing this rhythm. That's because the rhythm is clearly irregular with a number of different elements.

• PEARL #1: When you encounter a rhythm with multiple different elements (some of which are clearly more complex than others) — Start with the EASIER part(s)!
• Save those parts of the tracing that are more challenging to interpret for later ...

I was initially uncertain as to what I was seeing in Figure-1.

• I focused my attention on the long lead II rhythm strip. I highlight in Figure-2 the one beat that caught my "eye".

Figure-2: The KEY to today's rhythm is beat #2.

The KEY to Today's Rhythm ...

Beat #2 is the KEY to today's rhythm.

• Beat #2 is a sinus-conducted beat (RED arrow in the long lead II rhythm strip showing an upright sinus P wave with normal PR interval preceding this beat). I've highlighted this beat #2 within a RED rectangle in the picture of lead II from the 12-lead tracing.
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• PEARL #2: The reason beat #2 is so essential to understanding today's rhythm — is that this is the only beat that shows us what a "normal T wave" looks like!

QUESTION:

• Does PEARL #2 explain why the ST-T waves of beats #1,3,4,6,8,9 and 11 look different than the ST-T wave of beat #2?
• HINT: Check out Figure-3 ...

Figure-3: What is suggested by the YELLOW arrows?

ANSWER:

Beat #2 is the only sinus-conducted beat in this tracing.

• All beats in Figure-3 (with the exception of beat #10) manifest a narrow QRS complex. This confirms that all of these narrow beats are supraventricular. Since no P wave precedes beats #1,3,6,8,9 and 11 — these must be junctional escape beats.
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• PEARL #3 (Advanced concept!): Although the shape of the junctional escape beats looks similar to the shape of sinus-conducted beat #2 — the escape beats following each of the short pauses in the long lead II rhythm strip ( = beats #1,3,6,8,9,11) all appear to be slightly taller than the one sinus-conducted beat #2.
• On occasion in complex arrhythmias — it may be difficult to tell if one or more beats are sinus conducted vs escape beats from the AV Node. Awareness of a slight difference in appearance in the QRS complex of escape beats in such a tracing may provide a subtle clue as to whether or not these beats are being conducted. (An example of this advanced concept in which awareness of a slightly different QRS shape immediately tells you which beats are "escape" vs conducted beats — is seen in ECG Blog #63).

PEARL #4: The most plausible reason for the negative deflections that are highlighted by YELLOW arrows in the long lead II rhythm strip of Figure-3 — is that these negative deflections are the result of retrograde P waves that arise from the junctional escape beats.

• Note that the RP' interval (ie, the distance from the R wave of beats #1,3,6,8,9,11 in Figure-3 — until the negative deflection that follows) is the same!
• Note also that another QRS complex follows the retrograde P waves of beats #3, 6, 8 and 9 to produce beats #4,7, and 10. These beats #4,7 and 10 are called "Echo" beats (because rather than conducting down to the ventricles, the atrial impulses prior to these beats "turned around" and were directed back to the atria).

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Laddergram Illustration:

Today's rhythm is admittedly complex. That said — it offers a superb example of how drawing a laddergram serves to facilitate our understanding the mechanism of an "escape-capture" rhythm in which "capture" is the result of retrograde P waves that produce "echo" beats.

• I guarantee that this complex mechanism will become clearer by following my sequential derivation of today's laddergram — beginning with the legend in Figure-4.

Figure-4: As discussed above — beat #2 is the only sinus-conducted beat in today's tracing (as it is the only beat that is preceded by an upright P wave in this long lead II rhythm strip).

Figure-5: The next step in drawing today's laddergram — is to fill in the Ventricular Tier. I do this by drawing in near-vertical RED arrows to represent rapid conduction through the ventricles of narrow beats #1,3,4,5,6,7,8,9, and 11 (I'll explain beat #10 momentarily).
The large BLUE arrows in this figure schematically show that I timed each of the RED arrows in the Ventricular Tier to the occurrence of these beats in the rhythm strip.

Figure-6: As discussed in my explanation before beginning to draw this laddergram — We know that beats #1,3,6,8,9,11 are all junctional "escape" beats, because each of these beats follows a similar duration short pause, and none of these beats are preceded by P waves. I represent the AV Nodal origin of these escape beats by RED circles placed within the AV Nodal Tier.

Figure-7: I next represent the timing retrograde P waves by dotted BLUE lines that schematically show conduction back to the atria.

Figure-8: Conduction through the AV Node is slower than conduction through the atria. I schematically show this by the angled dotted BLUE lines that I've added in Figure-8 to represent retrograde conduction back to the atria from each of the junctional escape beats. This leaves me with having to explain how beats #4,5,7 and 10 come about! (which I do in Figure-9). 

Figure-9: The most plausible way to explain beats #4,7 and 10 — is that these must be "echo" beats, in which the retrograde impulse arising from junctional beats #3,6 and 9, turns around to produce forward conduction of beats #4,7,10. But this now leaves me having to explain how beat #5 comes about! (which I do in Figure-10).

Figure-10: Beat #5 must be another "echo" beat. And the only way I can explain how this might come about — is if during the forward path of conduction through the AV node on the way to produce beat #4 — there once again is retrograde conduction back to the atria (dotted BLUE line in this Figure-10).

Figure-11: The solid BLUE line that I've now added in this Figure-11 completes the laddergram. Thus, this figure shows the presence of 2 successive "echo" beats ( = beats #4 and 5).
Note in this Figure-11 that retrograde conduction back to the atria, followed by forward conduction to produce another QRS complex does not always occur. The reason why it sometimes does, and sometimes does not occur is not clear.

Figure-12: This is the completed laddergram. All that remains is to explain why the QRS of beat #10 is wide? (which I do in today's Figure-13 by going back to the original 12-lead tracing).

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Putting It All Together: 

Whenever we encounter a complex arrhythmia — it's important to always go back to the clinical situation and the original 12-lead tracing.

• The only history we were provided with for today's patient — is that this man in his 60s presented with "dizziness" — and that he has a history of CKD (Chronic Kidney Disease).
• Armed with awareness that hyperkalemia is notorious for producing unusual arrhythmias that are often featured by bradycardia and unusual forms of AV conduction disturbances — checking the serum K+ level is essential in today's case.
• Although subtle — I thought a number of chest lead T waves to be more peaked than expected, leading me to suspect some degree of hyperkalemia as a contributing factor. (Unfortunately — I was unable to find out the serum K+ level in today's case).
• Attention to the ST segments in leads V3,V4,V5 (within the BLUE rectangle in Figure-13) — suggests ST segment straightening and some ST depression. Perhaps this represents a recent or acute posterior OMI that may be responsible for the bradycardia with junctional escape beats? (Unfortunately — I was unable to obtain follow-up regarding this possibility in today's case).
• Finally — If we look in Figure-13 at the simultaneously-recorded chest leads for beat #10 — it becomes apparent that this beat is conducted with RBBB aberration (rsR' for beat #10 in lead V1 — with a wide terminal S wave in lead V6 for beat #10). This explains why beat #10 in the long lead II rhythm strip looks different and is slightly wider than other beats in this tracing. 
• It also explains why beat #7 in the long lead II rhythm strip is shorter than other beats in this tracing (Beat #7 is being conducted with incomplete RBBB aberration — as suggested by the rSr' morphology of beat #7 in simultaneously-recorded lead V1).  

Figure-13: Returning to today's original 12-lead tracing (See text).

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Acknowledgment: My appreciation to 林柏志 (from Taiwan) and M Shah (from Srinagar, India) — for allowing me to use this case and this tracing.
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