EXTRA COPY — ECG Blog #546: Which Beat is Key?— EXTRA COPY

 

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. This figure shows the presence of 2 successive "echo" beats.

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-12 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 this 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 the cause (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-12) — 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-12 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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