Wednesday, July 19, 2017

What is this rhythm?

Unfortunately, I remember who sent this EKG to me, or the clinical information, but it is a very interesting tracing.

What is it?

Below is an annotated version:

There is isorhythmic dissociation (see below), with escape beats in complexes 1-4 and 9-14.  In beat 4, you can see a bit of a P-wave, and then 5 thru 8 have sinus rhythm.

Aside: "Isorhythmic dissociation:" the P-wave just happens to arrive at the same time as the escape beat, so that the P-wave and QRS are dissociated even though they are happening about the same time ("isorhythmic").   ........AV Dissociation does not always mean AV Block!! There can be dissociation without block, as here.  See more posts below.

Whenever the beat is fully sinus, the QRS is normal.

Whenever it is an escape beat, the QRS has a tall R-wave, along with an inferior and right axis, similar to a left posterior fascicular block (LPFB).  

But since there is no LPFB in sinus rhythm, the escape cannot be coming from the AV node; if it were, the QRS would not be different.  [The QRS in sinus rhythm and junctional rhythm are the same, because sinus rhythm goes through the AV node.]

The escape must be originating from a more inferior location that activates the left anterior fascicle preferentially, thus mimicking an LPFB.  

1. This confused me, because any beat that initiates in the left anterior fascicle should also have a RBBB morphology, which is not present here.

2. Also: why does the sinus rhythm come and go?

I asked my very smart friends Ken Grauer and Christopher Watford for their opinions.

Ken gave this great explanation, also referring to insights by Christopher:

"It’s a beautiful example of AV dissociation beginning with beat #4 where the P wave comes right at the onset of the QRS. There is underlying marked sinus arrhythmia with accelerated idioventricular rhythm (AIVR) at a rate of ~80/minute (arising from a septal focus) — such that when the sinus rate is faster (beats #5-thru-8), we have sinus rhythm — and when we have slight slowing of the sinus arrhythmia rate to below ~80/minute — then the accelerated septal focus takes over (beginning with beat #9). Note the sinus P wave peaking back again at the end of the tracing (at the onset of beat #14). So, the mechanism for AV dissociation here is a combination of “default” (when the sinus rate slows to below 80/minute) + “usurpation” (due to the slightly accelerated ventricular septal focus.

"The QRS morphology as Steve and Christopher have described is fairly narrow with left posterior hemiblock (LPHB) morphology, but lacking the RBBB characteristic that is usually seen in V1 with the more typical fascicular ventricular tachycardias...... I imagine, depending on where in the septal the focus is, that you might get a hemiblock pattern with narrow QRS and no RBBB (as we see here)."

Here are more posts on AV dissociation and AV block

AV Dissociation Lecture by K. Wang (28 minutes)

A Mystery Rhythm, Explained by K. Wang's Ladder Diagram.

Atrial Flutter. What else?? (AV dissociation with block)


  1. Is there not also De Winter waves?

    1. There is ST depression, probably ischemia, maybe posterior MI, but they are not deWinter's T-waves which are not associated with such high QRS (R-wave especially) voltage. Take a look at true de Winter's T-waves:

  2. V3, V4 looking like De Winter, probably that burried P Waves causing it?

    1. Martin,
      De Winter's T-waves do not look like that. see the link at the above comment for many cases of de Winter's T-waves, which have lower S- and R-wave voltage in V2-V4. This is ischemia, likely posterior.

  3. Why you considered the accelerated rythm at 80 bpm as ventricular and no supraventricular focus ?
    thank you for responding

    1. I think it is difficult to be certain as to what the site of the escape rhythm is. I agree with you that the escape rhythm is NOT wide — but it DOES look very different than the sinus-conducted beats ( = beats #5-8) — and the morphology of those sinus-conducted beats is not characteristic of any hemiblock pattern (ie, I would have expected some known form of conduction block, such as LAHB if beats #5-8 were aberrantly conducted). This led me to believe that the escape focus is unlikely to be from the AV node (since QRS morphology is so different from the sinus-conducted beats). As explained above (Comments also by Christopher Watford & Dr. Smith) — lack of rbbb morphology made it unlikely for the escape focus to be fascicular — so what’s left is perhaps somewhere in the His … (ie, somewhere in the septum). All of this said — NONE of this is really clinically important. We don’t know the history — but it is the HISTORY that will tell us IF this is or is not a rhythm to be concerned about (with an excellent chance that it will turn out to be benign pending this history).


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