Why? First degree as part of trifascicular block

[18G]

Most commonly seen with a trifascicular block is a RBBB, LAFB and 1st degree AV block.

How does a 1st degree block qualify as a fascicular block and fall within the "tri"?

 

[Brandon O]

It's a slangy term. Many people understand what it means but many don't; I don't think it's particularly professional to use. I believe Bob Page popularized it in EMS, especially the idea that it should be an alarming finding.

The idea is just that of the three main pathways meaning the right bundle branch and the two left hemifascicles, two of them are blocked and the last is delayed -- implying that it's also in danger, so you should be concerned about a complete block developing. Whether or not that's true is somewhat dubious.

Obviously a true complete "trifascicular block" would be, as you note, 3rd degree. But the answer to your question is that the 1st degree block is the third "block."

 

[18G]

Thanks for the reply. I've read several different sources including the AHA that said what you just said which is trifascicular block is bad terminology and should not be used. With that said, the term still seems to be quite popular.

I get the "tri" means three blocks (obviously) but am thrown by the inclusion of the 1st degree in the trifascicular description. It makes more sense to me to describe it as a bifascicular block with 1st degree.

Also, how does a first degree block put the left posterior fascicle at risk for blockage? You have a RBBB so the RBB isn't conducting. Makes sense. Then you have a LAFB. The anterior part of the left fascicle isn't conducting. Makes sense. And now you have a 1st degree AV block which is at the level of the AV node and doesn't involve the left posterior fascicle. This is where my confusion is.

Is it because the 1st degree may be low enough to eventually affect the tissue of the posterior fascicle?

 

[Brandon O]

Also, how does a first degree block put the left posterior fascicle at risk for blockage? You have a RBBB so the RBB isn't conducting. Makes sense. Then you have a LAFB. The anterior part of the left fascicle isn't conducting. Makes sense. And now you have a 1st degree AV block which is at the level of the AV node and doesn't involve the left posterior fascicle. This is where my confusion is.

Is it because the 1st degree may be low enough to eventually affect the tissue of the posterior fascicle?

The theory, I believe, is that in a true "trifascicular" block, the delayed conduction would be down at the level of the remaining fascicle. If instead the delay was intrinsic to the AV node (i.e. it is anatomically distinct from the other conduction blocks, and perhaps even unrelated), that would not be the true sense of the term, although of course it would be indistinguishable on the ECG.

As far as actual risk I have seen no good literature, but as an isolated finding nobody seems to have any good horror stories. Context is everything, of course. If you're in the presence of an acute LAD occlusion where you watch each conduction tract fail before your eyes, obviously that would be somewhat alarming.

 

[18G]

That makes sense and is where my thoughts were as well but wanted to see if I was missing something or if it was just the terminology that was confusing.

The literature I've seen said that the chance of progression to complete heart block is only about 1% from one study. Other studies determined the chance was higher at like 25-70% (it's in that range but don't quote me) within one to six years.

Based on that in asymptomatic patients, they don't get pacemakers. However, in the case of a symptomatic patient (ie syncope) that results from a transient complete heart block, a pacemaker is indicated.

Interesting stuff!

 

[Brandon O]

I think those are numbers for chronic cases, not so much for the acute finding as encountered by EMS -- but of course we could be the first to discover the former as well.

But it's clear anyway that anybody with an MI impacting the conduction system is already very high-risk in general, so it probably goes without saying that you'd be on tenterhooks for them.

 

[Christopher]

I think those are numbers for chronic cases, not so much for the acute finding as encountered by EMS -- but of course we could be the first to discover the former as well.

But it's clear anyway that anybody with an MI impacting the conduction system is already very high-risk in general, so it probably goes without saying that you'd be on tenterhooks for them.

I pretty much agree with everything you've said, and will second that the nomenclature is crappy. Twice I've had to ask that pads not be placed on my patient due to an overzealous subscription to the philosophy that, "if you say 'block' three times when reading the ECG your patient is in real trouble"...

BUT when you see that constellation of findings you need to consider the company it keeps:

- Chronic is the most common, and likely you'll find it is due to an old MI (or just age).

- Acute is the second variety, and sometimes MI's are more subtle than a so-called "trifasicular block". Patient's ECG has this finding and you don't have a definite cause, start looking for ST/T-wave changes that aren't so frank.

 

[Brandon O]

Yeah, that's what I was trying to get at. Just like I'd almost never get worked up about a "new or presumed new LBBB," but if you truly do believe the patient in front of you just now developed the block due to the gigantic MI they seem to be having, that's quite different.