cointosser13
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So there's RBBB, and there are LBBBs. But....if it's a LBBB it could be a LPFB or LAFB. How do you determine on a EKG if it's LPFB or LAFB?
Left BBB...which one?
- Thread starter cointosser13
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A left bundle branch block (LBBB) means that both fascicles are blocked. A right bundle branch block (RBBB) can present with a fascicular block which would make it a bifascicular block or a trifasicular block if there is a 1st degree AVB block too or prolonged PR interval.
When I think of fascicular blocks, I primarily think of the mean QRS axis.
Left anterior fascicular block (LAFB) / left anterior hemiblock (LAH)
LAFB is more common. It presents as left axis deviation. I have seen some people threshold for "pathological" left axis deviation being less than -30 degrees or -45 degrees. Left axis deviation on the ECG is going to present as a positive QRS complex in lead I and a negative QRS complex in lead aVF. -30 degrees is perpendicular to lead II. That would mean that if lead II is small in amplitude and/or equiphasic then the mean QRS vector is about -30 degrees. -60 degrees is perpendicular to lead aVR. That would mean that if lead aVR is small in amplitude and/or equiphasic then the mean QRS vector is about -60 degrees. The simple criteria I like too look for is that lead I is positive, and lead II, III, and aVF are negative.
Books I've read don't mention this, but I commonly see poor R-wave progression in the precordial leads. It'll look like an anterior wall MI without ST elevation to me personally. The R-wave progression is late.
Books I've read do mention that lead I will have a qR wave and inferior leads (particularly books will say lead III) will have rS waves. I personally do not usually look for this criteria. I am already sold on the axis usually and poor R-wave progression alone.
Another thing mentioned in books is that the width of the QRS will be wider a little bit, but not as much as a bundle branch block (0.12s). We just talked about this in another thread here. Like I mentioned earlier, I am usually sold on the axis and poor R-wave progression alone, haha.
Left posterior fascicular block (LPFB) / left posterior hemiblock (LPH)
Rare by itself. This will usually as a bifascicular block or trifascicular block. I've honestly never seen it in real life and haven't even seen a lot of online examples. I believe it is simply right axis deviation (+90 degrees or more) that has had all other causes of right axis deviation ruled out eg pulmonary disease, right ventricular hypertrophy, pediatric patient, etc. Right axis deviation is simply a negative QRS complex in lead I and a positive QRS complex in lead aVF. It's +90 degrees if the QRS is equiphasic/small in lead I, +120 in lead aVR, +150 in lead II, and +180 in lead aVF.
Books will mention that lead I will have a rS wave and inferior leads (particularly will say lead III) will have a qR wave.
Just like the other type of fascicular block, it'll have a wider QRS width.
Bifascicular block
RBBB + LAFB
-or-
RBBB + LPFB
I am not sure if it'll significantly effect the width of the QRS complex since the RBBB already does.
I've noticed that RBBB + LAFB will make it so lead I has primarily an RS wave or rS, which is weird looking to me. It's usually a pretty big clue to me that it is a bifascicular or trifascicular block. Normally lead V6 and I should look the same. In a RBBB, lead I and V6 will have an Rs wave.
Trifascicular block
Bifascicular block + 1st degree AVB
I'm not sure if this still counts with other types of AVB such as 2nd degree type I, II, or 2:1. Probably does? I've only seen ECGs with 1st degree AVB though. This obviously would not apply to a 3rd degree AVB since there is a block at or above both bundle branch blocks / all the fascicles.
This is all just fancy terminology for axis deviation really.
For more review on axis determination, you can read what I wrote here (no pictures) or read what @TomB wrote on ems12lead.com here (has pictures and examples). He also wrote LAFB here (also has pictures and examples).
Like how I put the useful links at the very end of my post? Haha.
When I think of fascicular blocks, I primarily think of the mean QRS axis.
Left anterior fascicular block (LAFB) / left anterior hemiblock (LAH)
LAFB is more common. It presents as left axis deviation. I have seen some people threshold for "pathological" left axis deviation being less than -30 degrees or -45 degrees. Left axis deviation on the ECG is going to present as a positive QRS complex in lead I and a negative QRS complex in lead aVF. -30 degrees is perpendicular to lead II. That would mean that if lead II is small in amplitude and/or equiphasic then the mean QRS vector is about -30 degrees. -60 degrees is perpendicular to lead aVR. That would mean that if lead aVR is small in amplitude and/or equiphasic then the mean QRS vector is about -60 degrees. The simple criteria I like too look for is that lead I is positive, and lead II, III, and aVF are negative.
Books I've read don't mention this, but I commonly see poor R-wave progression in the precordial leads. It'll look like an anterior wall MI without ST elevation to me personally. The R-wave progression is late.
Books I've read do mention that lead I will have a qR wave and inferior leads (particularly books will say lead III) will have rS waves. I personally do not usually look for this criteria. I am already sold on the axis usually and poor R-wave progression alone.
Another thing mentioned in books is that the width of the QRS will be wider a little bit, but not as much as a bundle branch block (0.12s). We just talked about this in another thread here. Like I mentioned earlier, I am usually sold on the axis and poor R-wave progression alone, haha.
Left posterior fascicular block (LPFB) / left posterior hemiblock (LPH)
Rare by itself. This will usually as a bifascicular block or trifascicular block. I've honestly never seen it in real life and haven't even seen a lot of online examples. I believe it is simply right axis deviation (+90 degrees or more) that has had all other causes of right axis deviation ruled out eg pulmonary disease, right ventricular hypertrophy, pediatric patient, etc. Right axis deviation is simply a negative QRS complex in lead I and a positive QRS complex in lead aVF. It's +90 degrees if the QRS is equiphasic/small in lead I, +120 in lead aVR, +150 in lead II, and +180 in lead aVF.
Books will mention that lead I will have a rS wave and inferior leads (particularly will say lead III) will have a qR wave.
Just like the other type of fascicular block, it'll have a wider QRS width.
Bifascicular block
RBBB + LAFB
-or-
RBBB + LPFB
I am not sure if it'll significantly effect the width of the QRS complex since the RBBB already does.
I've noticed that RBBB + LAFB will make it so lead I has primarily an RS wave or rS, which is weird looking to me. It's usually a pretty big clue to me that it is a bifascicular or trifascicular block. Normally lead V6 and I should look the same. In a RBBB, lead I and V6 will have an Rs wave.
Trifascicular block
Bifascicular block + 1st degree AVB
I'm not sure if this still counts with other types of AVB such as 2nd degree type I, II, or 2:1. Probably does? I've only seen ECGs with 1st degree AVB though. This obviously would not apply to a 3rd degree AVB since there is a block at or above both bundle branch blocks / all the fascicles.
This is all just fancy terminology for axis deviation really.
For more review on axis determination, you can read what I wrote here (no pictures) or read what @TomB wrote on ems12lead.com here (has pictures and examples). He also wrote LAFB here (also has pictures and examples).
Like how I put the useful links at the very end of my post? Haha.
Last edited:
DesertMedic66
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I was taught that if you have a LBBB you will not have a LPFB or LAFB.
Edit: can I change my answer to exactly what Aprz said or is it too late?
Edit: can I change my answer to exactly what Aprz said or is it too late?
TomB
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If it's complete LBBB it's both because the block is above the bifurcation of the left bundle branch into the left anterior and posterior fascicles.
TomB
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Simultaneous left anterior and left posterior fascicular block would be indistinguishable from left bundle branch block. By convention bifascicular block is right bundle brach block plus either left anterior fascicular block (by far most common) or left posterior fascicular block.