help with this question.

[ar78cx]

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Hey,
would anyone know the answer to this q .
* An analysis of an EKG shows P-waves occurring in the S-T segment. This would indicate ectopic beats orginating in the :
A- atria
b- A-V node.
C- lower portion of the bundle of His.
D- upper portion of the bundle of His.
Any input is appreciated
Thanx

 

[Idiopathic]

I would say B. This most resembles (in my mind) a junctional or 'nodal' tachycardia, such as W-P-W.

 

[HamOnWholeWheat]

I have to disagree on this one. My vote is "A". That sounds like a classic nonconductive PAC, or an AV-block, as there's no mention of a subsequest ventricular depol but a p-wave is seen, so you know you have atrial depol. I'd think if you had a junctional WPW you'd see ventricular depol as well. If the question asked, "where's the most likely block", I'd vote "B".

HamOn

 

[BlackNDecker]

Hey,
would anyone know the answer to this q .
* An analysis of an EKG shows P-waves occurring in the S-T segment. This would indicate ectopic beats orginating in the :
A- atria
b- A-V node.
C- lower portion of the bundle of His.
D- upper portion of the bundle of His.
Any input is appreciated
Thanx

The answer is B. Remember, when you have an ectopic beat it travels in 2 directions: TOWARDS the SA node & TOWARDS the AV node. You can tell the origination of the ectopic beat by the time it takes for it to travel backwards and cause depolarization of the SA node. Because the depolarization occurs late(i.e. after the QRS complex) the ectopic beat is closer to the AV node than it is to the SA node.

The bundle of His is nonsense...the fibrous bundle separating the Atria and the Ventricles does not permit the impulse to travel back up into the atria. In this instance the P wave would be dissociated from the QRS complex.

 

[BlackNDecker]

there's no mention of a subsequest ventricular depol but a p-wave is seen
HamOn

I think ventricular depolarization is implied by the mention of a S-T segment which is by definition "when the heart(& ventricles) are fully depolarized."

Maybe I am unclear on your arguement?

 

[HamOnWholeWheat]

I think ventricular depolarization is implied by the mention of a S-T segment which is by definition "when the heart(& ventricles) are fully depolarized."

Maybe I am unclear on your arguement?

Yeah, I think we just have a disconnect here.

When I say that it doesn't depolarize I mean this:
1) Depol. -> QRS -> ST to repolarize
2) While you're in the ST segment, you get a P-wave which does not cause a depolarization. Were this an ectopic pacemaker, I would expect, as you pointed out, that both the atria and ventricles would depolarize. So by virtue of the fact that the p-wave didn't cause a depolarization, either the ventricles are refractory (PAC) or there's an AV nodal block and the atria and ventricles are disconnected from one another.

It could be a WPW on the AV node and the ventricles are just in an absolute refractory period, but PAC just seems like the simpler answer to me. As I said before, if it were asking for a block, I'd say AV node.

HamOn

 

[BlackNDecker]

Yeah, I think we just have a disconnect here.

When I say that it doesn't depolarize I mean this:
1) Depol. -> QRS -> ST to repolarize
2) While you're in the ST segment, you get a P-wave which does not cause a depolarization. Were this an ectopic pacemaker, I would expect, as you pointed out, that both the atria and ventricles would depolarize. So by virtue of the fact that the p-wave didn't cause a depolarization, either the ventricles are refractory (PAC) or there's an AV nodal block and the atria and ventricles are disconnected from one another.

It could be a WPW on the AV node and the ventricles are just in an absolute refractory period, but PAC just seems like the simpler answer to me. As I said before, if it were asking for a block, I'd say AV node.

HamOn

I am trying to conceptualize WHY the beat could originate from the bundle of His? Devil's Advocate if you will...

If the beat originates from the bundle, the impulse would travel back towards the AV node; Is there anything to prevent this impulse from reaching the SA node? Is there no "gate" preventing a backwards impulse from the ventricles to the atria through the conduction system? (similar to the atrio-ventricular fibrous bundle blocking impulse spill over from atrial muscle to ventricular muscle?)

 

[BlackNDecker]

Inverted P waves are present when you have an ectopic beat originating from the AV node, correct? I think this may be the clue that "it" is not originating from the AV node...

This is my error.

If the beat originated from the AV node, the P wave would be obscured by the QRS complex(.03s to travel from AV >>> SA and another .03s for the atria to completely depolarize; .03s to travel from AV >> bundle and another .03 for the ventricles to completely depolarize.). Grand total of .06s in each direction. I believe this is the rationale for the bundle of his.

So how do you determine where in the bundle the beat is originating?

 

[HamOnWholeWheat]

Inverted P waves are present when you have an ectopic beat originating from the AV node, correct? I think this may be the clue that "it" is not originating from the AV node...

This is my error.

If the beat originated from the AV node, the P wave would be obscured by the QRS complex(.03s to travel from AV >>> SA and another .03s for the atria to completely depolarize; .03s to travel from AV >> bundle and another .03 for the ventricles to completely depolarize.). Grand total of .06s in each direction. I believe this is the rationale for the bundle of his.

So how do you determine where in the bundle the beat is originating?

Wow, you really lost me. How did we get on the bundle of His? Are you still just being the devil's advocate for the bundle, or did we shoot down the possibility of it being a simple premature atrial contraction (PAC) and I just missed it?

My vote's still "A", as to me a WPW->bundle depol.->overrides AV node->depolarizes atria is a bit of a stretch. I can't say for 100% certain, but the AV node should slow the conduction in either direction. But more importantly, using your explanation of the inverted p-wave if it were AV node, it would still be inverted coming from the bundle of His, as they're both depolarizing in the same "reverse" direction.

Again, I'm no cardiologist, but PAC or AV block are the two most obvious abswers to me.

HamOn

 

[RS6]

You can eliminate C and D because a bundle of His origin would only depolarize the ventricles and not any atria. Therefore, you do not see any p waves with a ventricular focus. And even if there were, you wouldn't be able to see them in the ST segment because the QRS complex is so wide and bizzare.

Are the P waves inverted? If they are and there are no other p waves besides in the ST segment then it is called junctional rhythm with retrograde atrial depolarization. Remember that junctional beats originate from the AV node, pacing at around 40 to 60 beats per minute. If the originating focus is high enough in the AV node (ie still able to depolarize the atrium) then you will see retrograde P waves as well as QRS complexes (ie the conduction can travel in both directions). The retrograde P waves can be right before, buried within, or right after (ie in the ST segment) the QRS complexes. I think this is the answer to the question. This is the only answer choice that explains P waves within the ST segment and not at any other location.

You could make an argument for PAC. Sometimes a PAC can come so early (in this case within the ST segment) that the AV node is still refractory that it cannot conduct to the ventricles and therefore not generate a QRS complex. By this same argument you could argue a second degree heart block. However, you would need to see the rest of the tracing. This is why I was asking if there were any other P waves. Generally speaking, you do not see P waves in the ST segments with these two conditions.

One could also make an argument for complete AV heart block with junctional escape rhythm. In this case the SA node fires and the AV node fires with no relation to each other. This is what they call AV node dissociation. This could generate P wave within the ST segment portion. HOwever, this would only occur for some of the ST segments. Remember that the intrinsic rate of the SA node is between 60 to 80 beats per minute, while the AV node is 40 to 60 beats per minute. Therefore, over time, the p waves and the QRS complexes would march out at different rates and only some of the p waves would be seen in the ST segments. Unless, the two node rates fired at exactly the same time all the time. Very unlikely.

This is not WPW. WPW is an abnormal accessory AV conduction pathway, called the bundle of Kent. This is a direct connection of the atrial conduction system with the ventricular conduction system. This effectively bypasses the AV node and you don't get that slowing down of the electrical impulse. You can see this as the delta wave (slurring upward QRS complex) on the EKG. The slurring is due to the kent pathway getting into the ventricles before the normal AV conduction pathway. I do not think or have seen any p waves within the ST segment in WPW. The only thing i know of is that it can cause a reentry tachycardia through this bundle of kent.

This is hard to explain without EKG drawings. Read Dale Dubins book. But wait until you start your medicine rotation. This is way above what you need to know for step 1.

 

[BlackNDecker]

You can eliminate C and D because a bundle of His origin would only depolarize the ventricles and not any atria. Therefore, you do not see any p waves with a ventricular focus. And even if there were, you wouldn't be able to see them in the ST segment because the QRS complex is so wide and bizzare.

Just out of curiousity, what prevents the impulse from traveling backwards thru the AV bundle and node towards the SA node? It seems odd that under abnormal conditions the wave of depolarization can travel in any direction it finds(i.e. circus movements) but cannot travel into the SA node if originating in the bundle of his...

 

[BlackNDecker]

Read Dale Dubins book. But wait until you start your medicine rotation. This is way above what you need to know for step 1.

Is this the book you're talking about?

http://www.amazon.com/gp/reader/0912912065/ref=sib_dp_pt/103-8486453-7503867#reader-link

 

[RS6]

Yes, that is the book. 300 pages. Easy read, 1 to 3 days, depending. Funny too. Do a google search on good old Dubbin and you'll find he went to prison for various offenses and that he is not even a cardiologist. However, the book is still good and is a good introduction. But just be aware that it is a programmed text. ie. it is a fill in the blank type of format.

 

[ar78cx]

Thanx to everyone who posted a reply. 🙂

 

[p53]

Hey,
would anyone know the answer to this q .
* An analysis of an EKG shows P-waves occurring in the S-T segment. This would indicate ectopic beats orginating in the :
A- atria
b- A-V node.
C- lower portion of the bundle of His.
D- upper portion of the bundle of His.
Any input is appreciated
Thanx

Do you have any more homework problems we can solve for you?

 

[BlackNDecker]

Do you have any more homework problems we can solve for you?

Why are you hatin'? ...because you've already taken the step? 👎

 

[p53]

Why are you hatin'? ...because you've already taken the step? 👎

Yes, I have taken Step 1 and this is a Step 1 forum. The question is not a USMLE Step 1 question nor a Step 1 concept. THERE is no reason to ask this question on this forum, PERIOD!

The OP knew this, yet wanted an answer (for his/her homework) rather than looking up the answer.

 

[ar78cx]

Yes, I have taken Step 1 and this is a Step 1 forum. The question is not a USMLE Step 1 question nor a Step 1 concept. THERE is no reason to ask this question on this forum, PERIOD!

The OP knew this, yet wanted an answer (for his/her homework) rather than looking up the answer.

p53,
I don't know where u come from , but where I come from we are not born as intelligent as u are ( unfortunately, ha!)
I can't understand why you are here if you are too knowledgeable.
Basically I would like to know where on god's earth is it a crime to ask for an answer to a q .
apparently you refuse any knowledge that comes from planet earth.
If u would think about it , it would have taken u less time to post a letter referring to the right answer than what u did.
I don't know if your big brain processed your thoughts before u typed what u did, but in case u have not noticed , no one else but u posted such a thing, which makes u different buddy, don't u think?
On top of all that check the name of the forum, I don't think it says ( Einstein's Forum ) .
Apparently u typed the wrong address.
Oh ya, by the way I am studying for my dental boards and came across this question , didn't know the answer, posted it in dental and again no one knew the answer, so I thought I'll post it here ( maybe now you know why I had difficulty answering the q )
Thanx to all who posted a positive reply, and p53 or whatever your name was , I really feel sorry for u, GET HELP.

 

[ANGII]

first you should tell us if the QRS complex is narrow or wide !!!
anyways it is mostly originating from the AV node ---but--- if you have a wide QRS complex then it is originating from the upper portion of the bundle of His ........
you all should not exclude the bundle of His as an origin for this ectopics .... as the higher portion of the bundle of His can give ectopics that very much resemble AV node ectopics ....
and by the way ectopic beats CAN go back to the SA node deploarizing it and causing an SA node silence period that is bigger that 2 RR time .

 

[ar78cx]

first you should tell us if the QRS complex is narrow or wide !!!
anyways it is mostly originating from the AV node ---but--- if you have a wide QRS complex then it is originating from the upper portion of the bundle of His ........
you all should not exclude the bundle of His as an origin for this ectopics .... as the higher portion of the bundle of His can give ectopics that very much resemble AV node ectopics ....
and by the way ectopic beats CAN go back to the SA node deploarizing it and causing an SA node silence period that is bigger that 2 RR time .

Hey ,
The q did not say wide or not , I listed it the way it was written in my papers.

 

[dynx]

p53,
I don't know where u come from , but where I come from we are not born as intelligent as u are

I wouldn't be so sure about that. Unless you're from alabama Im pretty sure the avg. IQ is close

 

[ANGII]

Hey ,
The q did not say wide or not , I listed it the way it was written in my papers.

if the Q didn't mention it , then it is mostly narrow QRS complex
==> the ectopics originated from the AV node ....

 

[BlackNDecker]

first you should tell us if the QRS complex is narrow or wide !!!
anyways it is mostly originating from the AV node ---but--- if you have a wide QRS complex then it is originating from the upper portion of the bundle of His ........

Please clarify. Why would an ectopic beat originating high in the bundle create a wide QRS complex? The way I understand it, something that slows or prolongs propagation of the impulse through the bundle(AKA bundle branch block) or ventricular hypertrophy leads to a lengthened QRS complex. I can't see why an impulse would travel more slowly thru the bundle simply because it originated there??

I can see how an ectopic beat originating low in one of the bundle branches would require more time to reach the ends of the other branch... more time would be required to completely depolarize all of the ventricular muscle.

you all should not exclude the bundle of His as an origin for this ectopics .... as the higher portion of the bundle of His can give ectopics that very much resemble AV node ectopics ....

Would the P wave not be characteristically different in each of these instances?
AV node ectopic - P wave would just precede the QRS complex; assuming .04s req. to reach bundle + .06s to depolarize the ventricles for the impulse traveling towards the ventricles(total .10s)
assuming .03s to reach SA node + .03s to depolarize atria for the impulse traveling in the wrong direction(total .06s)

bundle of His ectopic(upper portion) - P wave would follow the QRS complex;
assuming .03s required to spread throughout the purkinje/myocardium + .03s to reach outermost ventricular muscle(total .06s)
assuming .09s stall in AV bundle + .03s required to reach SA node + .03s required to depolarize atria for the impulse traveling in the wrong direction(total .15s)

and by the way ectopic beats CAN go back to the SA node deploarizing it and causing an SA node silence period that is bigger that 2 RR time .

What is an "SA node silence period" and what is a "2 RR time?"

 

[dr_siba21]

Originally Posted by ar78cx
Hey,
would anyone know the answer to this q .
* An analysis of an EKG shows P-waves occurring in the S-T segment. This would indicate ectopic beats orginating in the :
A- atria
b- A-V node.
C- lower portion of the bundle of His.
D- upper portion of the bundle of His.
Any input is appreciated



I WILL SAY THE ANSWER IS OVIOUSLY 'B' .IF SOME ONE WILL READ CAUSES DESCRIBED BY VARIOUS REPLIERS THEN HE CAN UNDERSTANDS .


ur time is important so is mine

 

[ANGII]

infact i really don't know !
the problem here is that you can NEVER say that it originated from here or there and being sure about it according to a classic EKG ...
if you do an EKG through a catheter , it will FOR SURE tell you where that ectopic came from ......
and eventually ..... it's just one damn ectopic !!! please don't make a big deal out of it !! maybe right now i'm having an ectopic or two due to coffee overdose 🙂
and if you are so LOST ... ask your professor about it , that's his JOB !
Best of Luck !!!!!

 

[ANGII]

this may help :
http://www.madsci.com/manu/ekg_rhy.htm