Atrioventricular reentrant tachycardia (AVRT) & AV nodal reentrant tachycardia (AVNRT)

Atrioventricular reentrant tachycardia (AVRT) & AV nodal reentrant tachycardia (AVNRT)


Atrioventricular Re-entrant Tachycardia is also known as AVRT for short. And it’s a type of
supraventricular tachycardia where you have an abnormal
loop of electricity, or a re-entrant circuit, going around and around
between two pathways. So you have to have two pathways, one being normal AV conduction system, so that’s one of the pathways. And the other pathway
is an accessory pathway. And that’s just an extra pathway that exists between the
atrium and the ventricle. And atrioventricular
re-entrant tachycardia. So I’m just going to
erase this real quick, cause we’re gonna
re-draw that in a second. So normally, signal goes from the SA node through the atrium, to the AV node. And remember, the AV node is sort of like the gatekeeper, or the bridge that bridges signal from the atrium to the ventricle. So in a normal heart, signal has to go through this AV node. And it goes through the AV node and down to the ventricles and causes ventricular stimulation and contraction. Sometimes there’s an abnormal path, or an accessory pathway, which is an extra pathway between the atrium and the ventricle. So it could be here, it could be over here, I just drew over here for convenience. But there’s this extra pathway. And signal can travel from the atrium to the ventricle through this pathway and excite ventricular tissue that way. Signal can also go from the ventricles through the atrium through
this accessory pathway. So it can either go forward, or in the anterograde direction, which is from the atrium to the ventricle, or backwards, which is
the retrograde direction. From the ventricles to the atrium. And the direction that it goes depends on a couple of things. It depends on the timing
of the refractory period of the accessory pathway. And the refractory period, remember, that’s the window of time right after a group of cells are excited, in which they can’t be excited again. It’s kind of like a recovery period. So say you sprinted 100 meters. You’re not gonna be ready to sprint another 100 meters unless
you take a little break. So the refractory period is sort of like a mini-recovery period. And again, whether or not the signal goes in this forward direction
or this backwards direction depends on the timing
of the refractory period of the accessory pathway, as well as the direction from which signal is coming towards this accessory pathway. So if you have normal conduction through the AV node, and you have
an accessory pathway, this can set you up to
have a re-entrant circuit, or this abnormal loop electrical activity going around and around and around, which could cause a tachyarrhythmia. So again, you need to have both a working AV conduction system and
this accessory pathway in order to have AVRT. I think one of the best ways to better conceptualize AVRT is by going through the most classic example of AVRT, which is Wolff-Parkinson-White syndrome. So again, Wolff-Parkinson-White syndrome is a classic example of AVRT. And sometimes it’s called WPW for short. So again, in
Wolff-Parkinson-White syndrome, you have this extra pathway or accessory pathway that exists between the atrium and the ventricle. And you have signal that
goes from the SA node to the AV node, and then to the ventricles and that signal can
also go from the SA node through this accessory pathway and can stimulate the ventricles that way. So you’re getting ventricular stimulation through the AV node and through this accessory pathway. Now something to note is that the AV node has special tissue that actually slows down conduction so when signal hits the AV node, the
conduction slows down. However, this accessory
pathway is just kind of like a hole between the
atrium and the ventricle. It’s not gonna slow down any signal. So the signal going from
the atrium to the ventricle through this pathway is actually going to stimulate ventricular cells sooner than it would the AV node, cause this AV node has this built-in mechanism that slows down conduction. So you’re going to see
some changes on the EKG. You’re going to see a
shortened PR interval, and you’re gonna see the slow rise in the slope of the QRS. Again, you have this
shortened PR interval, the PR interval is usually
less than 0.12 seconds. And that’s because you
have this pre-excited ventricular tissue
that’s getting stimulated before the normal conduction system has a chance to stimulate ventricular tissue. And because you’re getting ventricular stimulation over a longer period of time, you’re gonna have this
slow rise in your QRS. This slow rise is called a Delta wave. And again, this is classic for WPW. Now it’s important to note
that this here is not AVRT. You’re not gonna get a
tachyarrhythmia just from this. However, in the event that you have a premature beat coming from the SA node going to the AV node, and if this accessory pathway happens to be in a refractory period, meaning that the signal isn’t gonna travel this way through the accessory pathway, then you’re gonna have signal going down through the ventricles. It’s gonna travel back up. And by the time it reaches the accessory pathway, it will no longer be in a refractory period. So the signal can actually travel through the accessory
pathway and then go back and stimulate the AV node. This creates the re-entry circuit. So you’re gonna have the signal going around and around
and around and around. And this is gonna create
the tachyarrhythmia that you get in AVRT. Atrioventricular Nodal
Re-entrant Tachycardia is another type of re-entrant tachycardia like AVRT, but it has its differences. So people call this AVNRT for short. Remember, this is very
different than AVRT. It’s called AVNRT, N is for Nodal, because the abnormal loop of electricity, or that abnormal re-entrant circuit, directly involves the AV node. And the tissue right around it. There is no accessory pathway in AVNRT. So again, this is the AV node, and here I drew a bigger AV node. I kind of blew it up. So this is the AV node, and the His Bundle, and
the conduction system going down into the ventricles. So in AVNRT, there are two pathways that run through the AV node. There’s a slow pathway, where an impulse travels
more slowly down the path, and there’s a fast pathway where the impulse can zip through. Now I’m gonna erase this real quick, cause we’re gonna re-draw
these in a second. Okay, something else to note. Just because of this inherent make up, the slow pathway has a
shorter refractory period. Now remember the refractory period is that window of time
when cells can’t be excited again after they’ve already been excited. Whereas the fast pathway has a longer refractory period. I’m gonna abbreviate refractory period RP. Refractory period. So again: slow pathway,
short refractory period. Fast pathway, long refractory period. So a signal comes down,
and it’s gonna split. And it’s gonna rush down the fast pathway, reach this common final pathway, and then spread to the ventricles. Meanwhile, it’s gonna slowly go down the slow pathway. After this impulse has been transmitted through this fast pathway, it’s gonna go through
the refractory period. So these lines through it
mean refractory period. By the time the slow pathway signal makes it to the final common pathway, it’s gonna hit the refractory period of the fast pathway, and it’s gonna terminate. Because no signal can
be activated this way since it’s in refractory period. This slow pathway is gonna enter its own refractory period, a
shorter refractory period. So it’s actually gonna recover. And then the fast will recover. And both the slow and fast pathway are ready for business again. They’re ready for another impulse. Now let’s say that there’s an early beat, or a premature beat that comes in. Sometimes people call these extra beats. So there’s an early beat that comes in. And let’s say it comes in at a time when the fast track is still recovering from a refractory period, but the slow track has already recovered from its refractory period and is open. So this beat’s gonna send impulse down the slow track. As the slow track slowly
makes its way down the slow track, the fast track is gonna recover from its refractory period. So by the time the impulse reaches this final common pathway, it’s gonna send signal down. And because the fast track has recovered from its refractory period, this impulse can activate the fast track and send signal back up. If the slow track has already been through its refractory period and recovered from that refractory period, it can activate the slow pathway. And send signal back down. And what happens is, the impulse will continue to circle around and around. You’re creating this re-entrant loop. And as it circles around
and around and around, it’s gonna keep sending
signal down this way. So this loop is sending signal through the AV node at a much faster rate than a normal pace maker’s would, so you might see a heart rate between 100 to even 250 beats per minute. And again, it’s because you have this abnormal re-entrant loop, sending signal around
and around and around which is gonna spit off signal down to the ventricles
at a much faster rate than a normal pacer’s would. EKG is gonna look like a
supraventricular tachycardia where you have a narrow QRS complex, meaning it’s less than 0.12 seconds or three small boxes. And you’re gonna have a heart rate of greater than or equal
to 100 beats per minute. Because that’s what a tachycardia is. It’s greater than 100 beats per minute. So on this EKG here, you can appreciate a narrow QRS complex, and again the QRS complex is narrow because there’s normal activation of the His Purkinje system, and you notice that the heart rate is greater than 100 beats per minute. So the heart rate here
is somewhere between 150 and 300 beats per minute. By looking from here to here, you can tell that the heart rate is above 100 beats per minute. So this is definitely a tachycardia.

74 Replies to “Atrioventricular reentrant tachycardia (AVRT) & AV nodal reentrant tachycardia (AVNRT)”

  1. What is the difference between Sinus Tachycardia vs SVT? The absence of the Pwave or the combining of the P wave with the T wave??

  2. The video says that a supraventricular tachycardia involves a narrow QRS <~ 0.12sec.  In other readings, I find that normal QRS is < 0.12 seconds.  What distinguishes the two?  Is it, primarily, the heart beat?

  3. If you have AVNRT dont make ablation for it, doctors will tell u its a EASY fix, no complacations bla bla bla.

    What they dont tell you is that ablation creates a scar, the SCAR it selfs creates abnormal signals making you have PVC. 

    And you DONT want pvc, it the absolutely the worst thing u can have, u cant sleep, u cant eat, u cant enjoy anything because of them. If u go back to the doctor the will laugh and say " everybody has them" " in normal heart pvcs is not a issue"

    The doctors really dont know what a PVC are, they never experienced it, if they did they would never say something like that.

    I rahter have my AVNRT back with another pathway, so i have 3 path ways in my AV node rahter than these PVCS

  4. Thank You so much for the thorough explanation.  I was diagnosed in 1998 with AV Nodal Reentry Tachy, had an ablation shortly thereafter to treat the condition.  However, after the procedure my doc informed me that the affected area was too close to the a/v node, and he was not able to completely get all of it.  So fast forward to today, I average 4 to 6 skipped beats per minute, with the occasional runs of tachy occurring.  I am back on the heart meds, taking Metoprolol extended release 100mg 1x per day.  I just somehow feel like this will be the bugger that gets me in the end.  At least I now know what is happening inside my chest,  Thank You 🙂 

  5. Question:
    1.) In AVRT (WPW syndrome), since electrical signals are going to inevitably travel through both the AV node and the accessory pathway, does that mean the person will have a relatively higher resting heart rate even without a premature impulse? (owing to the lack of mechanism that slows down the signals in the accessory pathway)

    2.) In 5:12 you mentioned that IF there was a premature beat, it would eventually form a reentry circuit by the signal that undergoes the AV node back up to the accessory tract at the time its refractory period is over , so my question is what if there was a premature beat that travels to the accessory pathway that wasn't in refractory period in the first place? Now does it differ from normal condition without a premature beat? and what determines the refractory period of this accessory pathway? @khanacademymedicine  or Anyone kind enough to enlighten me please? Thanks alot in advance!

  6. 3rd question, since the reentry phenomenon is going to revolve around the AV node and the accessory pathway in a vicious cycle manner, there will be new electrical signals coming from above (SA node/atrial pacemaker cells) too right? wouldn't that cancel off the reentry mechanism and lead to a normal anterograde conduction again, if that's the case, how can it still be said "the reentry circuit phenomenon takes place over and over and over again".. because it doesnt..??

  7. Just had my 2nd ablation, 3 days ago, for WPW. 10 years between procedures. Thought we had it beat (no pun intended). Hoping this one is a permanent fix!

  8. Are the P waves absent in both AVRT and AVNRT? Please explain the mechanism for why the P waves are absent in either.

  9. Best video on the internet regarding this topic. Very few people can clearly and adequately explain it. Amazing job! Thank you!

  10. Wow I didn't know that the normal AV node has 2 pathways 😮 none of my books mentions it 😛 thanks!

  11. 8:14 so does that mean normally impulse conduction in avn down to bundle of h n so forth is done by fast pathway n not slow ? cuz u said slow comes down hits fast which is refractory n it goes refractory itself? plz any help

  12. i'm crying, literally. got incoming internal medicine exam in 2 days, found this video, really give me a hope to pass the exam with flying colours. thank you very much, ma'am.

  13. Just a friendly note: you're sometimes really hard to understand for someone that English is a second language for – as you sometimes really speed up the words you say, especially in long, complex words, that might be clear to you, but are not clear to me. Thanks anyway!

  14. wow… leaving Thursday morning for mapping study plus or minus ablation with possible cryoballon… my hearts insane… I hope it works. thank you for this video!! would avrt or avnrt cause 17 pauses in 48 hours with the longest being 8.7 seconds?

  15. Thank you, excellent video. My only comment is that the normal QRS width is .08-.12ms, this means that in order for it to be narrow complex the parameter should be <.08 not .12.

  16. Very good lecture. The only thing I would change is that the slow and fast pathways are not really "in" the AV node. The slow pathway is between the tricuspid annulus and the coronary sinus and the fast pathway is on the other side of the coronary sinus. Otherwise, excellent presentation.

  17. Is it normal for nodes to have slow and fast pathways?! In the first example, how come the slow pathway doesn't go down the tract? Why is it halted when the fast pathway is going through refractory period?

  18. at 5:36 she said that the impulse can travel from the accessory node back to the AV node and cause a tachyarrythmia, my question is, since A/V node has a refractory period and can regulate the rate and rythm, why will the A/ V node cause tachyarrythmia, as in why will it let the impulse from accesory pathway through to begin with?

  19. Stumbled across this topic while doing my own extra research study. Your presentation is absolutely fantastic, and I believed you just made my next three-and-a-half-year of miserable life a little bit brighter. Cheers!

  20. Thank for the video! Why does the AV node not work as "the gate keeper" for the conduction from the accessory pathway in AVRT?

  21. Holy shit what a video.
    Been trying to learn this for the last year.

    You have one very happy Paramedic student.
    Thankyou.

  22. Feeling uneasy with the amount of idiot DRs on here who are relying on a Youtube video to pass their exams. You can see why the medical system is so ridiculous.

  23. So In AVRT, there can be two menifestation?
    (1) SA send impulses, but accessory tract is in refractory.So impulses go to normal AV conduction.When impulses reach to ventricles, the accessory tract finish refractory period.Then, impulses go from ventricles back to atrai before SA node produes another impulse
    (2) SA node produces impulses, some go through normal AV nodal pathway. And some go through accessory tract. Those who go through accessory tract reach faster as well as AV nodal conduction lates causing delta waves in ECG.
    1.So what about ECG for 1st menifestation?
    2. Did I understand correctly?

  24. This is 100000 times easier than I thought. Most people do a poor job explaining it (or perhaps they do not understand it themselves)

  25. This is wonderful. I gotta give a seminar about this in about 9 hours and now I know how to do it well. Thanks a bunch.

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