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

Video transcript

atrioventricular re-entrant tachycardia is also known as a BRT for short and it's a type of super ventricular tachycardia where you have an abnormal loop of electricity or reentrant 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 atrial ventricular ranter tachycardia so I'm just going to erase this real quick because we're going to redraw 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 that goes through the AV node and down to the ventricles and causes ventricular stimulation and contraction sometimes there is 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 excitement Ric Euler tissue that way signal can also go from the ventricles through the atrium through the successor e 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 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 going to be ready to sprint a hundred 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 signals coming towards the successor II 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 reentrant circuit or this abnormal loop electrical activity going around and around and round which could cause a tachyarrhythmia so again you need to have both working AV conduction system and successor e pathway in order to have a BRT I think one of the best ways to better conceptualize a BRT is by going through the most classic example of a BRT which is wolff-parkinson-white syndrome so again wolff-parkinson-white syndrome is a classic example of a BRT and sometimes it's called WPW for short so again and 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 you can stimulate the ventricles that way so you're getting a 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 HTM of the ventricle it's not going to slow down any signal so the signal going from atrium to the ventricle through this pathway is actually going to stimulate ventricular cells sooner than it would the AV node because this AV node has this built in mechanism that slows down conduction so you can you see some changes on the EKG you're going to see a shortened PR interval and you're going to see the slow rise and the slope of the QRS again you have this shortened PR interval so the PR interval is usually less than 0 Oh point one two 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 going to have the slow rise and your QRS the 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 a BRT you're not going to get a tacky arrhythmia just from this however in the event that you have a premature beat coming from the SA node going the AV node and if this accessory pathway happens to be in a refractory period meaning that the signal isn't going to travel this way through the accessory pathway then you're going to have signal going down through the ventricles it's going to 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 reentry circuit so you're going to have the signal going around and around and around and around and this is going to create the tachyarrhythmia that you get in a BRT atrioventricular nodal reentrant tachycardia is another type of reentrant tachycardia like a VRT but it has its differences so people call this a V and RT for short remember this is very different than a BRT it's called a B NRT ends for nodal because the abnormal loop of electricity or that abnormal rancette 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 HISP undal and the conduction system going down into the ventricles so in avnrt there are two pathways that run through any node there's a slow pathway where an impulse travels more slowly down the path and there's a fast pathway where the impulse comes it through and I'm going to raise this real quick because we're going to redraw these in a second okay something else note just because of as an inherent makeup the slow pathway has a shorter refractory period number the refractory period is that window of time when cells can't be excited again after they've already been excited whereas a fast pathway has a longer refractory period I'm going to 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 going to split and it's going to rush down the fast pathway reach this common final pathway and then spread to the ventricles meanwhile it's going to slowly go down the slow pathway after this impulse has been transmitted through this fast pathway it's going to go through the refractory period so these lines through it main refractory period by the time the slow pathway signal makes it to the final common pathway it's going to hit the refractory period of the fast pathway and it's going to terminate because no signal can be activated this way since it's in refractory period the slow pathway is going to enter its own refractory period a shorter refractory period so it's actually going to recover and then the fast will recover in both the slow and fast pathway are ready for business again they're ready for another impulse now let's say that there's a 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 beats gonna send impulse down the slow track as the slow track slowly makes its way down the slow track the fast track is going to recover from its refractory period so by the time the impulse reaches the spinal column pathway it's going to send signal down and because a fast track has recovered from its refractory period this impulse can activate the fast track and send signal back up if the slow track is already been through trajectory 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 reentrant loop and as it circles around and around around it's going to 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 makers 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 reentrant loop sending signal around and around and around which is going to spit off signal down to the ventricles at a much faster rate than a normal Pacers would EKG is going to look like a super ventricular tachycardia where you have a narrow QRS complex meaning is less than 0.1 two seconds or three small boxes and you're going to heart rate of greater than 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 histor Kinji 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