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Current time:0:00Total duration:9:30

Video transcript

so I know we talked about different pacemakers in the body but I thought it'd be kind of fun to kind of revisit that and show you an interesting example so let's start out by kind of laying out the table we'd kind of set up before we talked about the heart rate in beats per minute and we talked about the heart beat itself you know the length of the heart beat and we had measured the heart beat in terms of seconds and you remember there's a nice little relationship between the two of these because if the heart beat actually gets shorter then you can have more heart beats in a minute and so of course then the heart rate goes up so that's a relationship that that explains how it is that our heart rate goes up and down and we talked about the SA node the AV node and the bundle of hist and we said starting with the SA node the heart rate was somewhere between 60 and 90 and I I think I had chosen 90 just because I was kind of a nice easy number to do math with and we had said that the heart beat is about point six six seconds so that's the length of a heart beat there and then we have the AV node I'm just going to quickly go through this I know this is kind of recap for for you if you've seen the other video if you haven't then these numbers come from basically dividing beats per minute down into seconds and so then each beat then would be one second for the AV node and finally we did the bundle of hiss and I think I've started trying to take a shortcut in writing bundle of hiss into just boah and that looks something like this so this is basically and those underlined numbers of the numbers I'm using to calculate the heart beat links so that's basically what we'd come up with and we had also talked about the idea of having kind of delays right you actually need time for the pulse to take to be in transit basically and so I'm actually going to add a third column tour to our little table here and there really is no delay here because the SA node is is where things are starting so let me actually just keep my colors the same and then the AV node we know that there's kind of a small all delay because things do move pretty quick so we said that you know here would be something like 0.04 seconds so you can see that it's actually pretty quick getting from the SA node to the AV node but then it gets even faster as you get down to the bundle of hiss it actually takes the only about point oh five seconds so it gets really really fast and remember that this transit speed this is really kind of related to conduction velocity so how fast is the signal getting conducted so we call this conduction velocity and the relationship between conduction velocity and the action potential is the slope of phase zero remember the more steep phase zero is the faster something is going to go from cell to cell to cell and actually that brings up a good point because you know the AV know there's a huge delay built in because the conduction is so darn slow and so you have to actually remember that there's this point one second delay and generally speaking I kind of think of point one seconds is almost nothing but when you're compared to point zero zero five seconds because that's the transit time how long it takes the signal to get down we said from the AV node down to our particular bundle of his cell then all of a sudden this delay is looking enormous right by comparison this looks like a really big big number and let me just write transit here as well so this is time for movement and then the delay is simply getting through the AV node itself so this is all kind of just rehashing what we've talked about before and finally just to get at least a drawing down because I like to draw we have our SA node here and we have our AV node here and we have our bundle of hits over here and let me draw it half the distance somewhere like this and remember this is the direction of flow we're basically trying to move this way and again this way so now let me actually jump into something slightly new right so let's assume for a second this kind of a thought experiment that instead of 0.04 seconds I'm just going to kind of focus on these two right now instead of 0.04 seconds let's say that it took a hundred times as long for some reason let's say that transit time for some reason we don't know why let's say it takes a hundred times longer so this ends up being four seconds right 0.04 times 100 is four seconds so let's say it takes about four seconds for some reason to get a signal from the SA node to the AV node well what would that mean for for us what would that look like exactly and I think you'll start seeing some interesting kind of lessons from this little thought experiment so if that was the case if it was actually taking about four seconds to get from one point to another let's now draw out a time line this is a little time line and this time line starts at zero seconds and then you have let's say one second here two seconds three I'm just going to see how far this goes four five and let's go to 6 so this is 6 seconds ok and we're going to kind of follow what happens over 6 seconds so let's imagine now we keep track of our SA node up here we're going to keep track of our AV node down here so at time 0 let's imagine that everything is kind of beginning and we watch our SA node let's start with that one first well at 2/3 of a second because that's about how long it takes we calculated we would get our first action potential or heartbeat would kind of go through right first beat and that would then try to make its way towards the AV node so this one is going to try to make its way towards the AV node but we know it takes 4 seconds to get there now what happens after that well you'd have another beat kind of let off first one hasn't actually made it to the AV node but the second one is already done by that point and you'd have a third beat that goes through by that point and so really we're counting these action potentials that are kind of going through the SA node and they're just keep going through right they're just going to keep flowing through here and they're going to all just you and basically just what are we going to get a total of probably nine right we're going to get nine signals sent off all right now they're all going to take each of them is going to take four seconds to get to the AV node so when will this first one get to the AV node this very first one will get to the AV node somewhere around here right somewhere around here because that's four and two thirds of a second so at four and two thirds of a second this one let me somehow kind of show you without making this too messy this one will make it to the AV node right here of course at that time the SA node itself is letting out its seventh action potential but that very first one will get there at that point now the AV node is it going to sit around and wait for four and two thirds of a second to just kind of go by and not do anything no way right there's no way because what it's going to do is it's going to say okay well let's wait for a signal from the SA node and at this point it's going to say well nothing arrived from the SA node so I'm going to let off my own signal and it's going to keep doing this so it's going to go on its own rhythm now so two three so all this time the AV node is kind of on its own rhythm right and then finally before AV node is able to fire off its own v action potential by itself a signal arrives from the SA node this red arrow that I drew in and so it'll say oh wait we just got some positive ion pass through the electrical conduction system so let's go with it so it'll have a signal there and then now it'll have another one here because what happens at that point well you have this guy arrives right he took four seconds and he arrives right there and then this guy is gonna arrive kind of after that he's gonna arrive right there so you see they start arriving and so once they start arriving then you kind of get back onto a normal what looks like a normal rhythm and so it's interesting because you basically as a result of this long delay have a phenomenon where for a while the AV node is kind of doing its own thing over here and then after that it catches up where the SA node catches up and then it continues on what we would look like or what would look like a normal sinus rhythm and so sometimes you'll hear the term escape beats or escape rhythm and so that's what these are these are escape beats meaning they've escaped this scaped the normal flow of electrical conduction which starts with the SA node so hope that was helpful