Thinking about heartbeats

So I know we talked about different pacemakers in the body, but I thought it'd be fun to revisit that and show you an interesting example. So let's start out by laying out the table we'd set up before. We talked about the heart rate in beats per minute, and we talked about the heartbeat itself-- the length of the heartbeat, and we'd measured the heartbeat in terms of seconds. And you remember, there's a nice little relationship between the two of these, because if the heartbeat 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 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 His. And we said starting with the SA node, the heart rate was somewhere between 60 and 90. And I think I'd chosen 90, just because that was a nice, easy number to do math with. And we had said that the heart beat is about 0.66 seconds. So that's the length of a heartbeat there. And then we have the AV node. I'm just going to quickly go through this. I know this is recap 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 His. And I think I've started trying to take a shortcut in writing bundle of His into just BoH. And that looks something like this. And those underlying numbers are the numbers I'm using to calculate the heartbeat lengths. So that's basically what we had come up with. And we had also talked about the idea of having delays. You actually need time for the pulse to be in transit basically. And so, I'm actually going to add a third column to our little table here. And there really is no delay here, because the SA node 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 a small delay, because things do move pretty quick. So we said that here, it'd 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 His. It actually takes only about 0.005 seconds. So it gets really, really fast. And remember that this transit speed, this is really 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 0. Remember, the more steep phase 0 is, the faster something is going to go from cell to cell to cell. And actually, that brings up a good point, because in the AV node, 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 0.1 second delay. And generally speaking, I think of 0.1 seconds as almost nothing, but when you compare it to 0.005 seconds, because that's the transit time-- that's 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. 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 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 His 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 let me actually jump into something slightly new. So let's assume for a second-- this is a thought experiment-- that instead of 0.04 seconds, I'm just going to focus on these two right now. Instead of 0.04 seconds, let's say that it took 100 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 100 times longer. So this ends up being 4 seconds, right? 0.04 times 100 is 4 seconds. So let's say it takes about 4 seconds, for some reason, to get a signal from the SA node to the AV node. Well, what would that mean for us? What would that look like exactly? And I think you'll start seeing some interesting lessons from this little thought experiment. So, if that was the case, if it was actually taking about 4 seconds to get from one point to another, let's now draw out a timeline. This is a little time line, and this timeline starts at 0 seconds. And then you have, let's say, 1 second here, 2 seconds, 3-- I'm just going to see how far this goes-- 4, 5, and let's go to 6. So, this is 6. Seconds And we're going to follow what happens over 6 seconds. So let's imagine now we keep track of our SA node up here. And we're going to keep track of our AV node down here. So at time 0, let's imagine that everything is 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 a heart beat would 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 let off. The 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 going through the SA node. And they just keep going through. They're just going to keep flowing through here. And they're going to all just continue and basically, just what are we going to get? A total of probably 9, right? We're going to get 9 signals sent off. Now, take each of them is going to take 4 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, because that's 4 and 2/3 of a second. So at 4 and 2/3 of a second, this one-- let me somehow show you without making this too messy-- 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 4 and 2/3 of a second to just 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 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 2, 3, so all this time, the AV node is on its own rhythm. And then finally, before AV node is able to fire off its own fifth 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 passed 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 . He took 4 seconds, and he arrives right there. And then this guy is going to arrive after that. He's going to arrives right there. So you see they start arriving. And so, once they start arriving, then you get back onto 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 awhile, the AV node is doing its own thing over here. And then after that, the SA node catches up. And then it continues on 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 have escaped the normal flow of electrical conduction, which starts with the SA node. So, hope that was helpful.