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So we've talked about pacemaker cells
and I thought it would be kind of neat to draw out again
exactly what these pacemaker cells do and compare them
to one another.
So we know we have different types of pacemaker cells.
And I'm going to use our millivolt scale just as we
usually do to kind of compare them.
So the first type is our SA node.
So let me put that up here.
SA node.
And those cells start out negative
and then they kind of slowly creep up positive.
And try to remember why that happens.
The main reason for that is that you
have the increasing sodium permeability.
So at this point, more sodium is kind of rushing into the cell,
sodium is getting in.
And if it's getting in quickly, let's say
it's like just gushing in, then that line would be very, very
steep, you know?
Something like this.
But if it was kind of getting in very slowly,
it would be a little bit more shallow.
So I've drawn it kind of the way that it usually is drawn.
Kind of somewhere in between.
And when that sodium permeability hits
or when that cell hits a certain threshold,
let's say somewhere in there.
Then it's going to fire an action potential, right?
So this is our threshold for firing.
And that just means that the channels for calcium open up
and so they open up and make the cell go positive.
And then eventually, the potassium channels take over
and it becomes negative again.
And this part right here then, we
think of as the action potential.
So this is what it looks like for the SA node.
But let me now do it again for the AV node.
So if there's an AV node, it's going to kind of get up
to that threshold a little bit more slowly,
but once it gets there, it's going
to look basically the same as the SA node.
Really no different.
And it's going to come down again.
So this might be the AV node.
And finally, we have, let's say, a cell
that takes even longer to get to threshold.
This would be like the bundle of His
and, again, it looks basically the same
once it does get there though.
So these action potentials don't really
look that different from one another, but the amount of time
it takes to get to threshold changes
because the bundle of His cells, for example, they
are going to be the least permeable to sodium.
An AV node would be somewhere in between
and the SA node are most permeable to sodium.
So that's why those lines are slightly less
steep as you go along.
So this is how it looks and the key difference here
is that you're basically extending this heartbeat out,
right?
This is one heartbeat and this would be like the SA node
heartbeat, but if your AV node was controlling your heartbeat,
it might take a little bit longer, something like that.
And if your bundle of His was taking control of your heart,
the heartbeat would be really long, something like that.
So it would take longer and longer
for the heartbeat in terms of time.
Depending on which part of the electrical connection system
is in charge.
So let's actually think about that a little bit more
carefully.
So let's say we have our SA node and you're
talking about heartbeats.
Let's actually write out, let's say, heart rate,
heart rate is HR.
And this is going to be in beats per minute.
And then let's say I flipped it around and wanted
to know how long one beat takes.
So one beat.
And that will probably have to be done in seconds.
So how would that be for the SA node?
Well, we know that the SA node, and this is just
a number out of books, you can find them saying something
like 60 to 90 beats per minute.
And if we took the upper range, let's say you took 90,
and try to figure out how long one beat would take,
well, you'd say, OK.
You have one minute gives you 90 beats, I'll put B for beats.
And then you have one, let's say minute, is 60 seconds, right?
And so the minutes cancel.
And now you're left with, the zeros cancel, 2/3 of a second.
Right?
So 2/3 of a second per beat.
And actually, I might even like to I'm going to erase 2/3
and just rewrite that as 0.66.
OK?
It's 0.66.
Something like that
So that's how long it takes for the SA node
to fire off one beat.
And, in fact, just to really hammer home the point,
that's this distance, right here, right?
That's 0.66 seconds.
So now for the AV node, we could do the exact same thing.
We could say, well, the AV node, we know usually
is somewhere between 40 to 60 beats.
And I'm going to use that number.
And this one's really easy, right?
Because if it's 60 beats a minute,
that means that one beat is 1 second.
So that was actually a really quick one.
So that's 1 second.
And finally for the bundle of His,
I'm going to write that as BoH again.
Bundle of His is going to be somewhere between let's
say 20 to 30 beats per minute.
And if we use the number 30, that
means that you get a beat every 2 seconds.
So every 2 seconds, this will go off.
And I know that my picture now, since you know those numbers,
it's not going to look as impressive.
Because I should have drawn the bundle of His even more
stretched out than it is, but just
assume that that's two seconds on that graph.
So if that's the case, now let's kind of jump
back to how we usually think about our heart.
And the fact that you've got four chambers, right?
And the conduction system is actually
going to go through all of that.
And starts here in the SA node, goes down to the AV node,
and then you've got the bundle of His somewhere down there.
And you've got connections down there.
And you might be thinking, well, wait a second,
you haven't drawn in all of the rest
of the electrical conduction system, and that's true,
but for right now, let's just focus on these three parts.
Right?
So you've got AV here, and you've got the bundle of His
over here, BoH.
So you've got these three parts.
And they're kind of spaced out.
Right?
Like this is 2 centimeters apart, let's say.
I'm just kind of guesstimating.
This might be even closer, let's say one centimeter.
So these are kind of anatomically
how they're laid out.
In terms of how far apart they are from each other.
So the question might come up, how exactly
do you explain the fact that it's always
the SA node that fires off?
Right?
It's never your AV node or your bundle of His.
We always say, well, he's in sinus rhythm, right?
And what does that mean?
Well, if someone says someone is in sinus rhythm,
all they're saying is that the SA node
is what's controlling their rhythm.
So sinus rhythm, you might hear that actually a lot on TV
shows, I've noticed, they like to throw that term around.
And it just means that you're in a rhythm controlled by your SA
node.
So how does that work exactly?
Because if it's firing every 0.66 seconds, that's fine,
but how come these two other pacemaker cells aren't ever
firing?
Well, it gets back to basically trying to beat them out.
So if you can get a signal from your SA node,
this is, let's say, your SA node from that drawing
above, if you can get it to your AV node
faster, if you can get that signal there faster, than it
would fire, then you've beat it out.
So basically, if you can get that signal from the SA node
over to the AV node, if this happens
in less than one second, then the AV node
is not going to get a chance to fire before you're already
firing for it.
So this is the race, right?
The SA node is basically trying to get a signal over
there quickly.
And these distances that it has to cover,
we said about 2 centimeters and about 1 centimeter.
So what is the math?
How does that work out?
So you can actually look up these numbers
and it turns out that if you check it out,
these conduction velocities are really, really fast, right?
So it's about 0.5 meters per second up here.
And it's gets even faster as you get along further.
So it's about 2 meters per second here.
So these are the velocities of the signal,
how fast the electrical conduction system is actually
sending along that signal.
And those are the distances.
So if you think about it, if you just multiply them,
you should be able to get a time.
How long it will take a signal to get from the SA
to the AV node.
So we know that the SA node fires every 0.66 seconds,
right?
That much we have figured out already.
So the question is can it get a signal to the AV node
before the AV node fires by itself?
Can it get a signal down there in less than 1 second?
You're trying to beat out this time.
And can it get a signal to the bundle of His
in less than 2 seconds?
You're trying to beat out that time as well.
So let's figure out.
So this math works out to, let's see, you've got 0.5 meters
and you're going to want to end with a time.
So I'm going to put 1 second up here.
And you have, let's say, 1 meter is 100 centimeters.
So the meters cancel.
And you've got 2 centimeters to cover.
So the centimeters cancel.
So you've got 2 divided by 50.
And that's seconds.
So it's 0.66 seconds, plus 1/25 That's 1/25.
Let me make a little bit more space on here.
Just so it doesn't feel as crowded.
There we go.
So 1/25 of a second is the same as 0.04 seconds.
And that is 0.7 seconds.
So, so far the signal has gotten here in 0.7 seconds.
I'm just going to write that in yellow
because this is the SA node signal.
0.7 seconds.
Wow, that's really fast, right?
Really fast.
Let's see how long it takes to get to the bundle of His.
To get to the bundle of His, I'll do that math over here.
You have to now add up 0.7 seconds
because that's how long it took to get to that AV node.
And then you have to add 0.1 seconds and what is that for?
This, my friends, is the delay.
This is the delay of the AV node.
Remember, the AV node creates this delay
so that the ventricles contract just a little
bit after the atriums do.
So this delay is actually built into the system.
The delay is about 1/10 of a second.
And then you have to figure out how long it takes to travel.
So how long does it take to travel that last little bit.
Well, it's going to be going 1 second, it covers 2 meters,
and we know that 1 meter has 100 centimeters.
And we know that we're trying to cover 1 centimeter.
So centimeters cancel, meters cancel.
And you're left with 1/200 seconds.
So that's how long it actually takes to travel.
Travel time, you can think of it as.
So that would be 0.005 seconds.
So in total, it's now taking us 0.805 seconds.
So this is how long it takes to get to the bundle of His.
So now, let me write that up here, 0.805 seconds, now we're
really happy because we were able to beat out both the AV
node-- and I guess from the perspective of the SA node,
if SA node cells got happy, that's what
they would look like.
So it basically gets there really, really quickly
is the point.
So 0.7 seconds and 0.805 seconds.
So that explains at least why you never really see
the AV node or the bundle of His cells firing, right?
Now going back up here, imagine for a second
that you actually had a problem.
Let's say you actually had some sort of disease
or some sort of issue with your cells
and let's say these SA node cells gave up.
Well, if they gave up, then no signal
would be coming into the AV node and the AV node
becomes your plan B. This is your plan B. The SA node,
of course, that's your plan A. That's
what you're usually doing.
But it's nice because you have this plan B
and if one second goes by without a signal,
then the AV node kicks in and that'll start firing.
And you'll have a new heart rate something closer to 40 to 60,
but at least your heart is beating.
Now let's say catastrophe strikes and your AV node
is down too.
Well, your bundle of His is your plan C.
And so now if 2 seconds go by and your bundle of His
have not gotten a signal, then they start firing
and your heart rate will be somewhere between 20 and 30.
So these are the backup mechanisms
your heart has to make sure it always beats.
And this is one of the neat things
that your heart has figured out, to create not just
a plan A, but also a plan B, and a plan C.