- [Voiceover] So what do we mean when
we say compensation?
Well, let's remember that heart failure,
you have this decreased ability
to meet your body's demands, right?
Well, your heart and body have this way
of trying to make up for this decreased ability
and increase the blood you supply,
and that's called compensation.
Because you're compensating for this decreased supply.
But in heart failure this can lead
to something called decompensation.
So eventually you can't compensate anymore
and your symptoms start to get worse
and heart failure gets worse.
So there are a couple main ways we try to compensate.
And each of these ways either increase your stroke volume
or increase your heart rate.
And remember that cardiac output
equals stroke volume times heart rate.
So if you increase either one of those
you're gonna increase your cardiac output.
The first thing we're gonna look at
is activating your sympathetic nervous system.
And this is like your fight or flight response.
So, for example, what if you're hiking along
in the woods and you see this bear?
Your sympathetic nervous system is gonna kick in.
And your heart rate's definitely gonna go up
and your stroke volume is definitely gonna go up.
And so your body is preparing you
to either fight or flight, or run away.
So when your cardiac output goes down
your sympathetic nervous system kicks in.
And it uses these receptors
on the heart that tell the heart to
one, beat or contract harder with more force
which increases your stroke volume.
And two, to beat faster,
which increases your heart rate.
But if you use these too much
they start to down-regulate.
Or decrease in amount.
So you know, there's less of them.
And less receptors means your heart
won't respond as it did before.
So think of it like this.
What if you're moving?
And you call a bunch of your friends over
to try to help you move your stuff.
And you know, they're glad to come over and help you.
And so you move your stuff
and then you decide next week,
oh, I want to move again.
So you call them up again
and say "hey, I need some more help".
And they're like okay, I guess it's kinda weird,
but I'm gonna come help you.
And then they come.
But then the next week you're like,
oh, I'm gonna move again.
And then you keep doing this week after week
and those friends are gonna start to
well one, question why you're moving so much.
And two, they're gonna stop
maybe answering your phone calls.
It's sorta the same thing
with your sympathetic nervous system.
When you activate it too much
it's gonna start helping less and less.
So the next big way we can compensate is by
increasing this thing called preload.
Now preload is defined as this pressure in the ventricles.
So in this lower chamber,
after it's filled but before it contracts.
So during diastole.
When it's filled, the walls and the heart muscle cells
are all stretched out
because it's like filling up a water balloon.
So you put the water in it and then the water
causes that balloon to expand, right?
So the more you fill in,
the higher this pressure or preload.
So as you keep filling, the walls stretch more and more
and we get higher pressures or preload.
To do this, to fill more,
to get more blood in those ventricles
your body releases these specific hormones
like antidiuretic hormone, or we call it ADH sometimes,
or aldosterone, to increase this filling volume.
So if the ventricles used to have about 100 milliliters
at the end of diastole,
now maybe they have a little more,
like 115 milliliters.
This extra little bit might not seem like a lot,
it's like one tablespoon,
but it's enough to stretch the heart chamber
and muscles just a little more.
So that's great, but why does increasing the preload,
or filling pressure, increase the force of contraction,
and so your stroke volume?
Well, think of it like stretching a rubber band.
The more you stretch the rubber band
the more force it'll snap back with, right?
Okay, so there's a lot going on here, right?
So let's go one step at a time.
So as you fill the ventricles with more blood
it stretches those muscles out, right?
And just like the rubber band did,
when you stretch it out it contracts,
or it snaps back with more force.
And when it contracts with more force,
you get more stroke volume,
you get more blood ejected.
And so this guy named Frank-Starling saw this
and kind of decided to cut corners
and create this law that says,
as you increase your pressure or your preload,
you also increase your stroke volume.
And that's the Frank-Starling law.
Sweet, so increasing preload increases stroke volume
and therefore, your cardiac output,
and is one way we can compensate.
But again, like over activation
of the sympathetic nervous system,
this is a real delicate balance.
So these muscles now contract with more force, right?
But that means that they use up more energy to do that.
And therefore, they need more blood.
And if there's no additional blood flow
coming to the heart muscle cells,
they can begin to die off.
So let's get back to that first analogy
where that guy's moving all the time.
But this time maybe he decides,
oh, I'm gonna pay my friends to help.
And so all the sudden they all come running back
and then they help him move in
and they kinda support his weekly moves.
But then all of a sudden he starts
moving every other day,
and he's not increasing their pay.
Well, they start to get overworked,
and without more money, they're gonna leave, right?
It's kinda the same thing with our muscles cells.
They're contracting harder and they're working harder.
So they need more oxygen and more blood,
but they're not getting that so they start to die off.
So we just saw that when you contract harder
you eject more blood.
Well, besides increasing preload,
there's one more way we can do that.
And that's gaining muscle, that's bulking up.
This is also called myocardial hypertrophy,
where your heart gains muscle mass.
And when it gains muscle it contracts harder
because it's stronger.
So to try to make up for this decrease in stroke volume
or this death of cardiomyocytes, or heart muscle cells,
these surviving cardiomyocytes
become elongated and they grow.
And this causes the heart muscle as a whole to get larger.
So you get this enlarged heart muscle.
So it's like the ones that don't die
are like whoa, I gotta start bulking up
so I can take over for these guys
that aren't contracting anymore.
And these bulked up muscle cells contract harder,
they eject more blood and they increase cardiac output.
Perfect.
But as we know, this usually comes at a cost, right?
Well, more work means more oxygen
and more blood supply.
But with heart failure we know that that's not an option.
And so without an increased supply
these bulked up cells get overworked
and then they start to die off.
Not only that, if the heart muscle gets too big
the chambers start to get smaller
and so there's less blood that can fill in the chambers.
So those were the three modes of compensation.
You can activate your sympathetic nervous system.
You can increase your preload.
Or you can gain muscle,
which is called myocardial hypertrophy.
And so for each of these,
our goal is to increase cardiac output.
By either heart rate or stroke volume.
But like we saw, when you use
any of these too much
it leads to unintended consequences.
And eventually heart failure gets worse.
And this is what we call decompensation.
So when you overuse the sympathetic nervous system
your receptors start to go away.
And you end up with a lowered response.
When you increase preload,
you muscle cells stretch and then contract harder,
but this costs more oxygen
and then eventually leads to cell death.
Similarly with hypertrophy,
these bigger muscles also use more oxygen.
And without that the cells die.
So not only that,
these three can often feed back on each other.
Like the death of muscle cells from an increased preload
might result in hypertrophy,
because those remaining cells start to bulk up.
And this increases cell death even more.
And then as the heart weakens from this
it might stimulate the sympathetic nervous system
to try and beat harder and faster,
which further progresses heart failure.
Most of the time it's really difficult to pinpoint
which one was the initial culprit.
But the major takeaway should be that compensation,
regardless of its intentions,
is often a major contributor to worsening heart failure
and eventually decompensation.