00:01
Cardiac mechanics is the next topic.
00:05
Here, we need to talk a lot about
how much cardiac output we have.
00:09
What is cardiac output?
It’s the volume of blood ejected
by the heart per minute.
00:16
How do we determine
what someone's cardiac output is?
You take the stroke volume,
which is the bolus of blood ejected per stroke
times the heart rate.
00:26
The reason why we go through this process is
because we have two main variables to alter.
00:33
Chronotropy, or heart rate, governs the pump frequency.
00:38
Stroke volume governs how much
push out we have per beat of the heart.
00:45
There are a number of factors that govern the
control of both stroke volume and heart rate.
00:50
Some of them are going to be
very similar between the two.
00:53
Other ones have some inherent differences.
00:57
Just to go through a few of the
stroke volume ones.
01:01
Autonomic nervous system,
such as increases in norepinephrine secretion,
that will increase contractility,
which causes a greater stroke volume.
01:10
Hormones,
such as epinephrine,
also can feed into that beta-1 adrenergic response
and increase the bolus of blood ejected per stroke.
01:20
Interestingly, there are some intrinsic factors,
besides nerves and hormones,
that can also increase stroke volume.
01:29
And that is the heart
has a very unique principle,
in which the more it is stretched,
the harder it contracts
and we’ll go through that in more
detail here in subsequent slides.
01:41
Heart rate is governed by both
autonomic nerves and hormones,
but doesn't have some of those
intrinsic factors that stroke volume has.
01:52
So, what constitutes a stroke volume?
How do you know how much
bolus of blood is ejected per stroke?
Well, for this,
you need to know what
the end-diastolic volume is,
Well, for this,
you need to know what
the end-diastolic volume is,
which is the maximum amount of volume
that you can have,
while the heart is filling?
End-systolic volume is the most blood you can push out
per beat of the heart.
02:15
You simply subtract the fill
from the squeeze
and you get stroke volume.
02:23
Now, what factors affect
both end-diastolic volume and end-systolic volume?
The primary factors that affect end-diastolic volume
is a concept called preload.
02:35
Now, what is preload?
Well, I will get into that in the subsequent slides,
but I want to give you first a quick definition.
02:43
That’s going to be how much filling there is
of the left ventricle.
02:47
It’s a little more complex than that,
but that's a good place to start.
02:52
End-systolic volume is affected
primarily by the afterload and inotropy.
02:58
Afterload is the resistance it's pushing against
and inotropy is how hard you’re contracting.
03:06
Now, notice, though,
that there are these primary factors,
but nothing in the body is absolute
or independent of each other.
03:15
And therefore,
there are some secondary effects
between preload, afterload and inotropy.