Alright everybody.
Welcome back to our next lesson on hemodynamics.
In this lesson we are going to talk about the delivery of oxygen, which is ultimately
the end goal of our hemodynamics.
The goal is to delivery oxygen as well as nutrients to perfuse our end tissues and also
taking away any waste products that are a result of metabolism.
But ultimately the most important thing is getting the oxygen to our tissues.
In this lesson we are going to talk about, and we are going to cover a few different
equations that are used in understanding our hemodynamics.
I do want to preface this and say that the point is not to have these as something that
you guys need to memorize, but really just to break it down and help you understand the
components that all play a factor in the hemodynamics, and ultimately the delivery of oxygen.
So the first equation we are going to look at is just that; our delivery of oxygen.
So this is going to be our DO2.
And the equation for our DO2 consists simply of our cardiac output ("CO") times the concentration
of oxygen in the arteries (CaO2) times a factor of 10.
The 10 is really just there to get our units of cardiac output and the concentration of
oxygen all on the same page.
Now you may look at this equation.
It may seem a little scary or overwhelming in trying to understand how this plays in
to hemodynamics.
Like I said, the goal is to understand the components.
Our end goal is delivery oxygen, and in this case we can see that the components that make
up our delivery of oxygen are "cardiac output / CO," which is going to be how much blood
we are circulating through our body, and our 'concentration of arterial oxygen / CaO2"
which is basically how concentrated is the oxygen in the blood that we are beating.
So the concept is pretty simple, but I do want to break this down a little further in
order to better understand what is happening.
The first one that I want to talk about is our concentration of arterial oxygen, so that
is going to be our CaO2.
And big surprise, guess what, there is an equation that we can use to determine what
this concentration is.
And this is going to consist of our hemoglobin times 1.39, times our arterial oxygen saturation.
Then we are going to add our partial pressure of arterial oxygen and multiply that by 0.003.
Now you're probably looking back at the DO2 equation, and initially we were thinking that
was kind of a complicated equation to look at, and now you're looking at this and you're
like "What the heck is going on here?"
Again my point of this is that I don't want you to memorize this equation.
There may come a point, later on in your nursing career, as you become more advanced and more
trained that this may play a role in some of your practice.
For the most part, the reason I'm explaining this here is I want you to understand a couple
key concepts that i want you to take away from this.
The first of these is that in order to determine how concentrated our oxygen is in our blood,
that there's 3 factors that go in to that.
We have our hemoglobin, we have our oxygen saturation, and we have the partial pressure
of oxygen, which is going to be our PaO2 off our ABG.
If you take a look and see where these components play in to this equation, you can see that
our PaO2 actually makes up a pretty insignificant portion of our concentration of oxygen because
we are going to multiply that by 0.003.
Where as our hemoglobin and our oxygen saturation we are multiplying by 1.39, so obviously we
are going to end up with a larger number.
So to really drive this point home, I do want to give you some values so we can plug it
in to the equation here and really see the point that I"m trying to make.
So let's say we have a hemoglobin of 7.
We would take that and multiply that by 1.39 then we would multiply by; lets say we have
an oxygen saturation of 0.95.
If we do the math on this, this give us a grand total of roughly 9.24.
Now if we move over to the other side of the equation.
Let's say we have a partial pressure of oxygen off our ABG that is 400.
We'll multiply that by 0.003 and that will give us 1.2.
So here you can see, even with very high partial pressure of oxygen, it still only composing
a tiny part of our total amount of concentrated oxygen in our blood.
And really the important takeaway from this is, the two biggest factors in determining
how much oxygen we are going to have in our blood is going to be our hemoglobin and our
oxygen saturation.
Well hopefully that makes a little bit more sense for you guys in understanding how we
determine how much oxygen is going to be in our blood.
If we go back and look at our delivery of oxygen equation, the other factor that is
used in determining how much oxygen we are delivery is going to be our cardiac output.
And so our cardiac output, if we go ahead and bring that down here... we do have another
equation for this.
And this is going to be "cardiac output / CO" is equal to our "heart rate / HR" time our
"stroke volume / SV."
And if we take a look at these components, essentially what this is saying is, if we
take how much blood is being ejected with each beat, which is our stroke volume, and
multiply that by how many beats per minute, which is our heart rate, it's going to tell
us what our cardiac output is in liters per minute.
So just to give you some normals on some of these numbers, our stroke volume is normally
going to be 60-120 milliliters per beat.
We should know our heart rate is normally 60-100 beats per minute.
And our cardiac output is going to be 4-8 liters per minute.
It is important to note that cardiac output doesn't take in to consideration the size
of your patient.
So if you have a small lady with 4 liters of output, that's going to be a big difference
when we are comparing that to 4 liters of output in, let's say, a 350 pound man.
So this is where there is another thing that comes in to play that we call cardiac index.
And essentially cardiac index is just going to take your cardiac output and your body
surface area and determine how that output relates to the size of the person.
And this way, no matter what the size the person is, we can talk about the same numbers
and have an idea of how sufficient or insufficient that cardiac output is going to be for that
patient.
For our cardiac index, our normal numbers are going to be 2.5 to 4, and this is actually
in liters per minute per meter squared.
Now finally from all of this we can break things down one more time.
And that's if we take a look at our stroke volume.
There are three factors that go in to determining what our stroke volume is going to be.
That is going to be our preload, our contractility, and our afterload.
Now in a little bit we will get to talking about these different components and what
they mean and how they impact our stroke volume and ultimately our cardiac output, but for
now i just want you to understand how, if we take a look at our hemodynamics, our ultimate
goal is about delivery oxygen.
And if we take that calculation and break it down we can see how our cardiac output
is composed as well as how the concentration of oxygen in that blood is composed, and to
give you a good understanding of those components, why they are important, and ultimately how
they effect our hemodynamics.
And that's going to conclude this lesson on hemodynamics, talking about the delivery of
oxygen.
In our next lesson we are going to talk about our cardiac output and all the different factors
that play a role in determining what that cardiac output is.