welcome to another MedCram lecture we're going to talk about delivery of
oxygen to the tissues and the pathway that it gets to the tissues through the
lung the bloodstream and then finally on to the tissues and this is important
because it involves a lot of physiology it involves how oxygen diffuses into the
blood and the equations that are used and this is very easily testable because
they can use equations to minimize the answers and try to get you to think
about these equations so let's go through this using the major equation
which I'm going to write for you so the delivery of oxygen which will abbreviate
D o2 is equal to the cardiac output which I'll say is Co times the content
of the arterial blood which I'll abbreviate cao2 so cardiac output is
basically in liters per minute and the concentration of oxygen in the arterial
blood is basically units of millimeters of oxygen and deciliters of blood so the
basic equation here is again the delivery of oxygen is the cardiac output
how fast we can deliver oxygen in this patient two times the content of that
fluid and we can break this down even further as we'll see the cardiac output
as you know is really the stroke M times the heart rate and the content the
oxygen content of the blood is actually an equation that you should know which
is one point three four which is a fudge factor basically times the humid lobe
and concentration times the saturation of that hemoglobin molecule plus point
zero zero three times the partial pressure of oxygen in the blood now that
sounds a little complicated but let's break this down okay so the cardiac
output as you know from any kind of a discussion in
cardiology is simply the stroke volume times health many times that stroke
happens in a minute and that'll tell you what the cardiac output is so basically
this is the how many liters per minute you're pumping and that many liters per
minute times the amount of oxygen in those leaders is going to tell you how
much oxygen you're going to be delivering to the peripheral tissue well
the content of blood the content of oxygen in that blood is an interesting
portion here there's two components notice there's the first component and
there's the second component and that's very important here because you see when
there is an oxygen molecule let's take a look here at the alveolus
there's the alveolus and the oxygen molecule which diffuses into the goes
into that alveolus is going to come in contact with the pulmonary artery and
then it's because they've become oxygenated and turn into the pulmonary
vein now when that happens this oxygen molecule is going to diffuse down into
the plasma in the a pulmonary circulation when that oxygen molecule is
in solution we'll say it's dissolved in the plasma we can detect that by
measuring the partial pressure of oxygen in the plasma this is known as the po2
the partial pressure of oxygen however something happens as soon as that oxygen
molecule goes into solution in that plasma what happens is that a red blood
cell comes by corpuscle there's a side view of it and it goes in to the bag of
hemoglobin known as the red blood cell and it binds to your hemoglobin molecule
when it does that it essentially comes out of solution and precipitates with
that hemoglobin molecule the only way you can detect that now is by detecting
the saturation of this hemoglobin molecule so there's two forms of oxygen
in the blood there's the oxygen which is dissolve
solved in the plasma and then there's the oxygen molecules which are combined
or connected or precipitated to the hemoglobin molecule and that here is
what is going on this part of the equation in terms of oxygen in the blood
is the part that represents that oxygen which is bound to hemoglobin so you can
see there's a human globin concentration factor and there's a saturation factor
so the amount of oxygen in the blood is directly proportional to a fudge factor
of 1.3 4 times the concentration of hemoglobin that's in the blood to begin
with times the saturation of that hemoglobin molecule in the blood and
that tells you how much oxygen is there the other portion of it which is this
part down here tells you how much oxygen is carried in the blood by simply the
amount that is dissolved in the blood and as you can see it's a very small
proportion so therefore the majority of the oxygen which is carried in the blood
is carried in the form of oxyhemoglobin and not in the form of partial pressure
of oxygen that is dissolved that's a key point here it's a key point because
they're going to ask you where most of this oxygen is coming from now let me
rewrite that equation or that part of the equation again we've said that the
content of oxygen in the blood is equal to one point three four times the
hemoglobin concentration times the saturation of o2 plus zero point zero
zero three times the pao2 so if I wanted to increase the
concentration or the carrying ability of oxygen in the blood in general would it
be more important for me to increase the saturation or the pao2
and it would be more important for you to increase the saturation just
algebraically based on where this equation is now how is that boiled down
well there's something called the hemoglobin binding curve which I'm sure
you've all heard of by this point okay this describes the relationship between
the po2 which is what we were just talking about or the pao2 and
the saturation percent saturation in the blood okay so it kind of looks like this
okay and this actually just keeps going on and on and it kind of asymptotes with
100% and you've got 50% kind of in this range right here the point of this is
that once your saturation gets up high enough into the 90s you can have so
here's 90% you can have increasing po2 levels which are only going to modestly
increase your saturation levels so it gets the point that increasing the PA o
2 will increase the oxygen content by a very small amount and it will only
increase this part by a very small amount and why is that important
that's important to understand that there is a diminishing marginal utility
on increasing the amount of partial pressure of oxygen that you're
delivering to patient because you're not going to get that much more saturation
because of the hemoglobin binding curve and that's important because it's the
saturation which is part of the big part of the equation which determines the
content of oxygen in the blood okay so let me show you where this would be
practical on a test let me rewrite that equation again so I can show you how
this matters so going back to the first equation the delivery of oxygen to the
peripheral tissue generally speaking is equal to the cardiac output times the
oxygen content of the blood let's expand that once again we know that cardiac
output is heart rate times stroke volume and that the content of oxygen is equal
to one point three four times the haemoglobin times this essay o2 or the
saturation plus zero point zero zero three times the pao2 and then all of
this is multiplied by itself okay so we can come up with an equation here let's
say we've got a patients whose saturation is
let's say it's 92% here's the question the question is which of the following
will increase the delivery of oxygen the most and then here are the answer
choices number one we can increase the stroke volume by 10% number two we can
increase the heart rate by five percent number three we can increase the humid
loeben by twelve percent number four we could increase the SAO to the saturation
by five percent or number five we can increase the pao2 by thirty percent so
which one of these is going to increase oxygen delivery to the tissues now to
those uninitiated or confused or didn't see this lecture you would be tempted to
say if I increase the PA o two by thirty percent just numerically that's gonna
improve my oxygen delivery obviously if I give more oxygen I'm gonna be able to
deliver more oxygen but however if you look at this algebraic equation
mathematically you can see that the smallest component of this equation is
this and so increasing this amount by thirty percent is going to be multiplied
by a very small number and you're not going to get the biggest bang for your
buck in terms of delivery of oxygen so that's not the right answer the rest of
these issues so stroke volume heart rates hemoglobin and saturation
are all pretty equivalent because we're multiplying all of these things together
okay so all of this is multiplied together so this going up by five
percent times this going up by five percent plus this going up by five
percent or this going up by five percent are all going to have pretty much the
same equivalence in terms of the improvement in the delivery of oxygen
and so in that case you'd have to say then that increase in the human
and concentration by 12% will have a greater effect on the delivery of oxygen
than then the stroke volume going up by 10% or the heart rate going up by 5% now
what about the saturation going up by 5% yeah if this went up by 5% that would be
nice but still not as good as increasing the humor globin by 12% and this is the
type of test question that you might see and it has to do with the fact that you
understand how most of the oxygen is getting to the peripheral tissues and
thanks for joining us