welcome to another MedCram lecture we're going to talk about hypoxemia and
its five physiological causes again anything with the ending of a emia
it's gonna have to be something that's related to the blood so hyponatremia
hypokalemia hypercalcemia again it's referring to the amount in the blood
this is as opposed to the term that we use see a lot which is hypoxia the
difference here is hypoxia has to do with the amount of oxygen in the cells
that's the target area but how the oxygen gets to those cells is in the
blood and that's what we're going to talk about today so what are the reasons
why you might have a low oxygen concentration in the blood well there's
five different mechanisms for that and they lend themselves to five different
answers on a test which is why this is very highly testable stuff and we want
to go over it so high altitude is one of those reasons we'll go over that today
the other one is diffusion the next one is hypo ventilation next is shunting and
then finally we're going to go over VQ mismatch and this will take a number of
lectures to go over but we want to talk about where we're gonna go with this so
these are the five different reasons for low oxygen in the blood let's go over
those now okay so we're gonna talk about high altitude this is exactly as it
sounds as you go up mountain the amount of oxygen in your blood
at the top of that mountain is less than at the bottom of that mountain
that's actually quite intuitive because you know that as you go up in altitude
it becomes harder and harder for you to do things so you become more shorter
breath and this is something that we can actually experience people who going up
to Everest base camp notice that they can't breathe as well as they can when
they're down at sea level so what is the reason for that well to find
out we have to look at what is the equation that gives us the amount of
oxygen that dissolves into our blood and so the way we look at that is by looking
at the total barometric pressure the total barometric pressure in terms of
millimeters of mercury is 760 millimeters of mercury this is the way
we measure it this is the air pressure at sea level and now we know that oxygen
makes up about 21% of that so what happens as you go up in
altitude does the fractional or partial pressure of oxygen go down no it doesn't
what actually happens is the total barometric pressure goes down and as a
result the 21% of that pressure becomes smaller let me give you an example so at
sea level again we've got 760 millimeters of mercury now as soon as
you inspire this air at sea level into your lungs
it becomes humidified and this is kind of a red herring we're throwing into
this but you should know that the saturated vapor pressure is about 47
millimeters of mercury and so right off the bat 760 - 47 times point 2 1 is
equal to 150 millimeters of mercury so this is the pao2 available to us at sea
level ok so this is at sea level now at 19,000 feet which is approximately just
above base camp for Mount Everest the the total barometric pressure at that
altitude is about three hundred and eighty millimeters of mercury we
subtract the 47 millimeters of mercury from the vapor pressure and what we come
up with is about 70 millimeters of mercury of P
Oh - and so here we clearly see that the reason why the pao2 is going down in the
blood is because the total barometric pressure has dropped not because the
fio2 which is this has changed in fact it is the same it's the same point - one
so to review that high altitude what we see there is it's a result of a decrease
in the total barometric pressure which is 760 millimeters of mercury at sea
level what we see here is that if we put on a hundred percent oxygen
the pao2 or the oxygen content in the blood actually goes up so it responds
which is good the other thing that we see here is something called a normal a
a gradient so what is an a a gradient capital a stands for the alveolus if you
know it a cross-sectional area of a LV olace looks like and there's a blood
vessel going by and a capillary the AAA gradient this is the capital a and this
is the lowercase a it's simply talking about the difference in the o2 here in
the alveolus versus here in the capillary and so if there is a big
difference between these two that must be mean because there's something going
on here or there's a barrier or there's something that's preventing this oxygen
from fully oxygenating the capillary in this case there isn't it's simply a
result of a low barometric pressure and as a result of that the AAA gradient is
normal so what does the body do in the situation to counteract the low pao2 in
the blood hyperventilation and as a result of that what we see is the co2
levels or the P co2 levels the partial pressure of carbon dioxide actually goes
down and this allows more room for the pao2 so again
high altitude is a result of total barometric pressure going down it
responds to 100% oxygen the pao2 goes up when you put someone on
100% oxygen in this case it does respond the AAA gradient is normal and the
body's response to this is to hyperventilate this is why you see
people hyperventilating at high altitude it's to get their PA o2 up okay so we're
gonna talk more about these mechanisms these five physiological causes of
hypoxemia in the next lecture thanks very much