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 going to 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 going to 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 going to 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 that you become more short of
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 total barometric pressure in terms of
millimeters of mercury is 760 millimeters of mercury this is the way
we measure this is the air pressure at sea level 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 two one is
equal to 150 millimeters of mercury so this is the pao2 available
to us at sea level okay 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 380
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 pao2
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 to
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 100% 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
note a cross-sectional area of a alveolus looks like and there's a blood
vessel going by and a capillary the AAA gradient this is the capital A 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 me and 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 a a 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 going to talk more about these
mechanisms these five physiological causes of hypoxemia in the next lecture
thanks very much
you