Hypoxemia – The 5 Causes & Treatment… #1 High Altitude


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

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