welcome to another MedCram lecture we're talking about hypoxemia and this
time we're talking about hypoventilation we've already reviewed hypoxemia high
altitude and also diffusion let's talk about hypoventilation so again let's go
back to our alveolus which we showed here and our pulmonary vein pulmonary
artery system okay so this is the pulmonary artery which is deoxygenated
and this is the pulmonary vein which becomes oxygenated and you recall that
we've got oxygen which goes in and carbon dioxide which comes out so how is
hypoventilation go to cause hypoxemia well hypoventilation is simply when the
lung or the respiratory rate or the tidal volume goes down so that the
amount of ventilation that's occurring the amount of air that's going in and
out of the lung is decreased now the major consequence of hypoventilation is
an increased co2 and basically that increased co2 is gonna force the oxygen
in the alveolus to go down and as a result this is going to cause the amount
of oxygen in the pulmonary vein or the pulmonary artery to go down as well now
remember there was a a gradient equation and let's go over what that a a grading
equation was again and this is the a a gradient equation and normally it equals
about sixteen or twenty or so where in the low numbers this factor right here
dictates what the alveolar concentration of oxygen is and of course this you can
get from a blood gas is simply the arterial or the pulmonary artery
pulmonary vein concentration of oxygen or partial pressure of oxygen you can
see here if you have hypoventilation you're gonna have an increase in this
factor here which is going to cause this factor here to be bigger which
subtracted from this factor is going to make this whole a smaller and as a
result the is going to be smaller as well and your
a a gradient actually in hypoventilation stays about the same so what you see
here is the a8 gradient in hypoventilation actually does not change
but what we do see is the alveolar concentration of oxygen goes down and
the arteriolar concentration of oxygen goes down therefore we have hypoxemia so
where do we see this we see this a lot in patients who hyperventilate so why
would patients be hypoventilating narcotics and basically the example that
I like to give is imagine that your resident or a physician go up onto the
wards and what do you see there if you've got a orthopedic patient
post-operative who got a lot of dilaudid the patient is hypoventilating their
respiratory rate is eight they're in bed they're not responsive and yet let's say
they have a continuous pulse oximeter so an S a o2 or an SP o2 and it says ninety
two percent on four liters of oxygen so the question is is this patient in
respiratory failure and the answer is yes they're in respiratory failure
because if you were to do a blood gas you would see that their pH is like
seven point one five they're pco2 is quite high let's say it's 60 or 70 and
their po2 is let's say about 70 and their bicarb is still normal because
this is acute what's happened here is you've got the pco2 going up the pH
going down but because you're on supplemental oxygen you're able to get a
reasonable oxygen on your pulse oximeter so this is another good reason why not
to just look at the pulse oximeter when you're looking and seeing somebody to
see if they're in respiratory failure because they could very well be in
respiratory failure the other aspect of this is that people who've got
hyperventilation and hypoxemia can very easily be fixed with supplemental oxygen
and so that brings us to the other characteristic of hyperventilation which
is it responds to oxygen very well so let's review what
we've already talked about so we've got hypoxemia remember emia means in the
blood and we're talking about hypo ventilation okay here are the key points
about that number one it responds to
supplemental Oh two we've noticed that also in diffusion abnormalities and also
in high altitude you've got a normal capital a lowercase a gradient the other
one that had a normal gradient so far is the high altitude remember that
diffusion had it increased a a gradient and what are the some of the causes
we'll see we'll have high co2 from narcotics so that's what you'll see in
hypoxemia secondary to hypoventilation thanks very much