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 going to 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 AAA
gradient equation was again and this is the AAA gradient equation and normally
it equals about 16 or 20 or somewhere 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 arteriole 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
here which is going to cause this factor here to be bigger which subtracted from
this factor is going to make this hole a smaller and as a result this 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 a a gradient and 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 you're a resident
or a physician you 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 8 they're in bed they're not
responsive and yet let's say they have a continuous pulse oximeter so an sa-2 or
an SP o2 and it says 92 percent on 4 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 7.15 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
their aspect of this is that people who've got hypoventilation and hypoxemia
can very easily be fixed with supplemental oxygen and so that brings
us to the other characteristic of hypoventilation 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 so here are the key points about that number one it responds
to supplemental o - 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 to 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
you