Finally, we are dealing with the gases, this
puts us into yet another formula and this
is one in which you can’t,
there is no way you can circumvent this topic.
A-a gradient is what we are looking at and
it is the big 'A' that you are paying attention
to and the little a. Literally. Have to. The
big 'A' represents PAO2. In another words, this
is the oxygen in your alveoli and then you
subtract from this, the oxygen that is in
your little 'a' and that then represents
the artery. If you take a look at the first
bullet point here, you have, PaO2 and from
hence forth, you are not going to hear me
say PaO2 anymore. I am just going to refer
to as being PO2, but to make sure that we
are clear and that our teaching points are
coming across. The PaO2 is a partial pressure
of oxygen in the artery and you will tell
me that is approximately PO2, there you go,
of 100. 95, depending on the little bit of
shunt. Now, the PAO2 would be the alveoli
and that is obtained from the alveoli, obviously,
but more importantly, let’s just talk about
what A-a gradient means to you.
What does that gradient even refer to, the
gradient coming out of the alveoli or going
into the alveoli? Put yourself in the alveoli
right now. There you are. You are sitting
in that sack. Are you there? Nice to see you.
Okay, now, in that sack, you are trying to
get that oxygen through the alveolar membrane,
through the interstitium and into the pulmonary
capillary. That is the gradient. Who are you?
Oxygen, okay. So, the gradient there should
normally be, well how much? What is your PO2
in the alveoli, do you remember from physio?
100. Good! And that PO2 that is being carried
by my pulmonary artery and pulmonary capillary
into the alveoli, the PO2 is how much? A measly
40, right? So, that will be the blood on the,
or oxygen content on the venous side, isn’t
it? Deoxygenated blood, pulmonary artery and
company. Therefore, the difference between
the two would then give you a gradient of
60. That is a pretty big gradient for the
oxygen to hop out of the alveoli and diffuse
through the membranes into the pulmonary capillary
and then bind to whom? Good. Haemoglobin.
Speaking of which, the PO2, the partial
pressure of oxygen, is this actually referring
to the haemoglobin attached or the oxygen
attached to the haemoglobin? Not at all. The
oxygen bound to haemoglobin, give me a test
that you are going to use for its percentage.
That is your saturation of oxygen, isn’t
it? So, your saturation of oxygen approximately
97% being the optimal percentage of your oxygen
bound to haemoglobin. Right? But, here, we
are referring to PO2, that little bit of oxygen
is dissolved in your blood. I hope that is
clear. I hope that you understand the diffusion.
I hope you understand the gradient. And what
that means to you clinically? Ah, that is
what we are getting at.