00:00
Let’s talk about
the equation, shall we?
Now, every step of the way, I want you to
be clear about what we are looking at. Your
PAO2, is your PiO2, which you
are inspiring and from this you subtract your
PaCO2 divided by aO2, excuse me, CO2 and divided
by the R quotient. We are going to put all
this together. What is your PiO2? This then
represents your FiO2 and your barometric pressure
and the water vapour pressure. Let's go
even one step further. Your FiO2 is, for the most
part, going to remain pretty constant on planet
Earth at approximately 0.2. Please use 0,
that will make your life easier. Now, what
is this whole thing that I have been repeating
over and over again about high altitude? What
does that mean? Now, you will see.
00:56
Now, if you are ahead of the game, then this
little part right here is really good review
and just positive reinforcement. So, let's begin.
Your PAO2 equals the following. So, why is it only
say PAO2. Where is little 'a'? Because
while let’s say that you are doing a question
on a computer screen, really difficult for
you to do an ABG on a computer screen. You
can’t take a syringe and put the needle in.
As much as you would like to, you can’t.
01:24
So, the little 'a', they will give
that to you. So, of the A-a gradient, all
you are focusing on right now is the big 'A',
is that clear?
So, Dr. Raj, this entire formula is only for
the big 'A'? Yep. That is exactly what
I am telling you. Now, if you are lucky, maybe
they will just give it to you. But, chances
are not. So, someone has to break that to
you and you are responsible for calculating
it. ........., as I said, they will do an
ABG and they will give you the PO2 in the
arterial side.
01:52
Let's talk about the alveoli now. So, you
have an FiO2 of 0, what does 760 mean to
you? Good. That is you barometric pressure
at sea level. Take that air, oxygen, put it
into your trachea. What is the trachea responsible
for? It has cartilage we talked about. We
talked about cilia, mucous.
This is then introducing water vapour. So,
that partial pressure of water is 47 mmHg.
You subtract this from the 760 and you get
approximately 713 or let’s say about 700
to keep things rather simple. Where you are
at this point? You are in the trachea, right,
in the breathing area, basically. Breathing
meaning strictly the conducting zone. What
does that conducting zone mean to you? Do
you see as to how that anatomy now is coming
into play? That conducting zone is going to
take this oxygen from the ambient air at sea
level and put it into the alveoli. What
about that alveoli? Your oxygen has a roommate.
And what about these roommates? Well, they
don’t exactly get along. And actually, when
one is in, the other one is out. That way,
they actually maintain homeostasis. What am
I talking about? Carbon dioxide and oxygen
in the alveoli, keep it simple. So, whenever
there is carbon dioxide, inverse relationship
with oxygen.
03:13
So, now, let's start way back. Let's start
back up in the ambient air and we have 760.
03:20
And all we have done with 760 multiply by
0.2. What do you get? 160. Is it 160 of PO2
that you have in the alveoli? Not at all. That
is ambient air. What’s next? You put in
the trachea. Once you put in the trachea,
the PO2 in your inspiratory air will be 760
minus 47 which gives you approximately 700
and you multiply that by 0.2. That will give
you 150, isn’t it? Physiologically, remember
this conversation? That 150, is that
PO2 of alveoli? No. What do
we say PO2 alveoli is? 100. So, what’s left?
Oh, the carbon dioxide. What’s your carbon
dioxide approximately in the alveoli? Remember,
you are taking the carbon dioxide from the
pulmonary capillary, putting into alveoli
and you are blowing it off. So, CO2
approximately 46-47. Hmm, what are you going
to do with this? You subtract it from the
150 and you get approximately 100. Stick to
that and you will be fine. So, PiO2 equals
the partial pressure of oxygen in the central
airways. Isn’t that interesting?
So, take a look at that equation that we see,
the second bullet point there. PiO2 is only
taken into consideration the air that is in
the airway. What is the difference between
the second and the third equation? That third
big equation which is the complete equation
for alveolar gas equation is the actual oxygen
in the alveoli. Do you see how beautifully
that worked? You literally are moving from
compartment to compartment to compartment,
from the outside world externally to the alveoli
and every step of the way, you want to be
asked questions. So, what is this fraction
of inspired oxygen fraction? 0, I told you
about, room air. I don’t care for up in
the mountains or if you are down in the sea
level. PaCO2, value from your ABG, they will
give that to you. What’s normal? Approximate
please. We just had a huge discussion forever
on acid-based disturbances, 40. Now, the only
thing is just make sure a week from today,
you come back and you still say, “Oh, yeah,
PCO2 is 40.” Yes,
you want to test your short term memory. But,
you don’t rely upon your short term memory
exclusively. Every so often, you keep coming
back and you review, and it’s the only way.
It’s about reinforcement. Here is the barometric
pressure at sea level, 760. Your water vapour
pressure, 47 at 37 degree Celsius. Your respiratory
quotient, about 0.8. You can use 1.0 for calculation
purposes. And the ratio of carbon dioxide
production to oxygen consumption is what this
is. May I ask you something? I want you
to go down to tissue. Are you there? Tissue.
Really, how do you get there? You have the aorta,
systemic arteries and you end up getting into
arterioles and the capillaries and I am at
the tissue. Okay, what are you doing at the
level of tissue? That dissolved oxygen,
or first diffused through, followed by?
Good. That oxygen that is coming off of my
haemoglobin. Are we clear? One oxygen comes
off the haemoglobin and all of a sudden, my
PO2 drops down to 40. How many oxygen have
on your haemoglobin? 4. So, Dr. Raj,
you are saying 1 comes off, I have 3 left
on my haemoglobin, there is 75% and that is
equivalent to a PO2 of 40? Yep, amazing. That
is still lot of oxygen on your haemoglobin,
isn’t it? And that's still, you
call that deoxygenated? Yes, you do. That
oxygen is being consumed by the tissue to
do what? Daily activity. How about just to
breathe, huh? Daily activity. And in the mean
time, if that oxygen allows for you to go
through aerobic glycosis, TCA cycle, electron
transfer chain and you are producing
carbon dioxide. There you go.
07:26
Now, we go to systemic veins. Systemic veins,
they have carbon dioxide being added to my
40 to give you 46. Are we clear? The ratio
of carbon dioxide production at the level
of tissue to oxygen consumption at the level
of the tissue. It makes no sense for you to
say there is oxygen consumption at the lung.
All you are doing at the lung is to deliver
the oxygen for Pete’s sakes. I will give
you exception for the carbon dioxide, but that statement
right here, do you understand that? Consumption
and production at the level of tissue.