Great! Now, that we have the acid base box down.
Let’s talk through some disorders that could cause
an acidosis from the respiratory side of things.
If you have a decrease of alveolar ventilation
that will cause a respiratory acidosis.
There are some examples of these such as if
you have a neuromuscular disorder such as ALS.
If you have, Chronic Obstructive Pulmonary Disease
could also cause a decrease in alveolar ventilation.
You can also have decrease in the diffusion capacity
of the lung such as interstitial fibrosis.
would cause this particular response.
Another way you can have a respiratory acidosis
if you have a severe ventilation-perfusion mismatch.
Severe ventilation-perfusion mismatches is
sometimes occur in things like chronic bronchitis,
or very severe asthma, or even acute edema.
So this are all potential ways
you would get respiratory acidosis.
But on something like an acid-base box
what will we be concerned about
for this respiratory problem?
So if you have a respiratory acidosis
you’ll be building up carbon dioxide.
So you would have a high PACO2.
and that is what you are looking for,
for respiratory acidosis.
So why do I worried about ventilation
when I am thinking about CO2 values?
Well, alveolar ventilation (VA)
and PACO2 are inversely related.
What do I mean by inversely related?
If one goes up,
the others gonna go down.
If one goes down,
the other ones gonna go up.
So lets take a look at this.
If you have a hypoventilation, you have
a decrease in alveolar ventilation,
automatically you have an increase in PACO2.
The opposite also occurs.
If you have, a hyperventilation, you have
an increase in alveolar ventilation,
automatically you have a decrease in PACO2.
Now, it doesn’t matter if this
is alveolar CO2 or arterial.
So we could use PACO2 or PaCO2,
either would work fine.
There is an inverse relationship.
Why is this so handy?
Because if you measure someone’s
PACO2 with an arterial blood gas,
you automatically know what
there ventilation status is.
If you measure the low PACO2, you know
that their alveolar ventilation rate was high.
If you measure a high PaCO2, you always know
that there alveolar ventilation rate was too low.
Respiratory acidosis and alkalosis
always works in this manner.
You notice that haven’t really gone through
a lot of respiratory alkalosis problems, right?
We went through three respiratory acidosis, right.
A decrease in AVO ventilation,
a problem with the diffusion capacity,
and a severe ventilation of perfusion
mismatched. All respiratory acidosis issues.
Respiratory alkalosis, the only thing that happens
is you breathe too much. You hyperventilate.
That is why you would have
a respiratory alkalosis.
So you could force yourself
to do that by deep breathing.
Those large breathes and deep breathes
you could breathe out a lot of your CO2
cos you increase alveolar ventilation, PCO2 will drop.
Okay, so lets now use a different
tool rather than in the acid-base box.
We’ll use an acid base diagram.
Now this has a lot of information
on it and its really nice
this allows you to predict what
might happen in response to a change
in something like ventilation rate.
So we have three different axis to deal with.
We have Plasma pH,
we have Plasma bicarb
and then along this isopleths, which is just
a fancy word for this, curvilinear line that goes up.
PCO2 and this is PaCO2 or the partial pressure
carbon dioxide in the arterial blood.
Normally, you have a PACO2 of 40 mmHg.
You have a plasma pH of 7.4,
a bicarb of 24.
So that’s your normal.
The normal is now seeing here as a green dot.
So the normal condition, PCO2 of 40,
Plasma pH of 7.4, bicarb level of 24.
So that is where our starting point is.
If we are going to increase our PaCO2,
that is an increase in carbon dioxide.
That happens if you don’t ventilate enough.
So that is a decrease in alveolar ventilation.
We also call that hypercapnia.
So in these case, you are travelling from
the normal line which is the green dot
up to the hypercapnia line which
is now at 60 mmHg.
We travel up via a linear line.
That linear line is known as blood-buffer line.
That blood-buffer line was previously define for us
as 25 millimolar per unit of pH (25 mM /pH unit).
This is the buffering ability of whole blood.
So things like hemoglobin and other plasma proteins
always allow you to travel not linearly
but having a small slope associated with it.
The other thing I will point out is that bicarb levels
rose slightly but not a lot. Just a little bit.
That little increase in bicarb is simply
the product of the carbonic anhydrase equation.
And I’ll explain it a little bit more after
we go through to the hypocapnic condition.
So now, let’s increase alveolar ventilation.
So now, we are going to go and breathe off more CO2.
In these case, we are gonna move now to these
blue line still following the blood-buffer line.
You’d increase alveolar ventilation which
blows off more CO2 and causes hypocapnia.
You notice in this condition
you have an increase in plasma pH.
That’s because if you look at the dot,
the blue dot, drop the vertical line down,
you would see that pH increased
from 7.4 to very close to 7.6.
Now, lets deal with this bicarb issue again.
Because when they said the blood-buffer line
doesn’t have bicarbonate,
you still might get changes in bicarb.
it’s a very small change but there
is a small decrease in bicarbonate levels.
And that is a hard thing to
kind of wrap over your head around.
Because anytime that you have
an increase in hydrogen ions,
you will always have a small increase in bicarb.
Anytime you have a decrease in hydrogen ions
you will always have a small decrease in bicarb.
Because you’re following the same equation.
What I mean by the same equation?
You take carbon dioxide plus bicarb.
You get carbonic acid and then that product
yields a hydrogen ion and a bicarb.
So however this formulate its
traveling in one direction,
if you get a change in hydrogen ions you’re always
gonna get a change associate with its product.
Now a large change, what a change?
That is why you give a small change in each of
those variables as you go from hypo to hypercapnia.
In terms of how the body responds
to a change in alveolar ventilation.
So these is the hyper and hypocapnia
that we’ve talk about.
How does the body deal with these?
You usually deal with the
problem by using another system.
So if it’s a respiratory problem you
try to compensate with the renal system.
So lets go through an example for that.
If you have a hypercapnic condition, meaning
you haven't a decrease an alveolar ventilation.
You’are gonna go from the normal
line up to point number one.
However, to then try to compensate for that,
you are going to now travel up the PC02 line.
As you travel up the PCO2 line,
this is important,
you are not travelling along the
blood-buffer line anymore.
you are traveling up the PCO2 line.
These dramatically increases bicarb.
not just a little bit but a dramatic increase.
If you take point number two
and drop a vertical line down,
you will notice that pH is now
returning towards normal.
So if you started off with the pH of 7.4,
maybe moved to a pH of maybe 72.5.
Now, you’re moving back closer to 7.4 again.
The same thing can happen in a time
in which you have a low PCO2.
So if you now move from normal to 0.1,
that is moving down the blood-buffer line.
How does the body try to compensate for that?
You dump bicarb.
And you do that at the level of the kidney
and you go from 1 to 2.
That is a response a renal compensation
to a respiratory problem.
If we use the acid-base box, we can
do this also with acidotic conditions.
So here, we have an arterial blood gas of 7.34.
Red gnome, outside the box.
BICARB below 22?
Red gnome outside the box.
What do we have left?
Blue gnome outside the box
on the opposite side.
By knowing where the gnomes are,
I can name this condition
It is metabolic acidosis
with respiratory compensation.
How do you know that?
You always name the condition
base upon where the gnomes are.
If there are 2 gnomes on one side,
that’s how you name it.
Metabolic acidosis with
Lets take another example.
Lets do another acidosis.
So pH, I mean ABG 7.24.
Red gnome, outside the box.
So you know it’s an acidosis.
Right now, you know it’s an acidosis.
Now, were you doing is waiting
your waiting, you are at the edge of your seat.
Your thinking, okay is it gonna metabolic?
is it gonna be respiratory?
How would you know?
The gnome would be outside the box on the
same side is the red headed gnome.
So let’s look where we at?
the bicarb levels are of 23.
You have a gnome inside the box.
Here you are not sure what to do.
Green gnome inside the box.
It’s gonna be hard to name.
Red gnome outside the box respiratory weather
This is respiratory acidosis
With no metabolic compensation.