Our discussion earlier was dealing with bicarb.
Talked about the physio of it all. Let us
give it some clinical correlations like you should
be doing in every step of the way. Normally,
with this, we are talking about lowering the
renal threshold for bicarb. What does that
even mean? That means that you're actually not
reabsorbing bicarb. Listen to what I just
said. We are going to lower our standard
terminology, the renal threshold for reclaiming
bicarb, which means what? That means the more
the bicarb remains within the urine and you
are going to get rid of it. So, therefore,
your normal bicarb level, which is between
22 to 26, and we are using 24 has now dropped
down to 15 where, please? In the plasma, right?
Now you tell me, acid-base disturbances from
physiology. If you were to drop your bicarb
down to 15, what does that mean to you? Acidosis
because you don't have as much bicarb. You
have lost it. What kind of acidosis? Is this
respiratory acidosis or would this be metabolic
acidosis? Good. Metabolic acidosis. Are we
clear? If metabolic acidosis, let
me take one step further, just to make sure
we are good here. If you have metabolic acidosis
and how you are going to compensate? You are
going to try to blow off your carbon dioxide
very quickly so therefore within minutes,
you are going to start hyperventilating, so
that you can blow off that carbon dioxide,
aren't you? Continue. So now this results
with what? Loss of more of your filtered bicarb
than the normal renal threshold. Why might
you want to do this?
As a method of compensation maybe perhaps
you are doing this. Remember that high altitude
patient that we talked about acutely. In
acute high altitude sickness, in which that
patient is hyperventilating, blowing off the
carbon dioxide resulting in what, please?
A primary respiratory alkalosis. How can you
confirm that? The pH will be above 7.45. And
what is the kidney doing within days? Here
we go. You are going to decrease the renal
threshold for reclaiming bicarb. So what happens
to the pH in the urine? Stop. Listen. Urine
pH is going to increase above 5.5. Why? Understand
the concept first because there's more
bicarb that is left within the urine. What
happens to the activity of carbonic anhydrase
in the PCT? Upregulated or downregulated?
Downregulated because you are trying to get
rid of the bicarb. Do you see these questions that
are all related. Urine loss of bicarb, what
happens? Occurs until serum bicarb matches
the renal threshold. For example, let us say
that you do give your patient who has high
altitude sickness acutely, acetazolamide.You
inhibit that carbonic anhydrase so therefore
what happens to this threshold that are
reclaiming? The lowering the renal threshold
for reclaiming bicarb, your pH within the
urine is going to increase.
The bicarb combines with sodium and you get rid of your
sodium bicarb, acting in the PCT where we are
right now and so, therefore, you are going
to, in this particular case, what is my full
diagnosis if there was a pH of 7.6? Stop there.
7, alkalosis. You find that your carbon dioxide
is 25. Tell me diagnosis so forth, primary respiratory
alkalosis, correct? What is normal carbon dioxide?
40. As you blow off more carbon dioxide, what
did it drop down to? 25. What is the only
method of compensation? Is it minutes or is it days?
Well, the only method of compensation
would be kidneys, thus days. Correc,t good.
So, therefore, you are trying to get rid of
that bicarb. For diagnosis, you do have at
this point, days later, primary respiratory alkalosis with
compensatory metabolic acidosis. Clinical
correlations with everything we talked about.
Going to next point. Now you have raising
renal threshold for reclaiming bicarb,
the opposite and here we will give it a clinical
tag. What is it before one? You are lowering
the renal threshold for reclaiming bicarb.
But here, you are going to increase your threshold
so that you reabsorb more bicarb. Why might
you want to do this? Let's do vomiting. If
you are doing vomiting, what may then happen?
Well if you're vomiting, what you are doing is
you are then getting rid of your acid. And
so, therefore, if you are going to get rid
of your acid, then what might you be resulting
in? Watch this. Increasing threshold means
that the proportion more of the filtered bicarb
is actually being reclaimed. Increase
in the reclamation of bicarb is most important
contributing factor to increase the serum
bicarb and so, therefore, you are resulting
in metabolic alkalosis. So this is rather
interesting, why? Up until this point, you
might have learned in medical school, or maybe
you know sometimes you are in a habit of keeping
things so simple that you actually missed
the details. Or maybe you spent so much time
focusing upon the mnemonics, that you really
have forgotten the pathophys, or maybe even
really not even understood in the first place.
Listen. You know that vomiting is metabolic
alkalosis, isn't it? You might have learned
this as being what? Vomiting. Now, what do
you see on the floor? Acid. You think, and
that may be a good way of remembering if you
get rid of your acid, your body is in a state
of metabolic alkalosis. That is the wrong
method. I mean that might give you the diagnosis
quickly, but what if there is a physiology
question and that they really wanted you to know
what is going on? I'll walk you through this. So
when you vomit, you are getting rid of your
volume. Take a look at the volume depletion
as being our other example. Okay. So now volume
depletion, in this case we are using vomiting.
Okay. I want you to think of the Darrow Yannet box.
You know what am I referring to? Darrow Yannet box.
If you don't, that is okay. Because what
it does, the Darrow Yannet box, is divide your
total body water into ECF and ICF. Good. You
told me earlier that the ECF was 1/3
of your TBW and your ICF was 2/3. Correct.
Now, what are you paying attention to? First
and foremost, it will be your plasma, or in
other words effective circulating volume, right.
Earlier we talked about effective arterial
blood volume, effective circulatory volume,
all the same thing referring to the plasma.
Isn't that the most important component to
monitor at all times? How can you confirm that,
Dr. Raj? Carotid sinus with the baroreceptors
and down the afferent arteriole, we talked about
juxtaglomerular appartus. So far so good.
Okay, so say that you are vomiting, what are
you doing? You're volume depleting,
and so, therefore, the mark of ECF meaning
the plasma has actually decreased. Are we
good there? What is that called? Contraction.
So if you have enough vomiting taking place,
enough contraction of your ECF, who is going
to kick in? You are going to have your aldosterone,
aren't you? You are going to have some of
that aldosterone kicking in? Sure you will.
And if you have some of that aldosterone kicking
in, then at some point, what is is actually trying
to do? It is trying to get a point where, well
it is trying to retain everything that it
wants and so the aldosterone is going to try
to kick out the hydrogen and so, therefore,
that is going to contribute the metabolic
alkalosis. In addition, there is going to
be an increase in reclaimation of your renal
threshold of bicarb. So there is going to
be increased reabsorption of bicarb as well,
in the hopes of trying to restore some of
the volume depletion. All these, ladies and
gentleman, is then going to give you the consequence
or sequealae of what vomiting does to your
body? It is called metabolic alkalosis. From
henceforth, sure. The quick and easy you want
to think of vomiting as getting rid of acid
in a state of metabolic alkalosis. That is
a combination of aldosterone getting rid of
your hydrogen and also increase reclaimation,
increasing the threshold for reclaiming your
bicarb, which also contributes to what is
known as metabolic alkalosis or contraction
alkalosis. What does contraction mean? Volume
depletion, contraction of your ECF, specifically
plasma. We will talk more about the Darrow
Yannet as we move forward, but at this point
I am just introducing how important it is
for you to have that foundation of ECF and
ICF and which we're specifically focusing upon
in the ECF and that would be the plasma clinically,
and from there you can juggle things around.
Let us move on.
So now we have completed our entire discussion
of bicarb, beginning with normal phys and understanding
as to how you want to apply that in different clinical
situations. Now, in the PCT, we are going to wrap