Okay welcome to another MedCram video. We're going to talk more about
the Chem-7, specifically the basic metabolic panel.
And we're going to look at the chloride ion and also
what we called the bicarbonate or CO2, so this is bicarbonate, as we've talked
about in the last lectures. So one of the
things I want you to see is that the sodium is here. We've talked
about that extensively and the potassium is here, and if you
notice we've got the positive charges on this side and we've got the negative
charges on this side. What we're going to do is
we're going to look at these positive versus negative charges,
because we'll start to see that they're not going to add up completely to the
same. In other words, the number of positive charges in the Chem-7
doesn't exactly equal all of the negative charges. In fact, there's more
positive charges in this Chem-7 than there will be negative
charges. That's because there are negative charges
in the body that are not accounted for in this artificial
construct called the Chem-7, and you can probably think of some
negative charges that aren't here, tust thinking about it, right. So we can think
of sulfates, those are negative charges. We can think
of phosphates, well those are negative charges, and there's also
proteins that are negatively charged, like albumin,
right. What about other negatively charged things, like, for instance,
lactate? Lactic acid breaks off and forms lactate. Well that's a negative
charge that's not here in the Chem-7. What about
beta-hydroxybutyrate? In fact, anything that ends in an "-ate"
is usually the weak base of a strong acid.
So in other words, if you think about this, we've got
HA going to dissociate into H+ plus A-
and normally this is known as an IC acid, and then it dissociates into a
proton and an -ate. So let's think about some examples of this.
Lactic acid converts into a proton, and lactate beta-hydroxybutyric
acid dissociates into a proton and then beta-hydroxybutyrate
sulfuric acid etc., phosphoric acid etc., and so whenever we see an -ate,
that's usually because there is a negative charge,
and it is going to be a negative charge, but you're not going to see it here
in the chloride and the CO2. So how do we account for that and what's going
on? Let's talk a little bit more about that here.
So what I've done here is I've taken the Chem-7 and I've taken
those first two columns, if you will, so here's our Chem-7 here at the top.
And that second column I've put here, and that first column I've put here, so
in fact, this is the positive column and this is the negative column. As it
turns out, we don't really care too much about potassium, so all
you're going to see here is sodium, so we'll put sodium right here
and as you recall, the normal value is 140.
Chloride is on this side because it's negative
And the normal value is about 104. And then bicarbonate is down here. I'm
going to actually put it in as bicarbonate, HCO3-
and the normal value there is 24, and what you'll see is that if you
add 104 plus 24, these are the major negative ions in the
body, you'll get to 128, well 128
is just 12 shy of 140, and that's this area here. Now what's in
this area we'll talk a little bit about, but there's a gap,
and it's an anion gap, because these are anions,
and so this area is called the anion gap so we'll put that as AGAP,
and that's, generally speaking, normally around 12. So what makes up the anion
gap? Let's talk about that well what makes up
the anion gap, as we'll talk a little bit more about,
are any negatively charged particles that aren't accounted for by a chloride
and a bicarbonate ion. As we talked about before,
we've got sulfates and phosphates, SO4,2- and PO4,3-.
So there's a lot of negative charges there, but in fact,
anything that has an A-, as we just talked about, so any acid that
dissociates into an A-, where the A-
is not a chloride or bicarbonate, is going to be here in this anion gap.
And as these substances start to increase, you might notice that the anion
gap gets bigger. We'll talk more about that in a little bit.
So the first thing that I want you to be able to see from this is how to
calculate the anion gap. That's this portion right here. This
portion is called the anion gap, and as you can
clearly see the anion gap is simply going to be
the sodium concentration subtract the chloride and the bicarbonate.
And that will give you the anion gap, so basically
chloride plus bicarb, all of it, subtracted from the sodium. So 104 +
24 is 128. 128 from 140 is 12.
So the way you calculate the anion gap is 140 - (104 + 24).
So when you look at the relationship between the bicarb and the
chloride, it's going to tell you whether or not
there is an acidosis or an alkalosis occurring.
Let's first talk about an acidosis, and I will say that if you have any questions
about medical acid-base, please look at our
separate course, which is free on MedCram.com,
for Medical Acid-Base. It's an excellent review, and there are questions involved, too.
And as we talk about in that review, a
metabolic acidosis is where bicarbonate is lost.
And there's two ways of losing bicarbonate. That's this part down here.
And you'll know that you've got a metabolic acidosis, because
one of the ways of knowing that is that you'll have a low bicarb.
Yes, it is possible to have a low bicarbonate in a respiratory alkalosis,
but in fact, here we're talking about another cause of a low bicarbonate, which
is a metabolic acidosis. Again a metabolic acidosis is when your
body loses bicarbonate, and there are two ways of losing
bicarbonate. We'll go for those right now. The first way of losing bicarbonate,
actually, you can do in two different ways.
Number one: you can lose sodium bicarbonate, or you can gain
hydrochloric acid. And in fact these are really
the same thing; losing sodium bicarbonate is almost the same thing as gaining
hydrochloric acid, because of what happens in the kidneys.
You can think about it as, in the proximal convoluted tube of
the kidney you lose all of your bicarbonate, and you reabsorb
it in the remaining portion of the
glomerulus. If you do not regain that sodium
bicarbonate, that is the same as gaining hydrochloric acid. So, for the
sake of clarity, we're going to look and see what happens
if you gain hydrochloric acid and lose sodium bicarbonate here in this
example. So in this situation, where you're losing
bicarbonate, let's say that your bicarbonate level drops down
to 16. Here you had a drop
of eight points from 24, where bicarbonate normally is, down to
16. And in this case, the anion that is lost
here is bicarbonate, but the same amount of
chloride is gained, and that's because, in this situation, losing sodium
bicarbonate is the same as gaining hydrochloric acid.
And so, in this situation, because the bicarb has gone
down to 16, it is replaced by chloride, and so
this line up here does not change. And that's the key point to understand,
is that the anion gap in a metabolic acidosis that just loses
bicarbonate stays the same. So what kind of acidosis
would this be? This would be a situation where you have
diarrhea. This is where your GI tract is losing
bicarbonate. Another one would be a renal tubular
acidosis type 2. Tenal tubular acidosis type 2 is
where you have the inability to reabsorb sodium bicarbonate at the
proximal convoluted tubule. Another thing that could do this would
be a carbonic anhydrase inhibitor. That could do it. Another thing that
could cause a acidosis like this where the bicarb is
lost is if you were to add normal saline to a patient in large
amounts. Normal saline is sodium chloride, and
you're adding a lot of chloride, that's going to increase the chloride
concentration and drop your sodium bicarbonate
concentration. And so all of these here cause an
acidosis, that's known as a non-anion gap
metabolic acidosis. They say "non-anion gap" because the anion gap
has not changed from the baseline. It was 12 before,
it's 12 afterwards, and that's known as a non-anion
gap metabolic acidosis or "enagma." But there is another way to
get a metabolic acidosis, and that is not in the first case, where
you just lose bicarbonate or just gain hydrochloric acid.
No, in fact, there's another way you can actually have
a gain of HA, because, remember, HA dissociates into an
H+ and an A-, and the key here is the A-. Here, you could have the
same magnitude of acidosis, so that you would have a drop, so that
your bicarb level would be down to 16.
And the reason why your bicarb drops is because, again,
this proton is going to combine with the bicarb
and turn it into H2O+CO2, and it would basically go away.
You would breathe it out, but you're still left with less bicarbonate and
that's a drop down, but what happens that's different
is that this A-, which is part of the anion gap,
as we talked about here, accumulates here up at the top and it will drop here.
It dropped by eight points. This will also increase by eight points,
and so now the anion gap is no longer 12, but in fact, it is
20 and the chloride is the same; it is 104.
So, what kind of acidosis will cause this to occur? Well, anything that has
an A- that is not a chloride or a bicarbonate,
and the classical mnemonic for this is 'MUDPILES.' 'M' standing for
methanol, and methanol is converted into formic acid,
and formic acid goes into formate, which is an
A-. 'U' stands for uremia, because uremia causes accumulation of
sulfates and phosphates, both of which would be an A- and
would increase your anion gap. 'D' stands for DKA. DKA leads to
beta-hydroxybutyric acid, and that dissociates into beta-hydroxybutyrate,
which is an A-. 'P' stands for peraldahide, which is an old
drug not used very often, and it's a great 'P' holder for MUDPILES.
'I' stands for ischemia, which can lead to lactic acidosis, but also can be iron.
Iron is a substance that can lead to an anion gap metabolic acidosis.
Lactate for 'L' or lactic acid, and lactic acid of course dissociates
into lactate and a proton. And lactate is an A-, which is going
to accumulate here in the anion gap. 'E' stands for ethanol or ethylene glycol,
both of which cause A-'s. And then, finally,
'S' stands for salicylate, which is aspirin, and salicylic acid is
dissociated into salicylate, which is an A- once
again. So again, if you have an anion gap which is
greater than what it should be at 12, and you have a bicarbonate, which is low
of course, which is what it will be. You are going to have not a non-anti-gap
metabolic acidosis, but an anion gap metabolic
acidosis, or an AGMA. So the key to understanding this
is to know how to calculate the anion gap.
And again, the anion gap is equal to the sodium minus
the chloride, plus the bicarb parentheses, and the normal
should be around 12, if the albumin is normal. Going back to the
NAGMA, there is also another way as we talked about,
you can lose sodium bicarbonate or gain hydrochloric acid, but remember in the
distal convoluted tubule of the kidney is a
receptor for aldosterone. Now, aldosterone, as you may recall,
reabsorbs sodium, but it dumps potassium and protons into the lumen
of the distal convoluted tubule. If that, for some reason,
is not happening, either because there is an inhibitor like spironolactone
or because the zona glomerulosa of the adrenal cortex (which makes aldosterone)
is not working, such as in Addison's Disease, you're
going to get a gain, again, of hydrochloric acid, but in this
case, you're going to actually gain potassium,
because it's that enzyme's responsibility, the one that is stimulated by
aldosterone, to get rid of potassium. And if it is not, then you're going to
have an increased amount of potassium in that situation, and that is also
associated with RTA type 1, and you could also see this
with RTA type 4 (there is no type 3).
So therefore you could also see this with spironolactone,
but just remember that the ones that I have circled here
in orange will give you elevated potassium,
and also remember Addison's Disease, which of course will give you an
elevated potassium. The ones not circled will give
you a lower potassium, so when you see the Chem-7, you should
look at that bicarbonate level there in the
middle at the bottom, and if it's low, if it's
less than 24, you should calculate the anion gap.
If the anion gap is normal, then you may be dealing with a
non-anti-gap metabolic acidosis. If on the other hand, the anion gap is elevated
more than it should be, then you could have something called
an anion gap metabolic acidosis, and you might ask:
how high should an anion gap actually be?
Well, if you must know the albumin level makes up the lion's share of this anion
gap, and normally the albumin level is usually
about 4.0, and that correlates approximately with a anion gap of 12.
So if your albumin for instance is half of that let's say it's 2.0,
then your anion gap should be around 6. If it's more than that, then that would
be abnormal. If your albumin is 3.0, then your anion gap should be around
9, and if your albumin is 1.0, your anion gap should be around
3, but of course this is very low, so these are the thresholds that we use
for our albumin, depending on what it is. Okay, great. Now, this is just one video
from our video series, "Basic Metabolic Panel Explained Clearly,"
where, in addition to all of this acid-base stuff that we just talked
about, we also, in addition, go into the sodium, potassium, we look at
the kidneys, and also glucose, and on all of these we look at the
pathophysiology. We look at causes, manifestation, and
treatment and that course is available at MedCram.com.