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We have a number of different ways
to get a metabolic acidosis.
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So, what we have to do is have to diagnose
this to understand which one they might have.
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To do that, we use an Anion gap.
And what is an Anion gap?
And anion gap allows us to look at the
total number of positive charges in the blood.
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And compare that to the total number
of negative charges in the blood.
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So, it’s cations versus anions.
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They have to equal each other.
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The positive charges will always
equal the negative charges.
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And so, I know you’re wondering now, if they
are equaling each other, why they have a gap?
Well, there’s some amount of an anion
that you’re not taking into a count.
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So, let me explain this.
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Metabolic acidosis with a normal anion gap,
you have a certain amount of sodium.
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Sodium is your only cation
that you’re worried about.
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Potassium you’re not worried
about it, only sodium.
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Then you compare it to your anions,
chloride, bicarb and then something else.
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That something else is your gap.
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You don’t know what it is. It’s something out
there that it’s taking a part of that total
cation to anion comparison.
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What should your anion gap be?
Normally, somewhere between about 8 to 16.
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So, in our example here having an
anion gap of 10, perfectly normal.
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Let’s compare it to a problem
that has an abnormal anion gap.
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You have a sodium value, you get this
from your blood chemistry.
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You put in your chloride, you put in
your bicarb, and then something else
is taking up a larger piece of those cations.
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Cos again, they have to equal each other.
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They call this a gap because it’s
not something that you measure.
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You’re only measuring the ions in the blood.
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And so, you don’t know what was there
that take up all the extra cations.
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If it’s above 16, now that you know
you have an elevated plasma anion gap.
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So, you’re looking for a
value between 8 and 16.
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Once you have an increase plasma ion gap
you know that you have a certain list of disorders.
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You either have ketoacidosis, lactic acidosis.
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You might have renal failure, aspirin overdose,
methanol poisoning, propylene and glycol poisoning
or maybe you’re not eating enough.
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All those would have an increased plasma ion gap.
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The ones you can eliminate if you
have an elevated plasma ion gap
are the ones that are associated with
diarrhea, a renal tubule acidosis,
you fear on certain drugs like
carbonic anhydrase inhibitors
or if you have a disease such
as Addison’s Disease.
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So, you can automatically, tease out which one of this list
you were on if you have a metabolic acidosis.
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After you’ve calculated the anion gap, there’s another
differential diagnosis you can go through,
and that is calculating on an Osmolar Gap.
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So, if you were looking at a difference
between two numbers,
just like you were between cations and anions.
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What you’re going to look at now is the
difference between your measured osmolality
to your estimated osmolality.
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How do you do that?
Estimate your osmolality you can take
two times your sodium value.
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You take glucose divided by 18 and then,
you take your blood urea nitrogen divided by 2,8.
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You sum those together and that should
be within 10 of your measured osmolality.
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If it is greater than that, something is
taking up that osmolality. And it’s usually a poison.
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Something like propylene glycol, there’s
some other item that is causing an osmotic pull
that normally would not be there.
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And that is another way you can further
differentiate if you have a metabolic acidosis
with elevated plasma anion gap to tease out
between those problems that were on that list.
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You can use the osmolar gap to figure out
if the person has ingest the poison.