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