Medical Acid Base and ABGs Explained Clearly by MedCram.com | 2 of 8


thank you for joining me for part two of
medical acid/base time to review what we were talking about before we were
talking about the Henderson-Hasselbalch which I’ll go ahead and write for you
here in terms of what that was it was the pH is equal to the 6.1 plus the log
of this big equation the concentration of hco3 minus over the pco2 times 0.03
and we just got finished saying that we talked about the two issues here are the
two variables in the equation one was the bicarbonate which was primarily
dealing with metabolic processes and the other one was the pco2 and we said that
if the bicarb was changed that was a metabolic process and if the pco2 was
changed that was a respiratory process if the bicarb went down that was a
metabolic acidosis if the bicarb went up it was a metabolic
alkalosis if the pco2 were to go down that would
be a respiratory alkalosis at the pco2 were to go up it’ll be a respiratory
acidosis so respiratory here on the co2 metabolic on the bicarb and i found
those things change then that would be an acid emia or an alkyl emia so I want
to split this up here into two categories the hco3 minus and the pco2
and I kind of want to just tell you the differences between these two as we
talked about the bicarb is primarily made in the kidney okay where and it can
be excreted in the kidney and change and all that sort of stuff whereas the pco2
is primarily a lung situation so bicarb can have processes happening all over
the body and many different processes occurring all at once you can have a
several different metabolic acidosis for instance diarrhea can cause a
metabolic acidosis and vomiting can cause a metabolic alkalosis and both of
them can be occurring at the same time ironically they would cause cross cancel
out each other interestingly but both metabolic acidosis and alkalosis both
metabolic problems so you can have multiple happening at the same time
whereas you only have one one and it can either breathe on the either fast or
slow so there’s a single you can’t have a respiratory acidosis and a respiratory
alkalosis at the same time as I mentioned your lungs can only breathe
fast or slow so this can only happen once this in terms of the kidney
modifying bicarbonate takes a long time so like days for that to happen whereas
the lung can start to breathe quickly or slowly almost immediately so this is
like minutes in terms of how fast things can happen so there are definitely
differences here between bicarbonate it’s a metabolic situation it’s kidney
multiple can happen at the same time and it takes days for the kidney to ramp up
or to change when we talk about compensation pco2 on the other hand
primarily is governed by the lung there’s only one lung well there’s
actually two but there’s only one organ it can only breathe so fast either fast
or slow and that change can happen in minutes I want to explore a little bit
about bicarb because bicarb is kind of an unusual
situation here so let’s go ahead and put it up here so hco3 – okay so there’s
there’s kind of two ways in which you can lose bicarb or bicarb can go down
and that would be a metabolic acidosis you can lose it because you’re combining with a proton okay and in which case
you’ll lose it because remember we talked about bicarbonate plus the proton
will turn into carbonic acid carbonic acid will then turn into a water
molecule and a co2 molecule which will breathe off and so bicarb can go down
because you’re combining with a proton and you’ll lose the bicarb molecule that
way or the other way you can lose bicarb is just simply loss and just lose the
bicarb so you’ll lose it because you pee it out
or you’ll lose it because you poop it out and so those are two different ways
now let’s talk about those two different ways of looking at this so if you lose
bicarb from simply just losing bicarb your bicarb will start to fall but what
will happen is because you’re actually losing negative charges okay so what
will happen is your chloride or your measure chloride will will go up in that
situation and why is that important well it’s important because all of the
positive charges in your body and all the negative charges in your body have
to remain the same maybe I can demonstrate here by drawing
a chem-7 okay you’ll see this over and over again in camp in medicine where we
have the sodium we have the potassium chloride
this says co2 but really it’s the bicarb okay this is the blood urea nitrogen
this is the crap name and this is glucose so I want you to notice this
because this is this will become important later is that the positive
charges are in the first column and the negative charges are in the second
column now why is this important well talk about this obviously the body has
as many positive charges in it as it does negative charges if you didn’t then
you would be statically electrically charged and the body can’t have that
so there are equal number of positive charges as our negative charges but the
problem with our little chem 7 as we call it here is that we’re not
accounting for all positive charges there’s calcium that we don’t have here
there is magnesium that we don’t have here and likewise we don’t have all the
negative charges here we don’t have phosphorus and sulfates and all sorts of
things that are negatively charged but we’ve got negatively charged chloride
item one got negative charge bicarbonate over here so if we were to add up in a
normal situation the sodium which is usually about 140 and the potassium
which is around 4 and the chloride which is you know around 100 or so and then
the co2 which is around 24 you can see very closely and very clearly here that
you your positive charges are going to outnumber your negative charges and when
that happens that’s a normal situation and in fact when we calculate this we
don’t even use the potassium in that calculation so what we’ve come up here
is with something called the anion gap now the anion gap is simply an
artificial measurement it’s artificial because we’ve chosen to include these
electrolytes in this chem-7 and what we’ve done is we’ve chosen to take the
chloride in the co2 and subtract it from the sodium and the normal anion gap is
and in a normal person 12:12 so that means that if we take the sodium and we
subtract out the total negative charges which is the chloride and the co2 we
will get an anion gap of 12 what does that mean that means there are 12
negatively charged units out there floating around in the body that we’re
not accounting for in our chem 7 and that’s normal that’s okay so what does
that mean that simply mean simply means that we can tell if there are other
negative particles that somehow come up all of a sudden and so what happens well
let’s go back to our bicarbonate thing again remember bicarbonate there’s two
different ways of losing bicarbonate one of the ways is if it combines with a
positively charged proton where is that proton going to be coming from well I
should recall from chemistry there is a negative a a negatively
or there’s a whenever there’s an acid okay
there’s always a proton and a conjugate base conjugate base and then when that
proton comes off the conjugate base becomes negatively charged and the
proton comes off and in this case it’s going to combine with the bicarb to make
co2 and h2o the key here however is this is that now we have a negatively charged
particle that is now floating around in the body so what is that going to do to
our anion gap it’s going to increase it and so your anion gap will increase
whenever you have a situation where bicarb is lost because it combined with
a proton that was attached to a conjugate base now what does that
conjugate base what could it be it could be anything it could in fact it’s it can
be anything any negatively charged particle that’s not either a chloride or
bicarbonate and that’s important to know because there are certain medical
conditions where you have these anion gaps that occur that tells you that’s
how the bicarb was lost okay so let’s talk a little bit more about the anion
gap we said before that the anti gap is usually 12 what does that mean what
makes up that what makes up the majority of that anion gap as it turns out the
molecule or protein albumin makes up the majority of that anion gap which is 12
what’s the normal what’s the body’s normal composition of
albumin how much albumin is there normally in the body well if you were to
measure in terms of milligrams per deciliter it would be four so four
causes you to have an eye gap of about 12
that’s a good way to calculate what the normal anti gap should be for somebody
because let’s say somebody had an albumin of two then what would their
normal anion gap be you can see that it would be about six that’s important to
note because if you know what somebody’s predicted anion gap is you’ll be able to
tell whether or not there’s an anion gap metabolic acidosis or something
occurring because if somebody has a process that is eating up bicarbonate
because there is an acid that’s giving off a protein and that bicarb is being
lost because it’s combining with the protein that was on a conjugate base as
we showed in the last slide that’s going to increase your anion gap so if you’re
if for instance somebody has an anion gap of 20 because we just calculate the
anti gap and remember what the anti gap is the NI gap is simply the sodium minus
the addition of the chloride plus the bicarb that will equal the NI gap okay
and if if it is 20 then we know that’s higher than what it should normally be
it should normally be about 12 okay if someone has an albumin of 4 that means
that there’s eight units there’s an eight unit jump that means
there’s eight units of conjugate base floating around that shouldn’t have been
there and it’s causing them it’s causing an acidosis and metabolic acidosis
however if we calculate anion gap in it’s 20 and our albumin is only two that
would mean our anti gap should be only about 6 and therefore the difference
between these two is actually 14 which means that there are 14 units of anion
gap that’s floating around that shouldn’t be there okay so let’s review again let’s go back
and talk about bicarbonate we said already that there’s two ways that you
can lose bicarbonate one is simply just loss of bicarbonate and the other one is
combining with a proton a proton that comes from a conjugate base if that
happens you’ll be left with the conjugate base that’s negatively charged
we can’t keep track of that negatively charged molecule in the chem 7 because
it’s not a chloride and bicarbonate therefore there’s going to be an
increase in something called the anion gap the anion gap is calculated by simply the equation of the sodium minus
the chloride plus the co2 or the bicarb hco3 minus so if you take the sodium and
you subtract the addition of the chloride plus the bicarb you’ll get the
anion gap the actual anion gap that actual anion gap if it’s greater than
what you would predict the anti gap to be and what is the predicted anion gap
we’ve predicted and I in gap should be the albumen times three normal albumin is 4 so 4 times 3 is 12 this whenever you get an increased anion
gap this is termed an anion gap metabolic acidosis it’s possible for you
to just lose bicarbonate through loss and not cause that in that situation
when you lose bicarbonate you will not be causing anion gap and this is
important because you’ll be able to tell the difference between an anion gap
metabolic acidosis in a non anion gap metabolic acidosis so now we’ll get into
a little bit more about the acid based disturbances and how to
calculate them in what’s compensation in our next part part 3 thanks for joining

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