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