okay welcome to another MedCram lecture now I've got a lot of requests out there
to talk about a very difficult topic we're going to talk about hyponatremia
okay low sodium now this is a very complicated topic because we have to
talk about water balance we've got to talk about the difference between
osmolality and tonicity and antidiuretic hormone and aldosterone and compartments
of fluids and the different types of hyponatremia are the isotonic are they
hypotonic hypertonic and then of the hypotonic hyponatremia is is it
hypovolemic hypervolemic a nice oval emic and it just goes on and on and on
but I think after you get done with my series here in fact I know after you get
done with my series you'll be able to work through how to deal with the
different types of hyponatremia so to do that we have to build a little bit of a
foundation so you can understand what we're talking about so bear with me and
go through these lectures and you'll see that it'll build on one another just
like our other series on acid-base okay so the definition of hyponatremia is a
sodium concentration of less than 135 milligrams per deciliter okay that is
basically the definition anytime you have any word that says amia at the end
it's relating to the concentration of sodium in this case sodium or any other
molecule in the blood so hypokalemia alkyl emia acidemia all of that is
related to the blood so let's go ahead and build the foundation the first thing
that you should know is what is the definition of osmolality so osmolality
OS m o l al ity so that's serum osmolality as an equation and it's two
times the sodium concentration plus the glucose concentration
/ 18 and that glucose concentration is in milligrams per deciliter plus the B
UN and please look at our acute renal failure lectures to get a update on the
B you win and that's again in milligrams per deciliter divided by 2.8 and the
normal for that is 285 millimoles per kilogram you can also say milligrams per
deciliter as well so that is the definition of serum osmolality 2 times
the sodium plus glucose divided by 18 + bu n divided by 2.8 and that'll tell us
if something has a basically a low osmolality a normal osmolality or a high
osmolality now something that's very similar to that is tonicity it's a very
similar concept and you'll see it looks very similar it's two times the sodium
concentration plus the glucose divided by 18 once again so why is it missing
the B UN component it's missing the b1 component because B UN can go freely
between the plasma membranes and so therefore if b1 can go between plasma
membranes it really doesn't have too much of an effect on the difference
between the inside of the cell and the outside of the cell so from now on what
you'll see is we'll use tonicity and osmolality interchangeably but just be
aware that if they ask you to calculate the osmolality of something this is the
equation that you should use okay we'll come back to that again and again the
other thing you should know about is two major types of hormones one of them is
ADH this is antidiuretic hormone antidiuretic hormone is secreted from
the posterior pituitary it's actually made in the hypothalamus but it's
secreted from the posterior pituitary and if you could remember where this
works it works at the collecting tubules of the nephron and the presence of ADH
at the collecting tubules of the nephron causes
water reabsorption and what's that going to do what's reabsorption going to do
it's going to cause water not to go out to the urine but to be reabsorbed into
the blood and basically it's going to have the effect of diluting out a lot of
your substances but specifically it's going to be used to retain water the
this is a protein hormone and so it works almost instantaneously as soon as
it's secreted the other hormone is aldosterone you'll if you look at our
adrenal gland lecture you'll see that aldosterone is a steroid hormone it is
secreted from the zona glomerulosa and it works primarily at thee
so we'll put down here adrenal cortex and it works primarily at the distal
convoluted tubules of the nephron and it's action is to cause reabsorption of
sodium and because of that water and it causes excretion of potassium and
excretion of protons so just be aware of those things we're going to talk about
that obviously because water and sodium are going to be intimately related to
the discussion that we're going to have today okay so any discussion regarding
intracellular extracellular fluid you've got to know about the different
compartments of the human body so let's get into a discussion of that and this
drawing is going to be pretty important so I want to make sure that you
understand the difference nuances are regarding this so this is a diagram
essentially showing you the different compartments of fluid in the human body
okay so this is the intracellular fluid so this represents this compartment here
represents all of the all of the volume inside of cells this is the entry
vascular volume this represents all the fluid insides arteries veins capillaries
etc and this is the interstitial volume so this is separated this intravascular
volume is separated by the interstitial volume by the capillary wall and we know
the capillary wall is not a barrier to the movement of electrolytes and fluid
and I'll represent that here by simply drawing little openings in that so
basically sodium and water can go back and forth between these two and so
basically this is effectively one chamber for or one component or one
container for electrolytes fluid things of that in other words this wall does
not separate their movement on the other hand this wall here that separates if
you will the extracellular fluid from the intracellular fluid is separated and
there is a a pretty tight barrier this is basically the the membrane of the
cell now remember animals don't have cell
walls like plants do but yet this is a pretty rigid wall when it comes to
electrolytes water is able to move through it very freely of course because
it's a not ionic compound but in terms of sodium sodium cannot go through this
it is not permeable to sodium if sodium is going to go through it's got to go
through channels and so this is very well regulated sodium is not gonna be
able to go through however water can go through all of this okay so water can go
through here and water can go through here and so because of that because
sodium is able to freely go through here and water is able to we'll call this
simply this whole thing the extra cellular fluid compartment so this is
intracellular fluid extracellular fluid and generally if you want to look at a
compartment this is about four liters this is about 10 liters and this is
about 28 litres of fluid so the key here is that there are three compartments the
intravascular and the interstitial combined to form the extracellular fluid
this is where sodium and water mix water is able to go across this semi permeable
membrane but sodium is not and that's it that's a key there so when we're talking
about drawing blood with needles okay this is the fluid compartment that
we're talking about when we're talking about hanging IV fluids okay it goes
into the vascular and extracellular fluid compartment so the question is
going to be is what happens to these fluid compartments when there are
perturbations in sodium and fluid management and we're going to talk about
that so the way we're going to represent this from now on in the lecture to
simplify it is basically two compartments and remember this is going
to be the intracellular fluid and this is going to be the extra cellular fluid
now why do I have it drawn this way as a vertical and as a horizontal well the
way I'm going to represent this if you can imagine these are X and y axis is
the x axis is going to represent volume so how much so simply this dimension
here represents how much volume is in there and the y axis is going to
demonstrate the sodium concentration so the higher this is on the y axis the
higher the concentration of sodium or the lower the concentration of sodium
and the farther out this goes on the x axis the more volume there is or
consequently the less volume this is now because of that you'll note that the
area then is simply represents the number of sodium molecules let me give
you an example if I were to add water to this just straight out water what would
that do the water would add volume to this compartment but at the same time
it would cause the sodium concentration to go down so instead of it instead of
it being these dimensions we would have something more along the line of this
dimension but you could see that the area in the box would be the same
because the number of sodium molecules wouldn't change but the way this is
depicted on the graph would change because we have a lower sodium
concentration and a bigger volume so that's how we're going to depict this
just to finish up the analogy if I did by the way add h2o to this you would see
what would happen here the first thing that you would see will happen is that
the sodium concentration would go down and that the volume would get bigger
that would be if there was absolutely no transport of water across this membrane
but we know that that's not the case because as soon as this sodium
concentration would go down remember this is a permeable membrane to water
and if water is on both sides of this and the sodium concentration goes down
on this side water is going to go to the side of the higher concentration that's
the law of osmosis so you would have water going across from the
extracellular fluid into the intracellular fluid and you would see
that would happen until what would happen until this would be diluted down
to a similar level well in fact what would happen is is that this would go
down and then this would go down to meet this until this was equal but because
this has a much larger volume in fact it's probably twice the volume of this
that about twice the volume of water that we're putting in here would go into
the intracellular fluid chamber and so you can see here that when you give pure
water or in medically speaking dextrose d5w when you give water to somebody you
can see why this not expand the intra the intravascular
volume it's because the majority of this volume as soon as it goes into the
extracellular fluid is going to go across the membrane and basically fill
up the intracellular volume rather than the extracellular there will be some
left in the in the extracellular volume but because this is a larger volume more
of this is going to go into the intracellular varm so to review free
water goes in it causes a reduction in the sodium concentration because of that
the law of osmosis means that water will keep going across this membrane until
the sodium concentration in the cell matches that in the extracellular fluid
which means that most of the water that you give the patient if it's in the form
of free water is going to go into the intracellular fluid compartment now
that's as opposed to giving someone normal saline or as we like to say 0.9%
okay or 0.9% normal saline so normal saline when is given it's basically as
if you're just adding it on to the edge and why is that because the fluid
concentration that is being added to the extracellular volume is already at
exactly the same concentration or very close to the same concentration of the
fluid that's already there because of that that will simply be added on to the
volume there is no change in the extracellular fluid concentration of
sodium because of that there is no shift of fluid over into the intracellular
fluid and that is why normal saline is the best type of fluid to give if your
primary purpose is to expand the extracellular fluid compartment and
that's how that is represented here we see that the volume has gone up but the
sodium concentration in that extracellular fluid is the same okay so
with these basics we're going to move on and talk about the different types of
hyponatremia the hypotonic hyponatremia the hyper
Tomic hyponatremia and the hypotonic hyponatremia so isotonic hypertonic and
hypotonic we'll talk about those in the next lecture thanks for joining us