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 gonna 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 they isotonic are they
hypotonic hypertonic and then of the hypotonic hyponatremia is it hypovolemic
hypervolemic and I saw bulimic 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 divided by 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 UN and that's again in milligrams per deciliter divided by 2.8 and the normal
for that is 285 mili moles 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 plus 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 2 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 ok 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 can 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 gonna 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 gonna 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 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 gonna 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 intravascular volume this represents all
the fluid inside arteries veins capillaries etc and this is the
interstitial volume 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 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 to 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 compartments this is about 4 liters this
is about 10 liters and this is about 28 liters 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 of 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
questions 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 gonna 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 does 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 it's a 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 person 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 gonna move on and talk about the different types of
hyponatremia the hypotonic hyponatremia the hypertonic hyponatremia and the
hypotonic hyponatremia so isotonic hypertonic and hypotonic we'll talk
about those in the next lecture thanks for joining us