Fluids and electrolytes? Help i'm so confused: Sodium and Fluid Volume

nurseprnRN, BSN, RN · Jan 14, 2012

na+ and water balance, or why you have to remember that serum sodium doesn't tell you anything at all about sodium , and that saline is not water and salt. (now, don't overthink this. of course if you want to make a bottle of saline, you will mix plain water with sodium chloride. but read this without thinking about that, because .... well, it works better if you think of saline as an entity for purposes of this discussion. read it and then ask me if you still have questions.)

ok, thought experiment time: draw pictures with little molecules or such if you like, it will help. you have a beaker full of salt water, with a na+ level of, say, 140 (hmmmm, what a coincidence). you pour half of it out. what is the na+ level in the remainder? right, 140, because that measurement is a measurement of concentration, not a count of the absolute number of sodium molecules. got that? if not, work on it, because you have to "get" it.

now you refill the beaker to its previous level, full up, with plain water... or, say, d5w, which is the same thing, physiologically. now what's your sodium level? right, 70, because you have twice as much water per amt of sodium.

go back to the half-full beaker again, the one with a serum (oooh, a freudian slip! i think i'll leave it. serum counts as saline.) sodium of 140. fill it up with an equal volume of....normal saline, which for purposes of this discussion has a sodium level about the same as blood serum. what's the serum sodium now? right, still 140. as a matter of fact, you can pour quite a bit of ns into a body and not really influence the serum sodium that much at all. the way you change the serum sodium is by changing the amt of water.

repeat to yourself: "serum sodium tells you about water balance." and "saline is not sodium and water." (i used to have a poster of this and have my classes chant it three times before going on ....i wanted to be sure they would remember it for later)

ok, deep breath. now we look at water balance from the other side.

saline pretty much stays in its vascular place (unless you cut a blood vessel and spill some out). but water....ah, water travels. as a matter of fact, that's the other poster. repeat three times: "saline stays, water travels." (think: rivers flow from place to place, but the ocean pretty much stays where it is.) what the heck importance is that?

back to your original beaker.... the one full of stuff with a serum na+ of 140. evaporate half of the water. what is the serum sodium now? right, 280 (whooee, bigtime dehydration) as a matter of fact, if you lose enough water from your body to get your serum sodium up to 170 or so (("serum sodium tells you about water balance")), you'll probably die, especially if you do it rapidly. why? because water travels in and out of all your cells. if you lose water from your intravascular space, sweat it out, or pee it out because your kidneys are unable to concentrate urine for some reason, thus making your bloodstream more concentrated, water molecules on the other side of the cell walls all over town say, "whoops! gotta go!"...because water travels across cell membranes from an area of more water per volume (lower salt concentration) to the area of less water per volume (higher salt concentration). so if you are de-hydrated, meaning water-poor, all your cells shrink. most importantly, if your brain cells shrink enough from water loss, they pull away from your pia mater/meninges and you have an intracerebral bleed. bummer.

(interestingly, this is why you have a headache with your hangover after an alcohol binge. alcohol temporarily disables your kidneys from retaining water, so they let too much out. you pee a lot, and your brain shrinks just enough to put a little tension on your pia mater/meninges. bingo, headache.) (ahhh, digressed again....)

ok, now put this all together and tell me why your hematocrit is a lousy indicator of water balance (as a matter of fact, a nigh-on useless indicator of dehydration), but a good indication of saline balance.

ok. you are walking down the street with a perfectly good crit of 40 and a serum sodium of 140 (and normal other lytes). you are accosted by someone with a sharp thing and before you know it, a whole lot of your circulating volume is running into the storm drain. fortunately, you are whisked into a nearby er immediately, having had your bleeding stopped by a nearby boy scout with good first aid merit badge training (ummmm, i teach that too). the er nurse draws a baseline crit and lytes. what are they?

ok, crit is still 40...because hct is a *percentage of the blood that is red cells*, not a count of the absolute number of red cells you have. so even if you lose a lot of your blood, your crit is unchanged. until they start fluid-resuscitating you with.... normal (not half-normal) saline (or rl, which acts like it for purposes of this discussion).

na+ is still 140, because you have lost saline (serum counts as saline) but not water.

thought experiment time again. take two tubes of whole blood, that is, serum and red cells. they both have a hct ( which is often spoken as "crit") of 40, that is, 40% of the volume of each tube is taken up solely by rbc's. we already know what happens if you add saline to one of them: the crit drops, right? but what happens to the crit of a tube of blood if you add water-- like d5w? answer: nothing. why? because the crit is a % of volume....and when you add water, the water travels into the cells too. so they swell up, and their %age size change means no change in the crit of the tube. they still take up (in this example) 40% of the volume. what happens if, instead of adding water to your original tube of hct=40 blood, you evaporate half of the water out of it? (the answer is not, "make gravy." shame on you.) no, the hct stays the same, because the cells lose water too, and they shrink as much as the liquidy part did. same percentage of red cells in the resulting volume = no change in hematocrit.

so. when you have someone dehyrated (as evidenced by their elevated serum na+), you give him water (or d5w). this dilutes his serum na+ back towards normal and allows his shrunken dehydrated cells to regain their girlish plumpness. normal saline will not help, as it will not substantially change the serum sodium level ("saline is not sodium and water") and will not move into cells to restore their lost water content ("saline stays, water travels.")

if you have someone who is hypovolemic, as evidenced by (hmmm? what? how do you assess hypovolemia? how about bp, cvp, jvd, pad, lvedp, etc? you pick 'em), you give him saline, which goes into his vascular space where you want it for circulating volume but doesn't go anywhere else. d5w will not do the job, as it will travel into cells (not just rbc's, but all cells, and most of it will thus not be available in the vascular space to make blood pressure).

so why do dehydrated old ladies have high crits and high serum na+'s? well, as i was fond of telling my students, it's perfectly possible to have two things wrong at once.

let's look at a couple of people and see if that helps.

1) serum na+ 140, hct 25, bp 110/60. ok, so this guy is relatively anemic, but his circulating volume is ok (as evidenced by what we assume is an adequate bp for him) and his water balance is fine (as evidenced by his normal na+). who does this? well, anemia can have many causes, but if he comes in with a hx of a recent bleed with fluid resuscitation, you could guess that he had a perfectly good crit until he lost some red cells out his gi bleed or stab wound or bloody ortho surgery or something, and we were stingy and just gave him ns back. his crit is called "dilutional," as in, "his red cells are floating in saline."

2) serum na+ 118, hct 40, bp 110/60. this guy has 'way too much water on board, as evidenced by his na+ that's 'way low ("dilutional" too). we call him hyponatremic, but it's not that he has lost sodium (in most cases), it's that he retained too much water. he hasn't lost saline, as evidenced by his decent bp ("saline is not sodium and water"). who does this? well, remember the dread "siadh"? "syndrome of inappropriate antidiuretic hormone"? lessee.... inappropriate, ummm, too much. antidiuretic, ummmm, doesn't allow diuresis, holds onto water.... bingo. he's retaining water, and his na+ is called "dilutional" because all those little na+s are floating around in too much water. some degree of siahd is actually pretty common--- you can do it with anesthesia, mechanical ventilation (there's stretch receptors in the lungs, see, and....oh, later), and a host of common meds. of course, you can also get a low serum sodium in a hurry if some fool tanks you rapidly with a liter or two of d5w, or , like that poor woman in a socal radio contest, you drink a ton of plain water over a short period of time. she died of acute cerebral edema when her brain swelled up faster than her skull would stretch to accommodate it.

hope this makes some level of sense?

Pneumothorax, BSN, RN · Jan 15, 2012

Hi there, it's my first week of the 2nd semester of the first year in nursing, and i'm having so much trouble trying to get these two in my head, because in my mind they contra-indictate each other. This is about Sodium and Body Fluid Volume.
Ok here we go, i hope you guys can understand what i'm getting at...
NA+ (Sodium) the ranges i was taught was 135-145 mEq/L
Anything higher is Hypernatremia
Anything Lower is Hyponatremia
and that there is two types of places where fluid can be at
Extracellular Volume (ECF) and Intracellular Volume
So lets say the serum sodium is 150 mEq/L (Hypernatremia)
so hypernatremia would mean dehydration
hyponatremia would be overhydration?
how would this relate to Hyper/Hypovolemia then?
So would Hypernatremia mean Hypovolemia?

less water more salt. makes sense!

newstudentrn · Jan 15, 2012

I believe the last poster said it best. I know the other poster said don't think about water with salt in it, but that's how it works for me. If you cook something like chicken in a sautee pan: at first while you are cooking you add a little water to it, then with the heat up the water starts to evaporate and the drippings thicken, if you were to taste the sauce when you first added the water it wouldn't be very salty. If you taste it after letting the sauce "reduce" down, it will be alot more salty. I guess that's how it makes sense to me. Hope I didn't confuse you further.

nurseprnRN, BSN, RN · Jan 15, 2012

hypernatremia doesn't necessarily cause hypovolemia, though they can be associated if there was a big enough water loss.

sometimes serum sodium is intentionally increased (to shrink cerebral edema postop or post trauma, for example) by increasing water excretion through the kidney; the resulting serum na+ increase decreases intracellular water (by sucking the water right across the cell membrane and out into the vascular space). you may or may not be hypovolemic (which refers to vascular contents) during this process.

it is perfectly possible to be hypovolemic and have a perfectly normal serum sodium, if, for example, you have a big blood loss.

hyponatremia most commonly indicates water excess, not a shortage of sodium. you may or may not be hypervolemic if you are hyponatremic.

Pneumothorax, BSN, RN · Jan 16, 2012

it is perfectly possible to be hypovolemic and have a perfectly normal serum sodium, if, for example, you have a big blood loss.

hyponatremia most commonly indicates water excess, not a shortage of sodium. you may or may not be hypervolemic if you are hyponatremic.

exactly.

grntea, you're too smart for your own good hahah :)

CrimsonAlchemist · Jan 18, 2012

GrnTea, that was amazing! :bow: I wish you were my instructor! Your explanation was much better than the handout my instructors use.

BUT, could you please explain in a bit more detail what you meant by "Saline is not sodium and water"...It doesn't really mesh with "saline stays, water travels". I get that water follows the sodium(where ever to goes), but saline stays in the intravascular system to increase blood volume and therefore B/P, right?

Fluids and Electrolytes are one of my weak spots...Thanks :bluecry1:

nurseprnRN, BSN, RN · Jan 18, 2012

i meant to highlight the behavioral and travel differences between saline (as an entity) and water (as in d5w, which is water for the intent of this concept). water travels freely(diffuses) through cell membranes to an area of higher concentration of solutes, all solutes, not just na+, from an area of lower concentration. serum sodium is the way measure the relative amount of intravascular water, and we infer the cellular increase because we know about the diffusion. it's a pretty good way to look at water excess/deficit.

on the other hand, normal saline, as an entity, doesn't travel so much because it is, well, normal, meaning that it has about the same concentration of solutes as the intracellular space. it pretty much stays intravascular because of that, and looking at serum na+ doesn't tell you a think about that. what tells you about saline excess/deficit is the hematocrit, and, if intravascular volume is low enough, low bp and measures of renal function and other such things that happen from hypovolemia. hypervolemia indicators are things like chf and pulmonary edema.

is that helpful? sorry to have been unclear.

CrimsonAlchemist · Jan 18, 2012

Yes! Thank you! =)