Why does IV NS cause urination?

[herecomesthesun]

Hey,
Many patient's complained of frequent urination after getting IV normal saline pre-operatively on my gyn rotation. Normal saline increases the intravascular volume, and assuming the patients have normal na status, has little effect on the osmolarity of blood. Why then do these patient's urinate more? My resident was unable to explain this effect. I have already forgotten renal, and it was only 6 months ago! Thanks.

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[BrianUM]

remember sodium balance determines volume, water balance determines sodium concentration

in other words, although normal saline is isotonic compared to body fluid you are nonetheless loading up the patient with sodium and water and increasing extracellular volume. more volume means that more has to be cleared by the kidneys, hence the increase in urination. There is a higher renal blood flow and a decreased increase in oncotic pressure as blood is being filtered.

 

[Blesbok]

If the person did not urinate, where do you think all that excess fluid goes? Do you think you can just pump 5 gallons of NS into a person and they will be perfectly fine with all that extra fluid? Osmolarity is not the only thing that determines urination. Fluid volume plays a huge role in that too.

 

[anon-y-mouse]

remember sodium balance determines volume, water balance determines sodium concentration

This is the key. Were you taught by that "Acids/Bases/Electrolytes Made Ridiculously Simple" guy? I vaguely remember reading that he's a nephrologist at UM...

another way to put it-

Sodium concentration is a reflection of water status. If you're hyponatremic, it's not that you have low total sodium, it's that you have tons of water kicking around.

Water status is (a clinical determination) is a reflection of total AMOUNT of sodium (not concentration)... dehydration - low skin turgor or as determined through a central venous catheter-- that tells you that there's a low amount of sodium in the body.

try to separate sodium concentration (water status) from sodium *amount* and you'll be ok.

anyway, back to the original question, when you're giving someone NS, yes, it is an isoosmolar solution... if they're dehydrated, it goes to replete their vascular tree, but if you're overloading someone and their kidneys are fine, then obviously they'll pee more to maintain homeostasis. i think you are overthinking this.

 

[Noeljan]

Hey,
Many patient's complained of frequent urination after getting IV normal saline pre-operatively on my gyn rotation. Normal saline increases the intravascular volume, and assuming the patients have normal na status, has little effect on the osmolarity of blood. Why then do these patient's urinate more? My resident was unable to explain this effect. I have already forgotten renal, and it was only 6 months ago! Thanks.

no offense, but it is hard to believe your resident could not understand this. Any time you increase fluid you increase preload and thus increase CO and pressure..more blood flow to kidney....excretion back down to maintain normal pressure.

 

[herecomesthesun]

Thanks for your responses. It is all coming back to me now. I consulted BRS physiology which helped as well-->an increase in ECF leads to decreased peritubular protein concentration/oncotic pressure and increased hydrostatic pressure, and voila, there is less tubular reabsorption. Thanks.

 

[indo]

good lord.

 

[BrianUM]

This is the key. Were you taught by that "Acids/Bases/Electrolytes Made Ridiculously Simple" guy? I vaguely remember reading that he's a nephrologist at UM...

another way to put it-

Sodium concentration is a reflection of water status. If you're hyponatremic, it's not that you have low total sodium, it's that you have tons of water kicking around.

Water status is (a clinical determination) is a reflection of total AMOUNT of sodium (not concentration)... dehydration - low skin turgor or as determined through a central venous catheter-- that tells you that there's a low amount of sodium in the body.

try to separate sodium concentration (water status) from sodium *amount* and you'll be ok.

anyway, back to the original question, when you're giving someone NS, yes, it is an isoosmolar solution... if they're dehydrated, it goes to replete their vascular tree, but if you're overloading someone and their kidneys are fine, then obviously they'll pee more to maintain homeostasis. i think you are overthinking this.

Yup..great guy too, Dr. Preston

he even gave us a free copy of his book

 

[Instatewaiter]

Another way to think of it:

You are putting volume into the ECF. Since it is isotonic there is not a (major) shift into the ICF so intravascular volume goes up as does pressure.

I dont think the major mechanism is increased blood flow to the kidneys like someone previously said. The kidneys maintain RBF along a pretty tight continuum. The mechanism I am pretty sure is the PP puts out less ADH in response to the volume expansion causing diuresis.

 

[Droopy Snoopy]

Thanks for your responses. It is all coming back to me now. I consulted BRS physiology which helped as well-->an increase in ECF leads to decreased peritubular protein concentration/oncotic pressure and increased hydrostatic pressure, and voila, there is less tubular reabsorption. Thanks.

You're giving extra fluids, so assuming the patient doesn't have introperative hemorrhaging that fluid has to get peed off. Think of things in simpler terms and you'll be more likely to remember the complex mechanism behind them. It seems to me you're doing the reverse.

 

[Noeljan]

Another way to think of it:

You are putting volume into the ECF. Since it is isotonic there is not a (major) shift into the ICF so intravascular volume goes up as does pressure.

I dont think the major mechanism is increased blood flow to the kidneys like someone previously said. The kidneys maintain RBF along a pretty tight continuum. The mechanism I am pretty sure is the PP puts out less ADH in response to the volume expansion causing diuresis.

Any increase in volume will increase your preload pressure in the heart, leading to an increase in SV>CO> which will increase blood flow and pressure. The kidney is the main regulator of arterial pressure and wants to do all it can to help maintain your pressure (along with autoregulation and starling in the heart). Which begins with increased diuresis. It becomes even more efficient due to neurohumoral influences (after the increased vol) Because of increased volume in the atria you also get inhibition of ADH and increase ANP, among other things. These actually are less immediate than the pure volume effects that occur. Both occur, but don't underestimate the immediate pure volume control that occurs.
Anyways, OP this is very simple. If you have an excess volume you want to get rid of it....just like if your volume is decreased you want to retain it. This is a very simplistic concept don't think too much into it.

 

[Noeljan]

You're giving extra fluids, so assuming the patient doesn't have introperative hemorrhaging that fluid has to get peed off. Think of things in simpler terms and you'll be more likely to remember the complex mechanism behind them. It seems to me you're doing the reverse.

 

[Noeljan]

...

 

[THP]

Think of it this way . . . what happens when you drink 12 gatorades back to back. In about 30 min you will be running for the bathroom. Its the exact same idea.

 

[45408]

the part of the hypothalamus responsible for releasing ADH is much more sensitive to osmolarity than volume, but it does respond to volume. If your osmolarity is appropriate but your volume is excessive, you're going to urinate more.

 

[Blesbok]

the part of the hypothalamus responsible for releasing ADH is much more sensitive to osmolarity than volume, but it does respond to volume. If your osmolarity is appropriate but your volume is excessive, you're going to urinate more.

I am not sure how much it actually responds to increased volume though, I think it is only a huge response to decreased volume.

 

[supernareg]

everyone's giving such a long and info loaded advice... here's the most simple thing...


drink a lotta water, piss out a lotta pee
IV a lotta NS, it has to come out right? or else why do we piss/crap?

goes in, must come out

 

[braluk]

Ibelieve its called pressure natriuresis. Correct me if I'm wrong.

 

[Noeljan]

Ibelieve its called pressure natriuresis. Correct me if I'm wrong.

yep you are correct.

 

[Wet Coma]

yes simple answer is you drink (or give NS) you pee

many factors go into it, which many have already been mentioned. one that has not been mentioned is the whole JGA RAAS system. Kidneys sense more fluid, activates the system, less aldosterone around, less Na and water reabsorption. Also, Increased ANP from the heart

 

[anon-y-mouse]

yes simple answer is you drink (or give NS) you pee

many factors go into it, which many have already been mentioned. one that has not been mentioned is the whole JGA RAAS system. Kidneys sense more fluid, activates the system, less aldosterone around, less Na and water reabsorption.

I was under the impression that the JGA senses only hypoperfused states, which is the exact opposite of adding a fluid load to your body. It works to increase BP in hypotensive states. So, I don't think the RAAS is relevant for this scenario.

 

[McMD]

Yup..great guy too, Dr. Preston

he even gave us a free copy of his book

Seriously?...he told our class to "buy my book." He's awesome though, best teacher EVER (besides Dr. Salathe)

 

[BrianUM]

eh salathe taught well but i felt his tests were excessively hard (no other teacher ever gave exams nearly as difficult as his)..i guess its a matter of taste...i happened to like dr. kupin