Saline (USMLE)

[hyrule]

Oops meant to post this in the USMLE section

What are some of the important rules to follow with patients w/ dehydration/various forms of fluid loss? (ones to know for the USMLE)
or even some of the mistakes physicians often make

One scenario I can think of is:
if patient has hypotonic fluid loss and low BP --> giving them regular saline will normalize BP but will not correct dehydration.

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[Knicks]

In for responses.


When to use which fluid?

 

[cpants]

Never correct hyponatremia faster than 10mmol/L/day to avoid central pontine myelinolysis.

 

[drizzt3117]

Fluids confuse a lot of people. I won't get into the "how much" question, as that's far beyond the scope of step 1, but as far as "what" here's an easy rule of thumb.

If the patient is dehydrated (i.e shows clinical signs of dehydration) then you give normal saline or lactated ringers.

Once they are no longer tachycardic and are making urine (most sensitive sign of euvolemia) then you switch to half-normal saline in adults and 1/4 normal saline in children. Remember that you probably need to add 20 mEQ of K and probably Mg because you're giving IV fluids, as well as D5 if the patient is NPO.

as far as specific solutions:

DKA: Normal Saline
Trauma: LR
Septic Shock: NS or LR, depending on the attending, probably LR
Interoperative Volume Loss: LR
Pancreatitis or SBO: NS or LR

NS has a tendency to cause hyperchloremic metabolic acidosis in high doses, so LR is generally used whenever there is any lactic acidosis or high quantities of fluids are used. In DKA insulin is given with the NS so it's a different situation.

 

[OveractiveBrain]

Fluids confuse a lot of people. I won't get into the "how much" question, as that's far beyond the scope of step 1, but as far as "what" here's an easy rule of thumb.

He's asking about step 1? I think its a Step 2 question.

O
What are some of the important rules to follow with patients w/ dehydration/various forms of fluid loss? (ones to know for the USMLE) or even some of the mistakes physicians often make

Ill add in what to do for Sodium

Hyponatremia:
- Slow and Gradual, Asymptomatic = PO intake
- Rapid (Hours), Confusion, Disorientation = IV saline, NOT hypertonic
- Acute or Severe (<110 or <hours), Coma/Seizures/Death = IV Hypertonic saline to 120, slowly to normal
- Mistake: Giving hypertonic saline to a HypoNatremic patient who doesn't need it causes central pontine myelinolysis

Hypernatremia:
- IV NS unless there was somehow an acute or severe change, in which case you can use dilute D5W, but that almost never happens.

And for Burns:
(Kg Body Weight x %BSA Burned x 4cc) LR = How much fluid their losing through the burns
2000cc D5W = Maintenance

Overall, the Parkland Formula is (Kg X %BSA x 4)LR + 2000 D5W
- Half is given in 8 hours
- Half is given in 16 hours

Calculating Maintenance Fluid:
0-10kg 100cc/kg
10-20kg 50cc/kg
21+kg 20cc/kg

So a 46kg teenager would need 10x100 + 10 x 50 + 26*20
So a 46kg teenager would need 1000 + 500 + 520 cc every day

 

[Knicks]

Fluids confuse a lot of people. I won't get into the "how much" question, as that's far beyond the scope of step 1, but as far as "what" here's an easy rule of thumb.

If the patient is dehydrated (i.e shows clinical signs of dehydration) then you give normal saline or lactated ringers.

Once they are no longer tachycardic and are making urine (most sensitive sign of euvolemia) then you switch to half-normal saline in adults and 1/4 normal saline in children. Remember that you probably need to add 20 mEQ of K and probably Mg because you're giving IV fluids, as well as D5 if the patient is NPO.

as far as specific solutions:

DKA: Normal Saline
Trauma: LR
Septic Shock: NS or LR, depending on the attending, probably LR
Interoperative Volume Loss: LR
Pancreatitis or SBO: NS or LR

NS has a tendency to cause hyperchloremic metabolic acidosis in high doses, so LR is generally used whenever there is any lactic acidosis or high quantities of fluids are used. In DKA insulin is given with the NS so it's a different situation.

2 questions:

1) why that switch?

2) 1/2 (or 1/4) NS is hypERtonic compared to NS, right? (I never got fluids down).

 

[Dwindlin]

2 questions:

1) why that switch?

2) 1/2 (or 1/4) NS is hypERtonic compared to NS, right? (I never got fluids down).

Hypotonic. They both have less NaCl than NS.

 

[Rendar5]

2 questions:

1) why that switch?

2) 1/2 (or 1/4) NS is hypERtonic compared to NS, right? (I never got fluids down).

1) no switch. just don't think about it because it's voodoo. D51/2NS is a perfectly adequate maintenance fluid for kids (and my picu will use D5NS for older kids, but that's just my house). I'm sure you'll run into some attendings who preach the voodoo and will have you be superselective in your fluid approach and offer you valid theoretical explanations for why one is better than the other. But honestly at this point in your education, just forget about it, you don't need to know it.

2). if you want to know more clinical info, NS = 154meq of both sodium and chloride. you keep giving it in enough quantities, it'll raise your Na and Cl to those levels, thus the hyperchloremic metabolic acidosis, and why NS isn't the best fluid for someone with hypernatremia.

 

[Knicks]

Hypotonic. They both have less NaCl than NS.

Thanks.

1) no switch. just don't think about it because it's voodoo. D51/2NS is a perfectly adequate maintenance fluid for kids (and my picu will use D5NS for older kids, but that's just my house). I'm sure you'll run into some attendings who preach the voodoo and will have you be superselective in your fluid approach and offer you valid theoretical explanations for why one is better than the other. But honestly at this point in your education, just forget about it, you don't need to know it.

2). if you want to know more clinical info, NS = 154meq of both sodium and chloride. you keep giving it in enough quantities, it'll raise your Na and Cl to those levels, thus the hyperchloremic metabolic acidosis, and why NS isn't the best fluid for someone with hypernatremia.

Hold up,, I understand the hyperchloremic part (since NS has Cl- in it), but how does the met. acidosis fit in?

Thanks.

 

[Rendar5]

Simple dilution of bicarb is what does it. Whether this is significant or not is unclear.

 

[OveractiveBrain]

Thanks.

Hold up,, I understand the hyperchloremic part (since NS has Cl- in it), but how does the met. acidosis fit in?

Thanks.

Why does normal saline cause acidosis, why doesn't Lactated ringers? Why do medicine people prefer NS while surgeons prefer LR?

Far more complicated than you actually want to know. But here we go.

(1) First, what happens when some one gets DEhydrated? Less flow through the kidney, more renin, more angII, more aldo. What does aldo do? Kicks out K and keeps Na. Most people know that. But what else does it do? It turns on acid secretion into the tubules and generates some bicarb. This is what is meant by contraction alkalosis. Most people are pretty comfortable with this, so I went through it quickly.

(2) Going with that, what happens when you REhydrate? More flow through the kidney, less renin, less angII, less aldo. Less aldo means less acid secreted into the tubule and less bicarb made in the blood. If you fix a contraction alkalosis with rehydration, there must therefore be an "expansion acidosis." Except the "acidosis" is just fixing the original problem. Follow the logic?

But also feel that if giving fluids caused a "correcting expansion acidosis" isn't it possible that simply giving a normal person fluids could cause an "inappropriate expansion acidosis?" Hold that thought.

(3) Now it gets tricky. The whole body consits of strong ions with fully dissociate (Na, Cl, K) and weak ions (proteins and H). There are more strong (+) ions then strong (-) ions, creating a strong ion difference. That is, there is more Na and K in the fluid then there is Chloride. Other ions play a part, but this is easier to understand if we deal with just these three. More Na+K (positive) than Cl (negative) means a net positive SID.

(4) Work with me here, this is weird. If you've got a bunch of strong cations (a high SID) what influence does that have on weak cations (like H+)? Its going to push them away, right? It's going to make it more likely for those weak cations to buffer out on weak anions. Like charges repel, dissociation curves for salts in chemistry. And you thought you could forget all that, huh? So that means, the HIGHER the SID, the LESS H+ there can be. And, conversely, the LOWER the SID, the MORE H+ there can be, and the more acidic the patient becomes.

(5) Normal saline has a SID of 0; its only got Na+ and Cl-; net neutral. So if you add it to a person's body the AVERAGE SID will go down. Don't start thinking numbers, yet. Just go with it. If the average SID falls, is there more or less room for H+? SID goes down, the amount of strong cation goes down, the force driving H+ away goes down, so H+ can go up. More H+ means acid. So I hope you can see that THEORETICALLY, adding NS to a person COULD produce an acidosis.

(6) Feel this though. We're talking TOTAL body fluid and electrolytes. For an average man, there's about 60L of water. To dilute ALL that water you need to literally THROW NS at this guy. It takes about 10L of NS in a day to dilute the patient's fluids enough to cause an acidosis. Even then, its mild.

(7) Lactated ringers have a body neutral SID. Most people think "LACTATE!" in "lactated ringers" somehow plays with pH. Nope. Body neutral SID. Why? Its got potassium. A strong cation. Na and Cl balance out, but the K makes it net positive SID. Its like it was DESIGNED that way or something. So if you take a 60L person with an SID of 30, and you add 10L with an SID of 30, how does that change the average? Damn. It don't. If the SID is the same, is there any more or less room for H+? Nope. No change in pH.

(8) Surgery is messy. The belly is open. There's blood loss. Do you think you'd need to use a lot of fluid on a guy in surgery? Oh yeah. They are cut open, their bowels being manipulated. Their temperature is all out of wack. You control literally every factor of their being. Does a surgeon, who MAY give 10 L of fluid in the course of the surgery, want to worry about causing an acidosis? Hell no. Fluids are to maintain perfusion. Period. So he uses Lactated Ringers.

(9) Medicine patients are sick. They've got heart problems, renal problems. Medicine deals with patients that surgery would never see. They couldn't get "medically cleared" for surgery. Will medicine be dumping 10L a day into someone? Of course not. What to medicine people worry about? Fluid overload. So no 10L/day. What else do medicine people worry about? Electrolytes. Remember, LR has potassium in it. Renal patients, Heart Failure patients, patients on diuretics already have problems with potassium. So medicine stays away from ringers. Medicine uses Normal Saline.

Long winded, but potent stuff. Hope this helps!

 

[OveractiveBrain]

1) no switch. just don't think about it because it's voodoo. D51/2NS is a perfectly adequate maintenance fluid for kids (and my picu will use D5NS for older kids, but that's just my house). I'm sure you'll run into some attendings who preach the voodoo and will have you be superselective in your fluid approach and offer you valid theoretical explanations for why one is better than the other. But honestly at this point in your education, just forget about it, you don't need to know it.

Probably true. But I certainly didn't go into medicine just to follow protocols. That sort of thinking is why NPs and PAs can take our jobs an do it for less (like the people of the future) "THEY TIRK R JURBS" or this one

The message is true, though. At a medical student level, just know which ones are good enough for rehydration and call it a day.

 

[Knicks]

wow, thanks guys

 

[bambi]

1) no switch. just don't think about it because it's voodoo. D51/2NS is a perfectly adequate maintenance fluid for kids (and my picu will use D5NS for older kids, but that's just my house). I'm sure you'll run into some attendings who preach the voodoo and will have you be superselective in your fluid approach and offer you valid theoretical explanations for why one is better than the other. But honestly at this point in your education, just forget about it, you don't need to know it.

2). if you want to know more clinical info, NS = 154meq of both sodium and chloride. you keep giving it in enough quantities, it'll raise your Na and Cl to those levels, thus the hyperchloremic metabolic acidosis, and why NS isn't the best fluid for someone with hypernatremia.

Maintenance should be 2 Dextrose, 1 normal saline (with an additional 40mmol of potassium added overall) or like you said yourself you will mess with the sodium levels. 1 bag of NS supplies the daily sodium requirement and dextrose is essentially water. In an emergency basically anything other than dextrose is fine, some things are better than others though. I wont go into it as a lot of the fluids have different names in different countries, we don't have lactated ringers for example- I think Hartmann's is our equivalent and I'm not sure about your other fluids.

 

[Knicks]

maintenance should be 2 dextrose, 1 normal saline (with an additional 40mmol of potassium added overall) or like you said yourself you will mess with the sodium levels. 1 bag of ns supplies the daily sodium requirement and dextrose is essentially water. In an emergency basically anything other than dextrose is fine, some things are better than others though. I wont go into it as a lot of the fluids have different names in different countries, we don't have lactated ringers for example- i think hartmann's is our equivalent and i'm not sure about your other fluids.

u.s.m.l.e. 😀

 

[bambi]

u.s.m.l.e. 😀

Yeah that's exactly why I didn't talk about the other fluids but maintenance and emergencies are the same as I said whatever country you are in!

 

[BigRedBeta]

Fluids confuse a lot of people. I won't get into the "how much" question, as that's far beyond the scope of step 1, but as far as "what" here's an easy rule of thumb.

If the patient is dehydrated (i.e shows clinical signs of dehydration) then you give normal saline or lactated ringers.

Once they are no longer tachycardic and are making urine (most sensitive sign of euvolemia) then you switch to half-normal saline in adults and 1/4 normal saline in children. Remember that you probably need to add 20 mEQ of K and probably Mg because you're giving IV fluids, as well as D5 if the patient is NPO.

as far as specific solutions:

DKA: Normal Saline
Trauma: LR
Septic Shock: NS or LR, depending on the attending, probably LR
Interoperative Volume Loss: LR
Pancreatitis or SBO: NS or LR

NS has a tendency to cause hyperchloremic metabolic acidosis in high doses, so LR is generally used whenever there is any lactic acidosis or high quantities of fluids are used. In DKA insulin is given with the NS so it's a different situation.

A note on this from the peds perspective: (though it goes beyond the OP's question)
1) The 1/4NS or 1/3NS has not been shown to affect outcomes in pediatric patients compared to 1/2NS, so 1/2NS is entirely appropriate in pediatric patients. The 1/4 or 1/3 variation is based on calculating the body's daily sodium needs - 2-4mEq/kg/day, so if you work out the math, 1/2NS is excessive. But it's largely an academic point so long as you have normal kidney function.

2) It's a rare hospital that stocks 1/4 or 1/3 NS on the floor, nurses may get confused or not recognize that it has to be ordered from the pharmacy.

3) Hyponatremia is absolutely an important clinical entity, one with known significance in morbidity (as compared to the hyperchloremic acidosis). There is a movement in many hospitals across the country to make NS the default fluid.

4) DKA is much more complex than has been presented, and because of concerns with dropping sugars too fast and raising risk for cerebral edema/herniation, it's a situation where you would not rapidly correct dehydration with a goal of correcting capillary refill or tachycardia. International protocols recommend a fluid resuscitation of no more than 20-30ml/kg over the first hour although they may be much more dehydrated than that, and always, always, always finish this fluid bolus before starting an insulin infusion. (And never give an insulin bolus)

 

[jdh71]

(3) Now it gets tricky. The whole body consits of strong ions with fully dissociate (Na, Cl, K) and weak ions (proteins and H). There are more strong (+) ions then strong (-) ions, creating a strong ion difference. That is, there is more Na and K in the fluid then there is Chloride. Other ions play a part, but this is easier to understand if we deal with just these three. More Na+K (positive) than Cl (negative) means a net positive SID.

(4) Work with me here, this is weird. If you've got a bunch of strong cations (a high SID) what influence does that have on weak cations (like H+)? Its going to push them away, right? It's going to make it more likely for those weak cations to buffer out on weak anions. Like charges repel, dissociation curves for salts in chemistry. And you thought you could forget all that, huh? So that means, the HIGHER the SID, the LESS H+ there can be. And, conversely, the LOWER the SID, the MORE H+ there can be, and the more acidic the patient becomes.

(5) Normal saline has a SID of 0; its only got Na+ and Cl-; net neutral. So if you add it to a person's body the AVERAGE SID will go down. Don't start thinking numbers, yet. Just go with it. If the average SID falls, is there more or less room for H+? SID goes down, the amount of strong cation goes down, the force driving H+ away goes down, so H+ can go up. More H+ means acid. So I hope you can see that THEORETICALLY, adding NS to a person COULD produce an acidosis. !

Nice to see the strong anions gap is finally filtering it's way into medical education after like, what? 20 years since Stewart derived the physical chemical basis for physiologic pH, none of which have anything to do with Bicarb itself. Makes it much more easier to understand hyperchloremic acidosis.

So the surgeons like a more "buffed" AM BMP, are completely convinced NS is poison, but I there isn't a single study (though last time I looked hard was end of 2009, but I've not seen anything new in the medicine critical care literature since that time, though I make no claim to omniscience) that has demonstrated that LR is superior to NS in those measures that we are all interested in mortality and length of stay. Ok, so there is more mild metabolic acidosis with NS? Does it matter?

 

[OveractiveBrain]

Nice to see the strong anions gap is finally filtering it's way into medical education after like, what? 20 years since Stewart derived the physical chemical basis for physiologic pH, none of which have anything to do with Bicarb itself. Makes it much more easier to understand hyperchloremic acidosis.

So the surgeons like a more "buffed" AM BMP, are completely convinced NS is poison, but I there isn't a single study (though last time I looked hard was end of 2009, but I've not seen anything new in the medicine critical care literature since that time, though I make no claim to omniscience) that has demonstrated that LR is superior to NS in those measures that we are all interested in mortality and length of stay. Ok, so there is more mild metabolic acidosis with NS? Does it matter?

Probably not.

But dogma and old wives tales, just like rumors, have SOME element of truth in them. If you don't have to take the risk, why bother? Both NS and LR are equally available in the United States, and since neither is shown to do WORSE than the other, why not just follow the rules as they are?

I suppose one could do an experiment to show how harmful NS is in surgical patients, or LR in dilaysis patients. But why? Thats just hurtful!

 

[Rendar5]

Probably not.

But dogma and old wives tales, just like rumors, have SOME element of truth in them. If you don't have to take the risk, why bother? Both NS and LR are equally available in the United States, and since neither is shown to do WORSE than the other, why not just follow the rules as they are?

I suppose one could do an experiment to show how harmful NS is in surgical patients, or LR in dilaysis patients. But why? Thats just hurtful!

I believe those experiments have been done on surgical patients, though I'd have to check to be sure. But just because something is well-established experimentally does not mean that people are aware or believe it.

 

[OveractiveBrain]

I believe those experiments have been done on surgical patients, though I'd have to check to be sure. But just because something is well-established experimentally does not mean that people are aware or believe it.

A case presentation in anesthesia (this was like 15 years old, or some such nonsense) showed a TURP patient get overloaded with NS (wide open instead of keep vein open), got acidotic, coded, and was resuscitated. This was a case we did in first year physiology.

There was another one where surgical patient was overloaded with D5W instead of NS. Obviously, thats much worse.

I'd love to see any recent case reports telling the same tales. Who knows how accurate they were with their depictions, comorbidtities or other blunders. If you know of any experiments in people (or know an easy way to find them) I'd love some references!