basic info about fluids

[obiwan]

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i know that this is pretty basic stuff but i can't really find anything on how you decide how much to give in terms of IV fluids and what types of fluid to give in like D5 or half normal saline etc.

thanks in advance

 

[dragonfly99]

It depends what you are giving the fluids for. If you are just making someone NPO (nil per OS - means not allow your patient to eat or drink for several hours to days) then you are giving "maintenance fluids", we usually give 1/2 normal saline or D5 1/2 normal saline (for babies/peds it is 1/4 normal saline I think, which we NEVER give in internal medicine, or at least I have never...). There are specific formulas to figure out IV fluid replacement (takes into account patients weight, etc.).

If someone comes in volume depleted (i.e. "dehydrated" in layman's terms) then they generally get normal saline (NS). If someone is dehydrated or in sepsis, usually you end up giving them several liters of fluid to get them tanked up back toward normal. You have to be careful in little old tiny ladies or people with CHF, because if you give too much fluid too fast, you can provoke CHF leading to pulmonary edema.

If you are trying to fix hypo or hypernatremia then they are specific formulas that you can do (if hypernatremic, you calculate the "water deficit", etc.).

If you really want good comments on replacing IV fluids, you should post in the general surgery or anesthesia threads...those guys LOVE this stuff (fluid replacement).

 

[mrantarctica]

this is why you learned fluid physiology so long ago

In a day (a 24h period) you need to consume a 2-3L 'bag' of water. In that bag, there has to be around 60-80 mmol of Na, and 60 mmol of K. Most people have an energy requirement. Depending on your gender or how big you are for men its about 3000 kcal/day and women 2000 kcal/day.

If we were to simulate this as accurately as possible. Knowing that most fluid preparations come in 1L bags. We we need 2 bags of 'something'. Each bag would have to have about 30-40mmol Na, and 30mmol K. Each bag would need to run at a rate of 1L/12hr. (2L in 24hr)

Conveniently we find this as a bag of 1/5 normal Saline (30mmol Na) with 30mmol K+ mix. Often such bags are mixed with 4% dextrose to prevent protein catabolism. Such a bag you CANNOT give quickly as the glucose will be quickly metabolised by cells, making it a hypotonic solution, which will result in cerebral oedema, coma and death. This is the basis of 'maintenance fluids'.

Replacement fluids are essentially to replace whatever losses you encounter - and the fluid should resemble the constituents you lose. The important thing to remember is that often you're correcting electrolyte imbalance, and more than anything electrolyte imbalances need to be corrected cautiously. Too fast or too slow can cause some permanent damage. Generally speaking you can only correct Na at a rate of 1mmol/hr or some even say as low as 12-24mmol/day. Similarly, K is even more tricky. Proceed cautiously and keep reviewing the patient/results.

Resuscitation fluids are almost always isotonic and theres a big debate over whether crystalloids (saline, hartmanns etc.) are better than colloids (starch, gelatin based etc.). So far the evidence seems to indicate theyre about the same, and you should follow local protocols.

Give resuscitation fluids fast. The idea is to expand plasma volume and improve myocardial function in hypovolemia (starlings curve). you wont be able to do this if you put people up for 3 hour bags or 4 or bags. So give them fast into big veins with good access through big lines.

In suspected hypovolaemia due to blood loss, you should always consider starting a transfusion. Usually you can afford to ram in 2L of saline before the blood arrives for you to put that into them too. It's always important to get some baseline bloods for a Hb, PCV, and some coag profile. Remember Packed red cells are just red cells. If they have a coagulopathy - you won't save them. If they don't have a coagulopathy, and you load them with fluid, and then with some red cells - they may develop one!! If they need other blood constituents, consider on a case by case basis and give the stuff.

 

[carrigallen]

Often such bags are mixed with 4% dextrose to prevent protein catabolism.

I personally do not think that D5 does much to prevent protein catabolism. Bacteria might like it though...and it may exacerbate their diabetes.

 

[mrantarctica]

I personally do not think that D5 does much to prevent protein catabolism. Bacteria might like it though...and it may exacerbate their diabetes.

Glucose starvation and the consequent metabolic changes resulting in protein catabolism have been well documented. In practice, most patients are able to tolerate oral intake, or are only fasted for such short periods of time that catabolic events are not clinically evident. Dextrose is also added to low sodium solutions to make them isotonic. The dangers of infusing hypotonic fluids have also been well documented.

Most patients with bacteraemia needing IV fluids will probably need resuscitation fluids under the setting of sepsis. Like I mentioned - maintenance fluids are never used for resuscitation. Fluids such as Normal Saline, Hartmann's, or Colloid solutions are used. These solutions do not contain glucose or equivalent.

As far as I'm aware, the use of dextrose containing solutions have never been associated with an increase incidence of nosocomial infection or sepsis, above and beyond the association that exists with IV line placement. This could be because no (or very few) studies have actually investigated this!!

As for worsening hyperglycaemia. Well yes, I think that is obvious, theoretically. Whether or not patients have symptomatic hyperglycaemia, or damaging hyperglycaemia on maintenance solutions is questionable. I haven't read anything on the topic, I'm afraid. I have heard that tight glycaemic control on medical wards (but not ICU, where it's preferable) has been associated with a poorer prognosis. Though, I haven't reviewed this material with my own eyes - just spoke with a specialist about it that's all.

 

[viostorm]

I was taught the 4/2/1 rule that is weight based for maintenance fluids

4 cc/hr for first 10 kg
2 cc/hr for second 10 kg
1 cc/hr for every kg after that.

So 70 kg person: 10 * 4 + 10 * 2 + 1 * 50 = 110 ml/hr.

For the adult patient an easy shortcut is: weight in kilos + 40 is the maintenance rate, works out the same as 4/2/1.

As far as what to give most people here give D5 1/2 NS with 20 meq of K as maintenance.


As far as a resuscitation for volume depletion: 10-20 ml/kg of NS bolus (not a real bolus but run in over 1-2 hours), may repeat up to 3 times while being cautious with heart failure and elderly.


I'm interested to see some other people's views on fluids that are likely much more educated then I.

 

[mrantarctica]

interesting about the 4 2 1 rule. Sorta used to prescribing fluids for children with that nifty little tool.

 

[mrantarctica]

still not sure I understand the rationale with D5% and 1/2 NS. That is hypertonic fluid. 1 bag of that stuff will deliver 70mmol of Na + Cl, but only 1 L of water. Which means, to meet someones daily water requirment you will also have to give them 140mmol of Na and Cl, which is really overcooking them.

that said, most kidneys will handle that insult. but you run a fine line of hyperchloraemia, and overloading their sodium.

beware that 1L of normal saline will take at least 20 mins to STAT into someone, depending on what size line you have. Some of the newer fancier pumps can deliver 1L in less than that.. but on a ward, thats pretty much the fastest you can run a bag into someone.

In the resuscitation setting, your patient will be shocked. Certainly in purely cardiogenic and obstructive shock, fluids are not a major concern. However in hypovolaemia and distributive shock, running in fluids over 1-2 hours is just too slow. The point of resuscitation fluids is to treat aggressively. intravascular volume depletion is serious, and if cardiac output is compromised, then it is fatal if not corrected rapidly. Theres no point dillying about worrying that they will develop failure. Most cases of untreated hypovolaemic shock, or distributive shock will eventually develop some degree of cardiogenic shock or cardiac arrest, for which the prognosis is then exceedingly poor. So don't wait around.

At least in the surgical setting, in a patient with very very high suspicion of a post operative bleed, certainly 1-2 hours is the longest time before weve had them in theater. Most of the time, the boss was called, the senior resident scrubbed and patient wheeled in inside 60 mins. By the time that happened, on the ward we'd already loaded them with 2-4L of NS and perhaps even started a transfusion by then.