Sodium and Potassium Requirements
Quickcheck
It is clear that there is no strict daily sodium requirement since, in the normal individual, homeostatic mechanisms will instruct the kidney to conserve or excrete sodium and keep total body sodium content within the normal range.
More info: The Consequences of Pizza
Salt intake ==> increase in plasma Na and osmolality ==> increased thirst and increased ADH secretion ==> chug-a-lug ==> water intake and water retention, plasma dilution ==> plasma Na and osmolality decline almost to baseline at the expense of expanded plasma volume (I cannot take off my rings; socks leave deep marks on shin!) ==> the kidney stops making renin ==> no renin, so no angiotensin and no aldosterone ==> Na re absorption declines in the collecting duct ==> urinary Na excretion increases until all the salt from the pizza has been excreted ==> plasma osmolality falls as Na is excreted ==> ADH shuts off ==> water is excreted until plasma volume has declined to baseline...
I feel better.
Whew!
Based on recommendations made by Holliday and Segar, the daily sodium requirement was estimated at 3 mEq/100 ml of water water requirement. Holliday and Segar decided on this number by looking at the sodium content of human and cows' milk. So, Na is added to maintenance fluids at a concentration of 3 mEq/100ml or 30 mEq/L. For an adult, this will provide about 75 mEq of Na/day, equivalent to approximately 4.5 G of salt. Click for flashback to chemistry.
When we speak about adding sodium to IV fluids, we talk about it in terms of normal saline. Normal saline is isotonic to plasma. Normal saline is 154 mEq/L. So if we wanted to add 30mEq/L of Na, we would need 1/5NS. (154 mEq/L divided by 5 is roughly 30 mEq/L).
For decades, our maintenance IV fluids have ranged anywhere from 1/5NS to 1/3NS to Ѕ NS. Note that all of these are considered hypotonic to plasma. Based on current research, it is determined that giving hypotonic solutions as maintenance IV fluids is associated with severe morbidity and even mortality due to hyponatremia.
reference: AAP Guidelines on Maintenance IV Fluids in Hospitalized Children
We know that kids in the hospital are stressed. They are vomiting, or have respiratory illness, or require surgery, or have fever. All of these things cause an increase in ADH release. The more ADH, the more water is reabsorbed from the collecting duct of the kidneys. Combine this with hypotonic IV fluids, and you have a perfect formula for hyponatremia.
The maintenance K requirement is estimated at 2 mEq/100 ml of fluid or 20 mEq/L. Potassium either comes pre-added or can be manually added to any intravenous solution at a concentration of 2 mEq/100 ml or 20 mEq/L to provide the appropriate amount of K for maintenance. This was estimated by Holliday and Segar to again reflect the composition of human and cow milk and has remained the same since then.
In children who have a condition that might predispose to renal failure, such as dehydration, K is not added to intravenous fluids until the presence of renal function has been established.
Note: K concentration in IV fluids of up to 40 mEq/L is used for correction of hypokalemia. A concentration exceeding 40 mEq/L is irritating to the veins and may be dangerous!
