# Intravenous Fluids: The 4 Rs

Assuming NaHCO3 comes in a stock solution of 1 mEq/mL, then [C] = solute osmolarity / solution volume

= solute osmolarity / (solute volume + solvent volume)

= 150 mEq / (0.150 L + 1 L) = 130 mEq NaHCO3/L

In 850 mL of solvent, then [C] = 150 / 1000 = 150 mEq Na HCO3/L

Assuming total body water [L] = 0.5 x wgt [kg] (acutally TBW fraction varies between 0.45 and 0.6 depending on gender and age). We want to change the current plasma sodium by 10 mEq/L/24 hours (i.e. the maximum safe change in serum sodium that will not precipitate central pontine myelinolysis) and recall that the concentration of sodium in 0.9% saline or NS is 154 mEq/L, then:

infusion rate = 0.5 x wgt [kg] x 10 / 154 (L x mEq/L/24 hr) / (mEq/L)

= 5 x 1000 x wgt [kg] / (154 x 24) mL/hr

= 1.35 x wgt [kg] mL/hr for NS

If we are using 3% saline, multiply by 0.9/3 (0.9% / 3%) or 0.3

In the case of hypernatremia, we know that 1/2 NS is half the concentration of NS, such that if we did a similar infusion rate calculation the denominator would be half what is above, or simply multiplying by 2.