Thread by @NephroMD

How to estimate the increase in plasma sodium with oral urea? #ureaforhyponatremia

A 50 y/o woman with SIADH has a weight=70kg, UOP=1L/day, plasma sodium (PNa)=125 mEq/L, and UOsm=500 mOsm/kg. Which of the following is the expected PNa one day after fluid restriction 1 L/day and urea 15 g BID? (assume baseline solute balance and insensible losses of 0.8 L/day)

Step 1: Calculate baseline solute intake
Under solute balance conditions, solute intake = urine solute excretion (USE). We know UOsm=USE/V...(1), where V=24h urine volume. Reordering the terms of (1): USE= UOsm x V. USE = 500 mOsm/L x 1 L/day = 500 mOsm/day.

Step 2: Calculate V after urea
15 g bid of urea = 30 g or 500 mmol of urea
Reordering the terms of (1) above: V=USE/UOsm...(2). New USE is now 500 (diet) + 500 (urea)=1000 mOsm/day. Replacing in (2): V = (1000/500) = 2 L/day

Step 3: Calculate total body cations
PNa = (NaE+KE)/TBW (simplified Edelman equation). Where NaE is total exchangeable body sodium, KE is total exchangeable body potassium and TBW is total body water. Replacing values in Edelman:
125 = (NaE+KE)/(70x0.5)
(NaE+KE)=125x35=4375 mEq

Step 4: Calculate plasma Na after 1 day of urea.
NaE+KE remains constant as only water loss with urea. TBW has changed because of fluid intake (+1L), insensible losses (-0.8L) and UOP (-2L).
New PNa = (NaE+KE)/(TBW + 1 – 0.8 - 2)=4375/(35 - 1.8) =131.7 ≈ 132 mEq/L.