Thread by @aishaikh

💥 A Case of Hyponatremia

⚡️It is cool to see the ⬆️ in Plasma Sodium be close to the predicted value based on our calculation

⚡️Let’s review the case

💥An elderly man with metastatic cancer presents with Hyponatremia
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💥Hyponatremia work-up is consistent with SIADH due to the malignancy

⚡️⬇️ Plasma Osmolality
⚡️Euvolemic
⚡️⬆️ Urine Osmolality
⚡️⬆️ Urine Na
⚡️Normal Uric Acid, TSH, Cortisol
⚡️On no medications that ⬆️ ADH
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💥Day # 1
⚡️Plasma Na =124 mEq/L
⚡️Urine Output/24 hrs = 1 L
⚡️Urine Osm. = 604 mOsm/kg
⚡️Wt = 59 kg
⚡️TBW = 35 L (0.6 x 59 kg)

💥Urine Osmolality x Urine Volume = Urine Solute Excretion
⚡️604 mOsm/L x 1L= 604
⚡️Pt’s daily solute excretion is 604 mOsm
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💥Pt. is started on urea 15 grams po bid = 30 grams/day
💥 30 grams of urea = 500 mOsm

(In comparison:
6 grams of salt tabs = 205 mOsm)

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💥If you add 500 mOsm of urea to the patient’s daily solute excretion of 604 mOsm, then the daily solute excretion will ⬆️ to 1104 mOsm

Daily solute excretion =
604 + 500 = 1104 mOsm
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💥Urine Osmolality is fixed in SIADH so expected ⬆️ in Urine Output after starting urea 30 gram/day would be:

Daily Urine Solute Excretion/ Urine Osm. = Urine Volume per day

Let’s plug in the numbers:

1104 mOsm/ 604 = 1.8 L
UO on Day # 2 is 1.8L‼️
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💥Now let’s calculate the expected rise in patient’s plasma Na with the ⬆️ in Urine Output due to urea administration

⚡️Edelman Equation:
Plasma Na = Na(e) + K(e)/TBW
-Na(e) is exchangeable Na
-K(e) is exchangeable K
-TBW is Total Body Water

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💥Let’s plug in the values from
Day # 1 into the equation first:

Day # 1
Plasma Na = Na(e) + K(e)/TBW
124 = Na(e) / 35
Na(e) = 124 x 35 = 4340

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💥On Day # 2 the UO ⬆️es to 1.8L
So what would be the change in TBW?

35L plus water intake (1.5L in this patient) minus 1.8L (urine output) minus 0.8 L (insensible water loss)

So the new TBW = 33.9 L

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💥 So what would be expected ⬆️ in Plasma Na on Day # 2

⚡️Urea does change Na intake so exchangeable Na is the same = 4340 But as shown above the TBW has changed due to ⬆️ in Urine Output

💥Day # 2
Plasma Na = 4340/ new TBW
= 4340/33.9 = 128 mEq/L

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💥Our patient’s Plasma Na was
127 mEq/L on Day # 2 - Labs👇🏽

💥This case demonstrates how you can predict the ⬆️ in plasma Na in SIADH with urea administration

💥The patient tolerated oral urea well but sometimes it can be a limiting factor

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💥Here is a reference for the use of urea in patients with SIADH due to an underlying malignancy

👉🏽pubmed.ncbi.nlm.nih.gov/30868608/?i=1&…

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