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Time for another #tweetorial on fluid restriction in #hyponatremia from SIADH. Is there a way to predict which patients with SIADH will respond to fluid restriction?
1/It has been said that the first line therapy for SIADH is fluid restriction but 60-70% of patients with SIADH will not respond to fluid restriction alone and they require the addition of a drug (e.g., urea). How can we tell who will respond and who won’t?
2/We can predict responsiveness to fluid restriction by using the concept of electrolyte-free water clearance. 
3/This formula will calculate how much of the dairy urine volume is electrolyte-free water. To calculate it you need “V” which is the 24h urine volume, UNa is urine sodium, UK is urine potassium, and PNa is plasma sodium.
4/However, a lot of times we don’t have “V” as it is not usually recorded. In those situations we just need to calculate the ratio (UNa+UK)/PNa.
5/This ratio (UNa+UK)/PNa is called the Furst ratio (based on the first author of the paper that described this (ncbi.nlm.nih.gov/m/pubmed/10768… ), or urine to plasma electrolyte ratio, or simply U/P ratio.
6/There are 3 distinct situations when calculating the U/P ratio: ratio=1, ratio<1, or ratio>1.
7/1st case: U/P ratio=1. Let’s say V=2L. Then CeH20 = 0. This means that the 2 L of urine are made of 2L of isotonic fluid. There is no net gain or loss of free water (assuming water intake=insensible losses) and PNa should not change
8/ 2nd case: U/P ratio<1, let’s say is 0.5, and V=2L. Then CeH20= +1 L. This means that of the 2L of urine, 1L is isotonic fluid and the other 1L is electrolyte free water. There is a net loss of 1L (again assuming water intake=insensible losses) and PNa should increase.
9/3rd case:U/P ratio>1, let’say 1.5, and V=2L. Then CeH20= - 1L. Negative 1L means pat is RETAINING 1L of free water and leaving the associated solutes to be excreted in urine. The urine contain 2L of isotonic fluid + 140 mEq of Na and K (hypertonic urine) and PNa should decrease
10/ Patients with U/P ratio > 1 are rhe ones that won’t likely respond to fluid restriction alone and should need the addition of a drug which could be tolvaptan, furosemide and salt tabs, or urea.
11/The validity of this ratio has been confirmed in the following study ncbi.nlm.nih.gov/m/pubmed/27481… where a UNa>=130 (effectively making the ratio >1) and Uosm >=500 (a surrogate for free water clearance) are good predictors of non response to fluid restriction.
12/These parameter have also been supported by the 2013 US Expert panel recommendations on the diagnosis and management of hyponatremia: ncbi.nlm.nih.gov/m/pubmed/24074…
13/It is important to recognize that in some situations where the U/P ratio is very high (e.g., 1.9) then even ZERO fluid restriction can make the hyponatremia of SIADH worse:
14/We developed a therapeutic approach to SIADH with fluid restriction and urea using these parameters:
