Physiology Pearls – The Edelman Equation

The “oysters and pearls” appetizer at Criollo.

If you’ve sent any considerable amount of time trying to figure out hyponatremia (or any sodium physiology) there is a good chance that you’ve ran into the Androgue-Madrias Formula. They were able to predict the changes in serum sodium based on the concentration of the fluid being infused and they were also able to the the same with the fluid you losing (urinating). See the two equations below:


One of the important factors in the equations that are often forgot by house-staff is potassium. The reason that potassium is important to sodium has to do with the Edelman equation. Originally describe by Edelman et al in 1958 as Formula . [Na+]pw is the plasma water concentration, Ke is the total exchangeable potassium, and Nae is the total exchangeable sodium. The Ke has a net incremental increase on [Na+]pw because majority of K+ is intracellular (ICF) and intrasitial fluid (ISF) and will cause movement of water from plasma space into the ISF and ICF (Fig 1. A)


Figure 1.A

A simplified representation of the Edelman equation is : [Na+] = (Nae + Ke)/TBW. It may be helpful to think of conditions that could change the numerator or the denominator when think about dysnatremic conditions.

For example Just to name a few with hyponatremia:

Edelman chart

For the full Adrogue-Madrias 2012 review of hyponatremia, pubmed (or if you have a account download here) .




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