Determinants of plasma water sodium concentration

Sir,

I read with interest the hypothesis regarding the determinants of the plasma water sodium concentration by Nguyen and Kurtz [1]. The authors have highlighted the importance of parameters other than total exchangeable sodium, total exchangeable potassium and total body water (TBW) as determinants of plasma water sodium concentration. However, there seems to be an error in the calculation of the y-intercept.

The authors have derived the equation:

According to Edelman's equation [2]:

Solving the two equations, we get:

From this it appears that even the sum of (Nae + Ke) contributes to the y-intercept of Edelman's equation.

The authors have used two examples to highlight the clinical application of their hypothesis. In both of the examples, the authors have not considered insensible water losses while calculating TBW2. If they had done so, then the predicted [Na+]p would have been higher than the actual value.

While the authors have highlighted some important concepts, these might require modifications to accurately predict the changes in plasma water sodium concentrations with therapeutic interventions.

Conflict of interest statement. None declared.

Rakesh Lodha

Department of Pediatrics All India Institute of Medical Sciences New Delhi India Email: rakesh_lodha{at}hotmail.com

References

  1. Nguyen MK, Kurtz I. Are the total exchangeable sodium, total exchangeable potassium and total body water the only determinants of the plasma water sodium concentration? Nephrol Dial Transplant 2003; 18: 1266–1271[Free Full Text]
  2. Edelman IS, Leibman J, O’Meara MP, Birkenfeld LW. Interrelations between serum sodium concentration, serum osmolality and total exchangeable sodium, total exchangeable potassium and total body water. J Clin Invest 1958; 37: 1236–1256[ISI]




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