The differential diagnosis of hypernatraemia in children, with ...

The Diagnosis of Salt Poisoning Leading to Hypematraemia in Children – September 2009
figures from the paper show clearly that there is no significant relationship between the
plasma sodium concentration and the ratio of total body water to exchangeable sodium
(found mainly in the extracellular fluid) (Fig 5), but a very good relationship between
the plasma sodium concentration and the ratio of total body water to the sum of total
exchangeable sodium (extracellular fluid) and potassium (intracellular fluid) (Fig 7):
Therefore, although administered sodium does not enter cells, any prediction of rise in
serum sodium following excessive sodium administration should be calculated on the
assumption that it is distributed throughout the total body water. 27
One gram of salt contains 17 mmol each of sodium and chloride. Total body water amounts
to approximately 65% of total body weight in children. 281 The extracellular space is
approximately 25% of total body weight.
So taking an example, how much salt would raise the sodium from 140 to 180 mmol/l?
Firstly, assume rapid assimilation (e.g. IV infusion) and with no correction for any of the
administered sodium that might have been excreted by the time at which the elevated
concentration was measured. This would require only 10 mmol sodium (equivalent to
0.59 grams of salt)/kilogram body weight [or 25% of 40 mmol/L] if the extracellular fluid
space is taken as the volume of distribution. However, it would require 26 mmol sodium
(equivalent to 1.53 grams of salt)/kilogram body weight [or 65% of 40 mmol/L], using
total body water, the correct procedure. Roughly speaking this is the difference between 1
teaspoon and 3 teaspoons of salt for a 10 kilogram child.
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