Basic Concepts of Fluid and Electrolyte Therapy

the plasma sodium has fallen, the balance of water is positive and
hyponatraemia is dilutional. The next prescription should include less
water and the same sodium intake as before.
An alternative approach to sodium balance is to measure intake and
the sodium content of all fluids lost. This however, is difficult to do
accurately as well as being more demanding in staff time and
resources.
A falsely low serum sodium may be caused by hypertriglyceridaemia,
since triglycerides expand the plasma volume but contain no sodium.
Similarly hyponatraemia occurs in the presence of hyperglycaemia as
in decompensated diabetes, since glucose also acts as an osmotic
agent holding water in the ECF. This effect disappears as soon insulin
treatment causes cellular uptake of glucose and lowering of its concentration
in the blood.
Potassium
The normal serum potassium concentration lies between 3.5 and
5.3 mmol/l. Concentrations rising above 5.5 mmol/l progressively
increase the risk of death from cardiac arrest and require urgent
treatment which may include extra fluids, intravenous glucose and
insulin, bicarbonate, calcium gluconate (to stabilise the myocardium),
intrarectal calcium resonium and even renal replacement therapy.
Conversely, concentrations below 3.0 mmol/l increase the risk of
arrhythmias and indicate the need for potassium supplementation by
the oral or intravenous route.
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