Basic Concepts of Fluid and Electrolyte Therapy

Treatment depends on severity and cause, but may involve vitamin D
replacement in the form of 1- cholecalciferol and/or calcium supplements
by the oral or intravenous routes.
Magnesium (Mg 2+ )
This is distributed mainly in bone (500-600mmol) and the ICF (500-
850 mmol). Only 12-20 mmol are in the ECF at any given time, at a
concentration of 0.7-1.2 mmol/l. It is an important component of
many enzyme systems and helps maintain cell membrane stability.
The following facts are important to remember.
Mg 2+ like Ca 2+ is bound to albumin and a low serum level should
be interpreted in the light of the prevailing albumin concentration
Mg 2+ concentration in gastrointestinal fluid varies according to
the distance along the intestine. In upper small bowel fluid it is
only present at 1 mmol/l, whereas in the distal small bowel it rises
to higher concentrations. Significant hypomagnesaemia is therefore
more likely to occur from chronic diarrhoea or from distal
stomas or fistulae rather than from more proximal GI losses. GI
losses are the most common cause of hypomagnesaemia in clinical
practice.
Hypomagnesaemia causes blood PTH levels to fall, with secondary
hypocalcaemia. In all cases of hypocalcaemia therefore, the serum
Mg 2+ should be measured. Replacement of Mg 2+ deficits restores
PTH and hence Ca 2+ levels to normal.
Overt symptoms of hypomagnesaemia, with neuromuscular irritability,
convulsions, and arrhythmias are not usually apparent
until the serum Mg 2+ falls below 0.4 mmol/l, although with milder
degrees of hypomagnesaemia patients may experience improved
well-being with adequate replacement, suggesting that even mild
hypomagnesaemia may cause sub-clinical symptoms.
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