The causes of magnesium deficiency are presented in Table ! Magnesium deficiency is often multifactorial, and many therapeutic measures taken during critical illness may exacerbate it further. For example, catecholamine infusions, diuretics, and aminoglycoside antibiotics all worsen hypomagnesemia.
As serum magnesium accounts for only 1 per cent of the total body magnesium, the serum concentration may be normal in patients with magnesium deficiency. However, most hypomagnesemic patients will have some degree of magnesium deficiency. The normal serum magnesium is 0.8 to 1.2 mmol/l depending on the laboratory reference range. This is affected by many factors including the release of magnesium from cells by acidosis and alterations in protein binding. If spectrophoretic methods are used, hemolysis and raised bilirubin will cause overestimation by absorbing light of similar wavelengths. If both the serum magnesium concentration and 24-h urinary magnesium are both normal, it is unlikely that significant deficiency exists. A magnesium loading test may be performed aimed at detecting increased magnesium retention in deficiency, but, as in the 24-h urinary measurement, it requires good renal function for reliable results. Therefore these tests are of very limited value in critical care.
Intracellular measurement of magnesium has been used experimentally for muscle, bone, and heart, but these tests are not used clinically. Some workers have tried using the magnesium content of erythrocytes and circulating lymphocytes as indices of tissue magnesium content, but these do not correlate well with the magnesium loading test.
Symptoms of pure magnesium deficiency are non-specific and very variable. Many may be due to coexisting hypocalcemia. They are difficult to separate from those commonly present in critical illness. They include lethargy, apathy, generalized weakness, anorexia, nausea, and vomiting. Signs and metabolic features of magnesium deficiency are summarized in TabieJ,.
Table 2 Signs and metabolic features of hypomagnesemia
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