Clinical features

• Arrhythmias (SVT, VT and Torsades de Pointes)

• ECG changes (ST depression, 'T' wave flattening, 'U' waves)

• Metabolic alkalosis

• Constipation

Management

• Wherever possible, the cause of potassium loss should be treated.

• Potassium replacement should be intravenous with ECG monitoring when there is a clinically significant arrhythmia (20 mmol over 30 min, repeated according to levels).

• Slower intravenous replacement (20 mmol over 1 h) should be used where there are clinical features without arrhythmias.

• Oral supplementation (to a total intake of 80-120 mmol/day, including nutritional input) can be given where there are no clinical features.

Enteral nutrition, p80; Parenteral nutrition, p82; Electrolytes

, p146; Urinalysis, p166; Crystalloids, p176; Diuretics, p212; Steroids, p262; Cardiac arrest, p272; Tachyarrhythmias, p316; Electrolyte management, p414; Metabolic alkalosis, p436; Diabetic ketoacidosis, p442; Post-operative intensive care, p534

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Hypomagnesaemia Causes

• Excess loss, e.g. diuretics, other causes of polyuria (including poorly controlled diabetes mellitus), severe diarrhoea, prolonged vomiting, large nasogastric aspirates

• Inadequate intake, e.g. starvation, parenteral nutrition, alcoholism, malabsorption syndromes

Clinical features

Magnesium is primarily an intracellular ion involved in the production and utilisation of energy stores and in the mediation of nerve transmission. Low plasma levels, which do not necessarily reflect either intracellular or whole body stores, may thus be associated with features related to these functions:

• Confusion, irritability

• Muscle weakness, lethargy

• Arrhythmias

• Symptoms related to hypocalcaemia and hypokalaemia which are resistant to calcium and potassium supplementation respectively

Normal plasma levels range from 0.7-1.0 mmol/l; severe symptoms do not usually occur until levels drop below 0.5 mmol/l.

Management

• Where possible, identify and treat the cause.

• For severe, symptomatic hypomagnesaemia, 10 mmol of magnesium sulphate can be given IV over 3-5 min. This can be repeated once or twice as necessary.

• In less acute situations or for asymptomatic hypomagnesaemia, 10-20 mmol MgSO4 solution can be given over 1-2 h and repeated as necessary, or according to repeat plasma levels.

• A continuous IV infusion (e.g. 3-5 mmol MgSO4 solution/h) can be given; however, this is usually reserved for therapeutic indications where supranormal plasma levels (1.5-2 mmol/l) of magnesium are sought, e.g. treatment of supraventricular and ventricular arrhythmias, pre-eclampsia and eclampsia, bronchospasm.

• Oral magnesium sulphate has a laxative effect and may cause severe diarrhoea.

• High plasma levels of magnesium may develop in renal failure; caution should be applied when administering IV magnesium.

Enteral nutrition, p80; Parenteral nutrition, p82; Calcium, magnesium and phosphate, p148; Diuretics, p212; Asthma — general management, p296; Tachyarrhythmias, p316; Electrolyte management, p414; Generalised seizures, p372; Pre-eclampsia and eclampsia, p538

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