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Related Subjects: |Calcium Physiology |Magnesium Physiology |Calcitonin |Hypocalcaemia |Hypercalcaemia |Hypomagnesaemia |Hypermagnesaemia |Primary Hyperparathyroidism |Familial hypocalciuric hypercalcaemia (FHH) |Sarcoidosis

Magnesium sulphate 50% injections must be diluted to a concentration of less than 5% (20mmol/100mL) for peripheral administration.

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• Magnesium sulfate is highly soluble in water
• Most of it is found in bone and the rest in cells

Mode of action

• Intracellular cation, Coenzymic activity, Alters ion channels
• NMDA receptors, Calcium metabolism

Indications

• Acute Severe Asthma (FEV1<25% predicted)
• Torsades de pointes, Hypomagnesemia
• Eclampsia, Atrial arrhythmias

Parenteral Dose varies with indication: Magnesium sulfate/sulphate 1g equivalent to 4mmol

• Ventricular tachycardia (Torsades): Magnesium 8 mmol (2 g bolus) over 10-15 mins in 100 ml 5% Dextrose or N-saline. Then give 72 mmol (18 g) over 24 hrs
• Asthma: Magnesium 8mmol (2 g) in 100 ml 5% Dextrose or N-saline over 20 mins
• Eclampsia: Magnesium 16 mmol (4 g) over 20 mins in 250 ml 5% Dextrose or N-saline and then 96-192 mmol (12-24 g) over 24 hours
• Hypomagnesaemia (< 0.5 mmol/L): Magnesium 20 mmol (5 g) over 1-3 hours in N-saline or 5% dextrose. Then commence oral if levels remain low. A maximum daily dose of 50mmol is recommended in 24 hours; a total of up to 160mmol may be required over 5 days to correct the deficiency.

IM injection

• May be considered but small risk of abscess formation

Interactions

• Prolong action of non-depolarising muscle relaxants

Oral preparations

• Magnesium (Mg) replacement should be prescribed for patients with a serum Mg level of 0.4mmol/l or less
• For patients with a serum Mg level of 0.4 to 0.7mmol/l, magnesium replacement should be considered if the patient presents with symptoms of hypomagnesaemia
• Standard dose of oral magnesium for hypomagnesaemia is 24mmol daily e.g. magnesium glycerophosphate (containing 4mmol Mg) 4mmol tablets ii TDS

Contraindications

• Renal failure - Do not exceed 20 g/48 hr. Magnesium is renally cleared and can therefore accumulate in renal impairment, causing hypermagnesaemia. It has been suggested that approximately half the normal dose or less should be administered, depending on the extent of renal impairment and whether the patient is symptomatic.
• Cautions with hyperkalaemia, Myasthenia Gravis ( increased weakness)

Side effects

• Hypotension, CNS toxicity, respiratory depression.
• Toxicity with renal dysfunction

Magnesium Toxicity

• Ensure that appropriate dosing given and that initial rates are not continued.
• Bolus doses should not exceed 6 g over 15-20 minutes, and continuous infusion rates should not exceed 3 g/hour
• Hypermagnesaemia can cause cardiac arrest
• Antidote is IV Calcium Gluconate for severe magnesium toxicity

References

• Administration of IV Magnesium Sulfate
• CLINICAL GUIDELINE FOR TREATMENT OF HYPOMAGNESAEMIA IN ADULTS NHS Cornwall