Magnesium Sulphate (IV — ICU/Anaesthesia)

How it works

Magnesium acts as a membrane stabiliser and physiological calcium antagonist, with anticonvulsant, bronchodilator and antiarrhythmic effects and cerebral vasodilatation relevant to eclampsia.

Prescribing in practice

• Watch for magnesium toxicity — loss of deep tendon reflexes is an early warning sign, progressing to respiratory depression, hypotension and ultimately cardiac arrest.
• Reduce the dose and monitor more closely in renal impairment, as magnesium is renally cleared and accumulates.
• Have intravenous calcium available as the antidote for significant magnesium toxicity.

Monitoring

Monitor deep tendon reflexes, respiratory rate, blood pressure and conscious level during and after infusion (with urine output in pre-eclampsia), and check magnesium levels where toxicity is suspected or renal function is impaired.

Counselling the patient

• Brief the team to check reflexes and respiratory rate regularly and to stop the infusion if reflexes are lost or breathing is depressed.
• Warn that a warm, flushed feeling during infusion is common, but breathlessness or marked weakness must be reported at once.
• Ensure intravenous calcium is to hand as the antidote, especially in renal impairment.

Evidence & guidelines

Recommended for eclampsia and severe pre-eclampsia (NICE NG133), severe acute asthma (BTS/SIGN; NICE) and torsades de pointes (Resuscitation Council UK).