Handbook of Drugs in Intensive Care

MAGNESIUM SULPHATE

Like potassium, magnesium is one of the major cations of the body responsible for neurotransmission and neuromuscular excitability. Regulation of magnesium balance is mainly by the kidneys.

Hypomagnesaemia may result from failure to supply adequate intake, from excess NG drainage or suctioning or in acute pancreatitis. It is usually accompanied by a loss of potassium. The patient may become confused and irritable, with muscle twitching.

Hypomagnesaemia should also be suspected in association with other fluid and electrolyte disturbances when the patient develops unexpected neurological features or cardiac arrhythmias.

Magnesium sulphate has long been the mainstay of treatment for preeclampsia/eclampsia in America, but the practice in the UK until recently has been to use more specific anti-convulsant and antihypertensive agents. A large international collaborative trial shows a lower risk of recurrent convulsions in eclamptic mothers given magnesium sulphate compared with those given diazepam or phenytoin.

Normal serum magnesium concentration: 0.7–1.0 mmol/l

Therapeutic range for pre-eclampsia/eclampsia: 2.0–3.5 mmol/l

Uses

Hypomagnesaemia

Hypomagnesaemia associated with cardiac arrhythmias

Pre-eclampsia

Anticonvulsant in eclampsia

Acute asthma attack

Cardiac arrest (p. 257)

Contraindications

Hypocalcaemia (further ↓ Ca2+)

Heart block (risk of arrhythmias)

Oliguria