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
Contraindications
Hypocalcaemia (further ↓ Ca2+)
Heart block (risk of arrhythmias)
Oliguria
Administration
Magnesium sulphate solution for injection
Concentration
(%)
|
g/ml
|
mEq/ml
|
mmol/ml
|
10
|
0.1
|
0.8
|
0.4
|
25
|
0.25
|
2
|
1
|
50
|
0.5
|
4
|
2
|
1g = 8 mEq = 4 mmol
• Hypomagnesaemia
IV infusion: 10 mmol magnesium sulphate made up to 50 ml with
glucose 5%
Do not give at >30 mmol/h
Repeat until plasma level is normal
Concentrations <20% are suitable for peripheral IV administration
• Hypomagnesaemia associated with cardiac arrhythmias
IV infusion: 20 mmol diluted in 100 ml glucose 5%, given over 1 h
Do not give at >30 mmol/h
Repeat until plasma level is normal
Concentrations <20% are suitable for peripheral IV administration
• Pre-eclampsia/eclampsia
Loading dose: 4 g (8 ml 50% solution) diluted in 250 ml sodium
chloride 0.9% IV, given over 10 min
Maintenance: 1 g/h IV, as necessary. Add 10 ml 50% magnesium
sulphate to 40 ml 0.9% saline and infuse at 10 ml/h
Newborn – monitor for hyporeflexia and respiratory depression
• Acute asthma: 2 g in 50 ml sodium chloride 0.9% IV, given over 20 min
Oral therapy
• Magnesium glycero phosphate (unlicensed product) 1-g tablets contain
4 mmol of Mg2+. Usual starting adult dose 1–2 tablets 8 hourly
Monitor: BP, respiratory rate
ECG
tendon reflexes
renal function
serum magnesium level
Maintain urine output >30 ml/h
How not to use magnesium sulphate
Rapid IV infusion can cause respiratory or cardiac arrest
IM injections (risk of abscess formation)
Adverse effects
Related to serum level:
• 4.0–6.5 mmol/l
Nausea and vomiting
Somnolence
Double vision
Slurred speech
Loss of patellar reflex
• 6.5–7.5 mmol/l
Muscle weakness and paralysis
Respiratory arrest
Bradycardia, arrhythmias and hypotension
• >10 mmol/l
Cardiac arrest
Plasma concentrations >4.0 mmol/l cause toxicity which may be treated with calcium gluconate 1 g IV (10 ml 10%)
Cautions
Oliguria and renal impairment ( risk of toxic levels)
Potentiates both depolarising and non-depolarising muscle relaxants
Organ failure
Renal: reduce dose and slower infusion rate, closer monitoring for signs of toxicity
Renal replacement therapy
Removed by CVVH/HF/PD. Accumulates in renal failure, monitor levels
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