Hypomagnesaemia

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Normal range: 0·7 to 1·0mml/L

This guidance is for magnesium DEFICIENCY only, for treatment of pre-eclampsia, cardiac arrhythmias or asthma refer to relevant specialists.

These guidelines do not apply to critically ill patients or to patients at risk of refeeding syndrome. For refeeding syndrome please see Policy for prevention and management of refeeding syndrome in adults

Aetiology
  • GI losses eg diarrhoea, malabsorption
  • diabetes
  • alcoholism
  • diuretics
  • other drugs eg PPIs, amphotericin, cisplatin, aminoglycosides
  • pancreatitis
Clinical Features
  • paraesthesia
  • cramps
  • twitching
  • carpopedal spasm
  • ECG changes – widening of QRS, prolonged PR interval
  • ventricular arrhythmias
  • increased risk of digoxin toxicity
  • apathy
  • depression
Hypomagnesaemia Treatment Regime

Via enteral feeding tube: 1 to 2 magnesium glycerophosphate [unlicensed] tablets (4 to 8mmol magnesium) 3 times daily. Crush tablets and dissolve in 50mL water.Adult with eGFR less than 30mL/min should have above recommendations halved. Check magnesium 6 hours after last infusion ends (t=30hours) unless eGFR <30ml/min. Following this check magnesium daily until stable and underlying cause resolved  For eGFR< 30ml/min check magnesium levels 6 hours after the end of second infusion prior to starting the third infusion. If levels are >0.7mmol/L retest 6 hours later and take advice from Consultant Nephrologist before administering third IV dose. 

Magnesium Level   Action  Monitor
More than 0·7mmol/L No supplementation.  
0·5 to 0·7mmol/L Oral: 5mls [7mmol magnesium] magnesium hydroxide mixture three times a day* (off-label). Check magnesium levels daily. Continue treatment for 48 hours after magnesium levels return to normal.
Less than 0·5mmol/L IV: Peripheral Administration –

Not for patients with heart block or existing myocardial damage 

20mmol (10mL magnesium sulfate 50% injection (2mmol/mL)) in 250ml glucose 5%** over 3 hours. Give a total of three doses at 12 hour intervals i.e. dose 1 at t=0 hours, dose 2 at t=12hours and dose 3 at t=24hours.

This may not be enough to replete magnesium stores and treatment may need to be continued****.

Adult with impaired eGFR less than 30mL/min should receive 50% of the recommended dose delivered over a longer period ie 10mmol (5mL) magnesium sulfate 50% (2mmol/mL) in 250mL glucose 5%** over 6hours. Give a total of three doses at 12 hour intervals.

Monitor BP, heart rate, respiratory rate, urine output and for signs of hypermagnesaemia*** during infusion.

All infusions must be administered via an infusion pump.

ECG monitoring is not routinely required in the stable patient.

  • The oral route is not always realistic where patient is symptomatic or where the deficit is severe as large doses will need to be given, which can cause diarrhoea. Start with a low dose and titrate up; doses more than 40mmol/day are likely to cause diarrhoea.
  • *Start with magnesium hydroxide (milk of magnesia), and move to magnesium glycerophosphate (unlicensed) if diarrhoea or intolerance occurs. Dose: magnesium glycerophosphate (1 to 2 tablets [4 to 8mmol magnesium] three times a day) (unlicensed).
  • **If fluid restricted dose may be diluted in 100ml glucose 5%.
  • *** Hypermagnesaemia symptoms include respiratory depression, loss of deep tendon reflexes, nausea, vomiting, flushing of the skin, thirst, hypotension due to peripheral vasodilatation, drowsiness, confusion, slurred speech, double vision, muscle weakness, bradycardia, coma, and cardiac arrest.
  • ****Symptomatic hypomagnesaemia is associated with a deficit of 0·5 to 1mmol/kg; up to 160mmol magnesium over up to 5 days may be required.
  • Toxicity is most likely to occur in patients with chronic kidney disease. Serious toxicity eg respiratory depression or arrhythmias may be reversed temporarily by 5mmol IV calcium.
  • Other parental routes can be employed if the IV route is not available- see advice Medicines Information 01463 704288.