In this section : Electrolyte Disturbances
Hyperkalaemia
Hypomagnesaemia
Hypophosphataemia
Hypernatraemia
Hypokalaemia
Hyponatraemia
Hypocalcaemia
Hypercalcaemia
Hypomagnesaemia
Last updated 9th April 2024
Content by Tina Grant
Introduction
- The body contains around 1000mmol Mg, 50-60% of which is in bone and the rest in soft tissues – serum levels do not necessarily reflect body stores
- Mg is a cofactor in more than 300 enzyme systems that regulate, among other things, muscle and nerve function
- Usual magnesium intake is 15-20mmol per day.
- Normal serum level is 0.7-1.0mmol/l.
Common Causes of Deficiency
- The three main mechanisms that lead to hypomagnesaemia are inadequate intake, gastrointestinal and renal losses
- Inadequate intake – one of the reasons why common in alcoholics
- Gastrointestinal losses – including malabsorption, prolonged diarrhoea and pancreatitis. Also seen with PPIs in a syndrome known as hypocalcaemic hypomagnesaemic hypoparathyroidism
- Renal losses – due to aminoglycosides, platinum, cyclosporin and high dose combination diuretic therapy
Presentation
- Most cases are asymptomatic until serum Mg <0.5mmol/L.
- Clinical features are neuromuscular eg paraesthesiae, muscle cramps, irritability and confusion; and cardiovascular eg atrial and ventricular dysrhytmias.
- Mg deficiency often associated with other electrolyte deficiencies – hypocalcaemia, hypokalaemia and hypophosphataemia – hence check serum Mg if serum Ca, K or PO4 are low
Serum Magnesium 0.3-0.7mmol/l and Asymptomatic
- If patient eating then try magnesium aspartate sachets (10mmol/sachet). Rx 10-20mmol as 1 sachet or 1 sachet twice daily in 50-200ml water, tea or orange juice.
- Reduce dose if diarrhoea occurs and halve dose if renal impairment
- May require IV replacement if NBM or develops diarrhoea
- Monitor Mg, Ca and K daily
Serum Magnesium <0.3mmol/l OR Symptomatic
- IV Mg best given slowly as at high infusion rates a high proportion of infused Mg will be excreted via the kidneys
- Give 20mmol Mg as 10ml of 50% Magnesium sulphate. Dilute to 50ml in glucose 5% for central administration or to 250-500ml in glucose 5% for peripheral administration. Infuse over 12-24 hours.
- IV Mg may be required for up to 3 days, depending on deficit
- Halve dose if renal impairment
- Monitor Mg, Ca and K daily
Other Indications for IV Magnesium
- Recurrent VT, torsade de pointes, status asthma
- Give 8mmol IV bolus then check serum Mg and follow with 20mmol over 24hrs if <0.7mmol/l
Links
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- Dynamed Plus from EBSCO– Search for “Evaluation and Treatment of Hypomagnesaemia”
- BMJ Best Practice – Approach to Magnesium Deficiency Click on ‘Access Through your Institution’, enter NHS in the search box and select ‘NHS Scotland’ from the options. Then use Knowledge Network credentials to login.
NB The cutpoints for treatment vary with different guidelines mainly because advice is based on clinical experience rather than trial data.
Content Updated by Chris Isles