Magnesium

Physiology

• Essential element for utilisation of energy - needed to release energy from ATP
• Also used to maintain the sodium-potassium exchange pump across cell membranes, and regulating calcium movement into smooth muscle cells
• More than half is found in bone, with <1% in plasma. Therefore serum magnesium is not actually that useful of a measurement.
• 67% of the magnesium in plasma is in the ionised (active) form, and 33% bound to plasma proteins or chelated. The standard assay measures all three.
• Normally, only small amounts of magnesium is secreted in the urine, and this is highly conserved when magnesium intake is deficient.

Hypermagnesaemia

• Aetiology
  • Almost always iatrogenic in setting of impaired renal function
  • Haemolysis
  • DKA
  • Adrenal insufficiency
  • Hyperparathyroidism
  • Lithium intoxication
• Symptoms
  • Muscle paralysis
  • CNS depression
• Management
  • Dialysis
  • IV calcium gluconate stabilises the cardiac membrane until dialysis is started
  • Aggressive volume infusion and furosemide can work in mild derangements and preserved renal function

Hypomagnesaemia

• Common in early recovery period after being critically ill - seen in 65% of ICU patients, and the true incidence is probably higher, because we can't exactly measure total body magnesium depletion.
• Predisposing factors
  • Drugs
    • Furosemide - seen in 50% of patients on furosemide
    • Thiazide diuretics
    • Aminoglycosides (seen in 30%), amphotericin, pentamidine
    • Digitalis
    • Cisplatin, cyclosporine
  • Diarrhoea (secretory)
  • Chronic alcohol abuse - generalised malnutrition, chronic diarrhoea
  • Diabetes mellitus
  • Acute MI
• Clinical findings
  • Neurologic
    • Altered mentation through to seizures
    • All are uncommon
  • Dysrhythmias
    • Torsade de pointes
    • Magnesium deficiency magnify the digitalis effect and promote digitalis cardiotoxicity
  • Other electrolytes
    • Hypokalaemia - need to replete mag prior to fixing K
    • Hypophosphataemia - this is a cause of magnesium depletion, not an effect
    • Hypocalcaemia - impaired PTH release
• Diagnosis
  • Urinary magnesium excretion in response to a magnesium load is a more sensitive test for hypomagnesaemia than serum mag level. This test can be useful for determining the end-point of magnesium replacement therapy.
• Treatment
  • Best done IV in the acute phase (oral magnesium salts can cause diarrhoea, and intestinal absorption of magnesium is erratic)
  • Chronic therapy can be given orally
  • Standard preparation is magnesium sulfate
  • Regime as per UTD:
    • <0.7mmol/L: 4-8mmol replacement
    • 0.4-0.6: 8-16mmol
    • <0.4: 16-32mmol