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hypomagnesaemia

hypomagnesaemia

introduction

  • magnesium is an important cation and often forgotten but low plasma levels may cause clinical effects, particularly severely low levels
  • unlike calcium, it is not under significant hormonal control and mobilisation of reserves from bone takes weeks

aetiology

  • decreased intake:
    • starvation
    • alcohol abuse - 30-80% of alcoholics are deficient
    • total parenteral nutrition
    • ICU patients - 50-60% are deficient
    • diabetics - 25% are deficient
    • hospital inpatients - 10-20% are deficient
    • general poor diet - 2% of population are deficient
  • GIT losses:
    • diarrhoea
    • vomiting
    • NG suction
    • GIT fistulae or stomas
    • Hypomagnesemia with secondary hypocalcemia (HSH)
  • redistribution from extracellular to intracellular:
  • increased renal excretion:

clinical effects

<0.5mM

  • tetany, seizures
  • ventricular arrhythmia, coronary artery vasospasm, sudden death

0.5-0.7mM

Rx

  • in the presence of hypocalcaemia, tetany can occur during the administration of magnesium sulphate if calcium is not also supplemented, as ionized calcium levels can drop acutely from complexing of calcium with sulfate ions and increased urinary excretion
  • magnesium sulphate infusion also causes potassium excretion and may exacerbate hypokalaemia
  • an abrupt elevation in the plasma magnesium concentration by infusing magnesium sulphate will partially remove the stimulus for magnesium retention, and thus up to 50% of the infused magnesium will be excreted in the urine
  • magnesium is subject to slow equilibration between serum and the intracellular spaces and tissues, the serum magnesium level may appear artificially high if measured too soon after a magnesium dose is administered.
  • severe magnesium depletion requires sustained correction of the hypomagnesaemia, preferably by sustained-release oral preparations
  • Rx underlying cause
    • those on diuretics which cannot be ceased, may benefit from use of a potassium-sparing diuretic as these increase magnesium reabsorption in the collecting tubule
  • if Mg < 0.4mM:
    • Rx with iv magnesium sulphate as correction will take 7 days instead of 6 weeks as with oral supplements.
    • One 5 mL ampoule of magnesium sulfate contains 10 mmol of magnesium ions.
    • 10—20 mmol of magnesium ions can be given in 100 mL of 0.9% sodium chloride over 1—2 hours.
    • hypomagnesemic-hypokalemic patient with ventricular arrhythmias:
      • 50mEq Mg over 8-24hrs, rpt x 1 prn to maintain plasma Mg > 0.4 mmol/L or 0.8 mEq/L
    • Patients with renal insufficiency should have their doses decreased appropriately, be monitored closely for decreased deep tendon reflexes, and have their magnesium concentrations checked regularly.
    • monitor calcium levels as sulphate anions however may bind calcium and aggravate existing [hypocalcaemia]].
  • oral magnesium supplements such as magnesium aspartate (1.66mmol Mg per tablet) ii-iv tablets per day in divided doses but risk of diarrhoea which will limit higher doses.
hypomagnesaemia.txt · Last modified: 2013/12/20 15:43 (external edit)