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Magnesium

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Magnesium metabolism

Hypomagnesaemia

Hypermagnesaemia

Pathophysiology

- 65% of body magnesium in mineral phase of skeleton, 34% in intracellular space and 1% in ECF
- serum Mg 0.8-1.2 mmol/l. 55% in ionized form.
- homeostasis maintained principally by renal and gastroenterological mechanisms.
- dietary Mg obtained from green leafy vegetables, grain products and dairy products. Absorption is related to dietary Mg availability (the smaller the amount presented to the small bowel the higher the proportion absorbed), plasma Mg, oral Ca intake and vitamin D
- kidney is primary affector of serum Mg. Diffusable Mg is filtered at the glomerulus. 10-20% is reabsorbed in proximal tubules, 60-70% in the thick ascending limb of loop of Henle and 5-10% in the distal loop with a resulting 5-10% fractional excretion.
- serum Mg levels directly affect tubular transport of Mg. High levels decrease effective net absorption with a resulting increase in fractional excretion. Low levels have the opposite effect.
- inhibition of sodium reabsorption in proximal tubule (eg by diuretics or fluid loading) inhibits Mg reabsorption
- hypercalcaemia inhibits tubular reabsorption, possibly by an as-yet unelucidated shared transport mechanism.
- PTH probably directly enhances tubular Mg reabsorption although this effect is usually overshadowed by the hypercalcaemia present.
- end result of all these mechanisms is a reasonable resistance of the body to hypermagnesaemia in the presence of normal renal function.

Aetiology

- usually iatrogenic and is frequently seen in conjunction with renal failure
Renal: creatinine clearance < 30ml/min
Non renal: iatrogenic - much more likely to occur in the presence of renal insufficiency
Both: excessive Mg in diasylate

Clinical features

Effects are essentially those of a calcium channel blocker combined with a membrane stabilizer. (Mg binds to many Ca binding sites and therefore blocks the effect of Ca in a number of enzyme systems). Serum concentrations of at least 2 mmol/l are necessary to produce clinical effects.

Cardiovascular

- ECG:

< 5mmol/l

delayed interventricular conduction
prolonged QT

> 5mmol/l

first and second degree AV block

> 12.5 mmol/l

complete AV block
asystole

- hypotension: usually only transient. Except in severe toxicity or following rapid parenteral administration of MgSO4 hypermagnesaemia does not usually produce a profound reduction in SVR.
- myocardial contractility is probably not affected by mild to moderate hypermagnesaemia.

Neurological

- varying degrees neuromuscular block by decreasing impulse transmission across the neuromuscular junction
- decrease in post-synaptic membrane responsiveness and an increase in the threshold for axonal excitation also occur
- one of the earliest clinically noticeable signs of Mg toxicity is diminution of deep tendon reflexes. Becomes apparent at levels > 2 mmol/l. almost always antecedes more significant symptoms
- hypoventilation due to respiratory muscle paralysis
- somnolence and coma at very high levels

Other effects

- prolonged administration of parenteral Mg lowers serum Ca. ? by impairment of peripheral effects of parathormone
- may impair clotting

Management

- determine cause and stop administration if iatrogenic
- if haemodynamically stable, no evidence of respiratory depression and reflexes present simply observe for further symptoms and maintain urine output
- if reflexes diminished treat with saline diuresis or loop diuretics. If symptoms persist or in massive overdose this should be followed by dialysis with Mg free diasylate +/- prolonged IV calcium (15 mg/kg as gluconate over 4 hours).
- cardiac arrest or resp. depression: calcium gluconate 1g over 3 mins

Hypomagnesaemia

Aetiology

GI disorders

- malabsorption syndromes
- GI fistulae
- short-bowel syndrome
- prolonged NG suction
- diarrhoea
- pancreatitis
- parenteral nutrition

Alcoholism

Endocrine disorders

- hyperparathyroidism
- hyperthyroidism
- Conn's
- DM
- hyperaldosteronism

Renal diseases

- RTA
- diuretic phase ATN

Drugs

- aminoglycosides
- carbenicillin, ticarcillin
- amphotericin B
- diuretics
- cisplatinum
- cyclosporin

Clinical features

- neuro: confusion, irritability, delirium tremens, fits
- tachyarrhythmias
- often associated with resistant hypokalaemia and hypocalcaemia

Treatment

- treat underlying cause
- IV MgSO4 10 mmol. over 5 min followed by 20-60 mmol/day


© Charles Gomersall December 1999

 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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