Magnesium

[Ref: KB2:p17]

Reference range

[HH26:p416]

• Extracellular [Mg2+] = 1.5 mmol/L
• Intracellular [Mg2+] = 30 mmol/L

[KB2:p17]

• Serum [Mg2+]
  = 0.7 to 1 mmol/L
  = 1.4 to 2 mEq/L

 

Function

1. Act as cofactors or catalyst

• Responsible for catalysing or activating over 300 enzymes
• Required for
  * All enzymes catalysing phosphate transfer (e.g. all reactions involving ATP)
  * All enzymes that require thiamine pyrophosphate as co-factor
  * Sodium pump, oxidative phosphorylation

2. Effects on nerves and muscles

• Mg2+ reduce nerve and muscle membrane excitability
  * Similar to effect of Ca2+, but less powerful
• Inhibition of neurotransmiter release
  * Opposite to Ca2+
• Inhibition of excitation-contraction coupling of muscles
  * Opposite to Ca2+

NB:

Increased [Mg2+]
--> Potentiation of succinylcholine and non-depolarising relaxant blockade

3. Effecs on smooth muscles

• Relaxation
  --> Vasodilation

4. Others

• Magnesium is needed for normal PTH secretion
  * Permissive [WG21:p395]
  * Low magnesium can lead to low PTH and thus low [Ca2+]

 

 

Hypermagnesaemia

Usually asymptomatic until Mg2+ > 4mmol/L

Main abnormality
= Neuromuscular transmission becomes impaired

Early symptoms

• Non-specific
• Nausea, vomiting, drowsiness

Serum levels

• 2.5-5.0
  = ECG changes
  * Increase PQ (????), wide QRS
• 5.0
  = Loss of patellar reflex
• 6.0 - 8.0
  = Respiratory paralysis
• 7.5
  = Complete heartblock
• 12.0
  = Asystole

NB:

• Loss of deep tendon reflex is used as a sign of significant toxicity which preceeds cardiorespiratory arrest