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