Magnesium

Intro

2nd most common intracellular cation after potassium
4th most common cation after sodium, potassium and calcium
½ in soft tissue, muscle and RBCs
Remainder in bone
ECF Mg <1% - ionized and unionized
Ionized portion active

Physiology

Ca antagonist

Cellular
Cofactor for ATP so needed for Na/K pump for maintaining electrochemical gradients
Needed for generation of cAMP via Adenyl cyclase
CVS
Direct myocardial depression
Blocks catecholamine receptors and release
Antiarrhythmic via Ca channel blockade and intracellular K regulation
Vasodilator – peripheral, pulmonary, cardiac, cerebral
RS
Bronchodilator
NS
Antiepileptic
Reverses cerebral vasospasm
Antagonises Ca at presynaptic membrane and → ↓ release Ach
↓ excitability of nerves and muscle
Muscle weakness if ↑Mg
Endocrine
Controls PTH so ↓Mg → ↓Ca
Glucose utilisation and protein synthesis
Haem
PLT inhibition – prolongs bleeding time

Homeostasis

Regulated by GI absorption and renal excretion
0.7-1

Hypomagnesaemia

↓ intake
Poor diet
Malabsorption
↑ loss
Drugs
Digoxin, gentamycin, loop diuretics, alcohol
ARF
Other electrolyte abnormalities
GI losses – diarrhoea, vomiting, NG drainage
Redistribution
Hypoalbuminaemia
Large fluid infusion

Likely to have low total body stores if plasma concentration low

Effects

CVS
Hypertension
Angina
Arrhythmias
Digoxin toxicity
ECG
↑ PR, ↑ QT, T-wave changes
CNS
Confusion
Fits
Coma
Muscle
Myoclonus
Tetanus
Stridor
Electrolyte
↓K, ↓Ca
Wernicke’s (direct cause or associated vitamin B def from poor nutrition?)

Hypermagnesaemia

Iatrogenic

Effects
N + V
Weakness
Loss of reflexes
Respiratory arrest
Cardiac arrest

Treatment
IV Ca 2.5-5mmol

Uses

Antiarrhythmic

  • Especially torsades
Pre-eclampsia (severe) and eclampsia
  • BP control and cerebral vasodilatation
  • 4g over 10 mins if fitting
  • Infuse aiming for 2-3
  • Monitor reflexes
Asthma
Obtunds stress response to laryngoscopy
Enhances neuromuscular block
Phaeochromocytoma surgery