Magnesium

Intro

2nd most common intracellular cation after potassium
4
th 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