Magnesium
28/10/10
Class – electrolyte (most abundant intracellular cation)
Mechanism of Action
- depresses neuronal activation
- essential cofactor in >300 enzyme systems & essential for the production of ATP, DNA, RNA
& protein function.
Pharmaceutics
- sulphate or chloride
- clear, colourless
- 10 mmol in 10mL
Dose – 5mmol bolus -> 20mmol over 60 min
Indications
- Mg deficiency (if develops in ICU treat as associated with increased mortality and prolonged
LOS)
- arrhythmias (post ischaemia/cardiac surgery)
- post MI
- asthma/severe bronchospasm
- pre-eclampsia/eclampsia
- SAH management
- tocolytic
- phaeochromocytoma surgery
- hypokalaemia (will need to treat hypomagnesaemia in this context)
Adverse Effects
-
nausea
flushing
CNS depression
coma
heart block
respiratory weakness
toxicity -> IV calcium
PK
Absorption – IV (via CVL)
Distribution – widely distributed, 30% protein bound
Metabolism - nil
Elimination – filled by kidneys
Jeremy Fernando (2011)
Evidence – see Mg2+ document in Electrolytes
Eclampsia
- standard of care
- halves rate of progression from pre -> eclampsia
- drug of choice in treating eclamptic seizures – more effective than phenytoin or
benziodiazepines (MAGPIE trial 2002, Cochrane review, 2003)
- dose: 4g over 5min -> 1g/hr (aim for a level of 2-4mmol/L)
Arrhythmias
- likely to be effective in a subgroup of patients with total body Mg deficiency, however this
group is hard to diagnose.
- post cardiac surgery -> meta-analyses have shown that IV Mg decreases occurrence of post
of AF and ventricular arrhythmias
- not yet currently endorsed by the AHA/European Heart Association
- may be as effective as amiodarone in treating rapid AF (Critical Care Med, 1995)
- recommended for treatment of Torsades De Pointes, but no RCT on this.
- effective in digitalis induced arrhythmias
Post Myocardial Infarction
-
controversial (not widely accepted)
conflicting evidence
early trials (LIMIT2) showed a mortality benefit
later trials (ISIS4, MAGIC) were unable to reproduce findings
Asthma/Bronchospasm
-
improves FEV1 and PEFR in some patients (those at severe end of spectrum)
no evidence to support improvement in mortality
of benefit in selected patients ?maybe more effective in paediatric patients
dose = 5-10mmoL over 20 min
Cochrane review, 2000
more trials needed
SAH Management
- rat models demonstrate effectiveness of IV Mg in reversing induced vaspasm (Stroke, 1991)
Jeremy Fernando (2011)