Magnesium

INTRODUCTION

• hypomagnesaemia = common electrolyte abnormality in ICU
• intracellular cation
• provides a major role in the transfer, storage and the utilisation of energy
• co-factor for membrane Na+/K+ ATPase -> membrane stabilising effects
• significant role in = nerve transmission, cardiac excitability, neuromuscular conduction, muscular contraction and vasomotor tone
• administered as IV MgSO4

CAUSES OF HYPOMAGNESAEMIA (RRID)

Reduced intake

• TPN
• malabsorption
• alcoholism

Redistribution

• insulin
• hungry bone syndrome

Increases loss

• diarrhoea
• N/G secretions
• RTA

Drugs

• amphotericin B
• aminoglycosides
• carbenicillin
• diuretics

EFFECTS OF HYPOMAGNESAEMIA

• confusion
• irritability
• delirium
• tremors
• tachyarrhythmias: wide QRS, prolonged PR, ventricular arrhythmias, Torsades
• refractory hypokalaemia and hypocalcaemia

THERAPEUTIC 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)
• Irukandji syndrome (unproven)

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 vasospasm (Stroke, 1991)
• several small studies showed additional benefit from Mg if added to nimodipine treatment (Neurosurgery 2000, Stoke 2005, Journal of Neurosurgery 2002)
• one study showed Mg to have comparable effect to nimodipine (Neurosurgery, 2006)
• remains controversial, latest trial (as yet unpublished) failed to demonstrate benefit.

REPLACEMENT

• IV in symptomatic patients
• oral sustained release preparations for asymptomatic and enterally fed patients
• correct hypokalaemia and hypocalcaemia

MAGNESIUM TOXICITY

• generally safe and well tolerated
• can cause hypotension if rapidly administered
• adverse effects of high levels = muscle weakness, respiratory muscle fatigue, slowed cardiac conduction (>7.5mmol/L) -> arrest (>12.5mmol/L)
• toxicity unlikely if tendon reflexes are present