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

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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