Magnesium

CLASS

  • electrolyte (second most abundant cation in intracellular fluid, after potassium)

MECHANISM OF ACTION

  • depresses neuronal activation
  • essential cofactor in >300 enzyme systems and 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
  • pheochromocytoma surgery
  • hypokalaemia (will need to treat hypomagnesaemia in this context)

ADVERSE EFFECTS

  • nausea
  • flushing
  • CNS depression
  • coma
  • heart block
  • respiratory weakness
  • toxicity -> IV calcium

PHARMACOKINETICS

  • Absorption – IV (via CVL)
  • Distribution – widely distributed, 30% protein bound
  • Metabolism – nil
  • Elimination – filled by kidneys

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 des 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)

References and links

LITFL

Journal articles

  • Wu J, Carter A. Abnormal Laboratory Results: Magnesium: the forgotten electrolyte. Aust Prescr. 30(4):102-105. 2007. [article]

CCC 700 6

Critical Care

Compendium

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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