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Magnesium

Magnesium is a versatile drug. In this article, we will discuss its role as an adjunct to TIVA.

Magnesium

Magnesium plays an important homeostatic role in both neurotransmission and neuromuscular activity. It is the fourth most common cation in the body and the second-highest intracellular cation. The reference range for adults is 0.65-1.05 mmol/L.

The addition of magnesium to propofol maintained TIVA has the potential to reduce maintenance infusions by 15-20%. This reduction occurs as magnesium potentiates the NMDA antagonism action of propofol – similar but not as powerful as ketamine. Propofol acts by both activating GABAA and inhibiting NMDA receptors and HCN-1 channels. Magnesium is a calcium antagonist and potentiates the effect of neuromuscular blocking agents. Therefore, we strongly recommend quantitative peripheral nerve monitoring. It reduces blood pressure and antagonises catecholamines.

Uses:

  • Intraoperative haemodynamic control (e.g. phaeochromocytoma excision, airway manipulation and pneumo-peritoneum)
  • Replacement in deficiency
  • Asthma
  • Arrhythmias (Torsades de Pointes)
  • Eclampsia and pre-eclampsia
  • Tocolysis

Perioperative magnesium demonstrated a reduction in both propofol and opioid requirements.

Preparations:

Clear colourless 20% or 50% solution. 10ml of 50% solution contains 20 mmol or 5g. Dilute with 0.9% sodium chloride or 5% glucose solutions prior to slow administration.

Actions:

  • NMDA receptor antagonist
  • Calcium channel blockade
  • Ach release reduction
  • Catecholamine release reduction
  • Promotes osmotic retention of fluid in the bowel

Benefits:

  • Analgesic and anaesthetic sparing effect
  • Reduction in postoperative shivering and PONV
  • Reduction in neuromuscular blocking agent. It can also reduce the onset of non-depolarising muscle relaxants and the myalgia associated with suxamethonium

Dose:

  • IV loading dose 20-50 mg/kg given over 15-20 minutes
  • +/- IV maintenance 10-20 mg/kg/hr

Onset:

Onset of action is rapid when given IV and lasts ~30 minutes.

Route:

  • Intravenous bolus and infusion
  • Intramuscular
  • Oral

IV Kinetics:

  • Half-life 4 hours – increased with renal impairment
  • Protein binding ~ 40% (similar to endogenous magnesium)
  • Not metabolised
  • Excreted in the urine

Caution:

  • Myasthenia Gravis and Muscular Dystrophy
  • Severe Renal impairment
  • Heart Block
  • Hepatic Encephalopathy

Calcium gluconate is used to treat magnesium toxicity.

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Introduction to Using Total Intravenous Anaesthesia (TIVA)

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