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 GABA_A 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.