Magnesium sulphate: Current application in anaesthesia

Reduction of hypertensive response to laryngoscopy

Magnesium sulphate can be used to suppress the hypertensive response to laryngoscopy during intubation; it causes cardiovascular depression by acting as a calcium channel blocker. The consequent inhibition of catecholamine release reduces plasma epinephrine and norepinephrine concentrations after endotracheal intubation and, therefore, reduces hypertensive responses during anaesthesia induction.^[4] However, it should be used with caution in hypovolemic patients and in patients with limited cardiac capacity. In addition, slow administration (>10 min) of a loading dose of magnesium sulphate may minimise cardiovascular side effects, such as hypotension and bradycardia.

Muscle relaxants property

Magnesium acts as a calcium channel blocker at presynaptic nerve terminals and decreases acetylcholine release at the motor end plate, which diminishes muscle fibre excitability and reduces the amplitude of endplate potential, resulting in the potentiation of a neuromuscular blockade by nondepolarising neuromuscular blocker.^[5] In contrast, others concluded that a perioperative magnesium sulphate administration reduced requirements for non-depolarising neuromuscular blockers^.[6-8]

Potentiation of muscle relaxation by magnesium sulphate can have diverse clinical implications. Magnesium sulphate can be used as an adjuvant for tracheal intubation. Until recently, when used during the induction of general anaesthesia, magnesium has been highlighted on its efficacy to attenuate cardiovascular responses associated with tracheal intubation.^[9,10]

In addition, Kim et al.^[11] reported that magnesium sulphate, when combined with rocuronium priming, improved rapid-sequence intubation conditions, compared with either magnesium sulphate or priming used alone.

In the operating room, the patient sometimes exhibits resistance to non-depolarising muscle relaxants, which may result from a drug or a disease of the patient. In such cases, magnesium sulphate can be used effectively.

Prevention of hypercalcemia with suxamethonium

Magnesium sulphate can also be helpful when suxamethonium is used for tracheal intubation. Magnesium may prevent hypercalcemia produced by suxamethonium.^[12]Sakuraba et al.^[13] reported that pre-treatment with magnesium sulphate is associated with less fasciculation induced by suxamethonium. However, magnesium was reported not to influence the clinical course of suxamethonium-induced malignant hyperthermia.^[14]

Reduction of incidence of shivering

It has been observed that administration of magnesium sulphate decreases the incidence of shivering by up to 70–90%.^[15-17] The fact that shivering is a leading cause of postoperative discomfort and increases oxygen consumption^[18] the prevention of shivering is one of the most important benefits of magnesium sulphate use in surgical patients.

As adjuvants in Bier’s block

The combination of magnesium sulphate with local anaesthetic agents has beneficial effects in intravenous regional anaesthesia (Bier’s block). Turan et al.^[19] reported that when magnesium was added to lidocaine for Bier block, the quality of anaesthesia and analgesia was improved. Specifically, the onset times of the sensory and motor blocks were shorter and postoperative analgesia was better when combined with magnesium sulphate. However, in this study, recovery after Bier’s block was prolonged in the magnesium group.

Seizure prophylaxis

Magnesium sulphate has seizure prophylaxis property as described by Kim et al.^[20] reported that valproic acid decreases rocuronium duration resulting in an increase in its requirement; however, magnesium sulphate administration attenuates this increase. Children with cerebral palsy show resistance to non-depolarising neuromuscular blocking agents. When magnesium sulphate was administered during operation in children with cerebral palsy, the rocuronium requirement was significantly decreased and postoperative opioid consumption was also decreased.^[21]

Reduction of the incidence of nausea and vomiting

Intraoperative use of magnesium sulphate can be associated with decreased incidences of postoperative nausea and vomiting.^[7,15] This could be due to the lower consumption of volatile anaesthetic (sevoflurane), rather than any antiemetic effect of magnesium sulphate. And again, because nausea and vomiting are one of the most common and distressing complications after surgery, this effect is helpful for surgical patients, especially patients undergoing day-case surgery.

Post-operative analgesia

In terms of postoperative analgesia, the use of magnesium sulphate during surgery can reduce opioid consumption in the first 24 h postoperatively and to a lesser extent, pain scores. A study by Ryu et al.^[15] compared magnesium sulphate and remifentanil during middle ear surgery. In this study, either drug, when combined with sevoflurane to provide adequate level of hypotensive anaesthesia; however, the magnesium group had more favourable postoperative courses, showing better analgesia and less shivering, nausea and vomiting postoperatively. In addition, there was less requirement of sevoflurane to maintain surgical anaesthesia in patients receiving magnesium sulphate than those receiving remifentanil.