Intravenous (IV) magnesium sulfate is an increasingly utilized perioperative adjuvant in the operating room (OR), valued for its relaxant, analgesic, and neuromodulatory properties. Magnesium exerts its clinical effects primarily through noncompetitive antagonism of N-methyl-D-aspartate (NMDA) receptors and blockade of voltage-gated calcium channels, thereby reducing presynaptic calcium influx and attenuating acetylcholine release at the neuromuscular junction (1). These mechanisms contribute to its role as a smooth and skeletal muscle relaxant, as well as its capacity to modulate central sensitization and acute nociceptive transmission. In modern anesthetic practice, magnesium is not used to replace standard anesthetic agents but rather to reduce their requirements and improve perioperative recovery profiles.
IV magnesium produces a muscle relaxant effect by decreasing skeletal muscle excitability and enhancing nondepolarizing neuromuscular blockade. In spine surgery, magnesium reduces paraspinal muscle tone and resistance to retraction, improving surgical exposure. In abdominal and laparoscopic procedures, it decreases abdominal wall tension and diaphragmatic reflex activity, improving compliance during pneumoperitoneum and stabilizing the operative field. These muscle relaxant effects contribute directly to smoother intraoperative conditions when used with appropriate neuromuscular monitoring.
By decreasing acetylcholine release and reducing postsynaptic responsiveness, magnesium also enhances the effect of nondepolarizing neuromuscular blocking agents, such as rocuronium. This interaction enables anesthesiologists to administer lower doses of neuromuscular blocking agents while maintaining optimal muscle relaxation, potentially reducing the risk of residual neuromuscular blockade after surgery (1).
IV magnesium is also selected as an adjuvant for its analgesic and opioid-sparing effects, which are highly relevant for OR settings. Through NMDA receptor antagonism, magnesium reduces central sensitization and wind-up phenomena associated with surgical injury (1). In a comprehensive meta-analysis, De Oliveira et al. found that systemic perioperative magnesium administration significantly reduced postoperative pain scores at rest and with movement, as well as decreased cumulative opioid consumption (2). These findings are particularly relevant in orthopedic and spinal procedures, where central sensitization contributes significantly to postoperative pain burden. By reducing opioid exposure, magnesium may also indirectly decrease opioid-related adverse effects, including respiratory depression, ileus, and sedation.
Beyond analgesia and muscle relaxation, magnesium has demonstrated additional perioperative benefits. A systematic review by Lysakowski et al. concluded that magnesium administration was associated with a reduced incidence of postoperative shivering and decreased anesthetic requirements (3). The attenuation of shivering is thought to result from central thermoregulatory modulation and reduced catecholamine release. Magnesium’s potential to reduce postoperative nausea and vomiting (PONV) may be due to its opioid-sparing effect and decreased volatile anesthetic requirements (3). These additional benefits support its inclusion in multimodal anesthesia and enhanced recovery after surgery (ERAS) protocols.
When administered in appropriate doses and with vigilant monitoring, magnesium sulfate has a favorable safety profile. Its hemodynamic effects may include mild hypotension due to vasodilation and dose-dependent sedation (1). Excessive dosing can lead to prolonged neuromuscular blockade, respiratory depression, and, in extreme cases, cardiac conduction abnormalities. Therefore, intraoperative hemodynamic monitoring and neuromuscular function assessment are essential when using magnesium as an adjuvant. Due to reduced magnesium clearance, dose adjustments and heightened caution are warranted in patients with renal impairment.
In summary, IV magnesium sulfate is an effective and safe perioperative adjuvant that provides clinically meaningful muscle relaxant and analgesic effects in the OR. Through NMDA receptor antagonism and calcium channel blockade, magnesium reduces the requirement for neuromuscular blocking agents, decreases postoperative pain and opioid consumption, and lowers the incidence of shivering and other recovery-related complications. When used judiciously with appropriate monitoring, magnesium can enhance multimodal anesthesia strategies and contribute to improved perioperative outcomes.
References
1. Fawcett WJ, Haxby EJ, Male DA. Magnesium: physiology and pharmacology. Br J Anaesth. 1999;83(2):302-320. doi:10.1093/bja/83.2.302
2. De Oliveira GS Jr, Castro-Alves LJ, Khan JH, McCarthy RJ. Perioperative systemic magnesium to minimize postoperative pain: a meta-analysis of randomized controlled trials. Anesthesiology. 2013;119(1):178-190. doi:10.1097/ALN.0b013e318297630d
3. Lysakowski C, Dumont L, Czarnetzki C, Tramèr MR. Magnesium as an adjuvant to postoperative analgesia: a systematic review of randomized trials. Anesth Analg. 2007;104(6):. doi:10.1213/01.ane.0000261250.59984.cd
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