Magnesium Sulfate (MgSO₄) in Anesthesia

Introduction

1. Mechanism of Action

• NMDA receptor antagonism: Magnesium blocks the NMDA receptor channel in a voltage-dependent manner, inhibiting calcium influx. This underlies its analgesic, antinociceptive, and anticonvulsant effects.
• Calcium channel blockade: Competes with calcium at voltage-gated channels, causing smooth muscle relaxation, vasodilation, and reduced synaptic acetylcholine release.
• Membrane stabilization: Decreases neuronal excitability by stabilizing cell membrane potentials.
• Presynaptic inhibition: Reduces acetylcholine release at the neuromuscular junction, potentiating neuromuscular blockade.

2. Uses in Anesthesia

A. Treatment and Prophylaxis of Eclampsia / Preeclampsia

• Mechanism: Reduces cerebral vasospasm (via calcium antagonism), limits cerebral edema, and raises the seizure threshold via NMDA antagonism.
• Dose (Pritchard regimen): Loading dose of 4-6 g IV over 15-20 minutes, followed by a maintenance infusion of 1-2 g/hour IV.
• When general anesthesia is chosen for cesarean delivery in these patients, MgSO₄ therapy can complicate management by causing uterine atony and maternal muscle weakness - both of which must be anticipated.
  • Rosen's Emergency Medicine, p. 2668; Barash Clinical Anesthesia 9e

B. Multimodal Postoperative Analgesia (Opioid-Sparing Effect)

• Mechanism: NMDA receptor antagonism inhibits central sensitization and suppresses neuropathic pain signaling - similar to ketamine but without psychomimetic effects.
• Effect: Decreases postoperative pain scores and reduces opioid consumption in the first 24 hours after surgery.
• Dose: Typical regimen is an IV bolus of 30-50 mg/kg at induction, followed by an intraoperative infusion of 6-15 mg/kg/h, continued into the postoperative period.
• Particularly valuable in opioid-tolerant patients where standard analgesic approaches are often insufficient.
• When combined with low-dose ketamine (bolus 0.2 mg/kg + infusion 0.15 mg/kg/h), magnesium further decreased postoperative morphine consumption and improved sleep quality in scoliosis surgery patients.
  • Barash Clinical Anesthesia 9e, p. 4692-4693
• Recent evidence (2024-2025 systematic reviews, PMIDs 40606621, 36087812) confirms benefit in spinal surgery, and as an epidural adjuvant to local anesthetics for postoperative pain.

C. Adjuvant in Neuraxial / Regional Anesthesia

• Intrathecal MgSO₄: Used as an adjuvant to bupivacaine for spinal anesthesia, particularly in cesarean sections - it prolongs the duration of sensory block and reduces analgesic requirements postoperatively.
• Epidural MgSO₄: Added to epidurally administered local anesthetics, it reduces postoperative pain and decreases supplemental analgesic requirements (2023 systematic review, PMID 36087812).

D. Prevention and Treatment of Emergence Agitation

• MgSO₄ infusion reduces the incidence of emergence agitation in adult surgical patients.
• Two recent meta-analyses (2024, PMIDs 39324601 and 38749290) confirm statistically significant reduction in emergence agitation incidence and severity with perioperative MgSO₄ infusion.
• Mechanism: Likely related to NMDA receptor antagonism blunting the hyperexcitable state during emergence from general anesthesia.

E. Tocolysis (Prevention of Preterm Labour)

• MgSO₄ is used as a tocolytic agent to arrest preterm labour, particularly in the context of fetal surgery or premature labour during pregnancy.
• It is given prophylactically for 24 hours post-operatively after fetal surgery to maintain uterine quiescence.
• Oral nifedipine is used after discontinuing MgSO₄ until 37 weeks gestation.
  • Miller's Anesthesia 10e; Creasy & Resnik's Maternal Fetal Medicine

F. Bronchospasm / Bronchodilation

• MgSO₄ produces bronchial smooth muscle relaxation via calcium antagonism, making it useful in the management of severe, refractory bronchospasm perioperatively and in acute severe asthma.
• A dose of 1.2-2 g IV over 20 minutes is used in acute asthma not responding to conventional bronchodilators.

G. Antiarrhythmic Use

• MgSO₄ is the drug of choice for torsades de pointes (polymorphic ventricular tachycardia associated with prolonged QT interval).
• Dose: 1-2 g IV over 1-2 minutes for acute treatment.
• Also used in refractory ventricular fibrillation and atrial fibrillation.
• Mechanism: Stabilizes cardiac membrane, blocks calcium channels, and reduces automaticity.

H. Attenuation of Pressor Response to Laryngoscopy and Intubation

• MgSO₄ given before intubation blunts the sympathetic cardiovascular response (tachycardia, hypertension) to direct laryngoscopy - particularly useful in hypertensive patients and those with preeclampsia.
• Acts by inhibiting catecholamine release and reducing peripheral vascular resistance.

I. Potentiation of Neuromuscular Blocking Agents (NMBAs)

• MgSO₄ potentiates both depolarizing and non-depolarizing neuromuscular blockers by:
  1. Reducing acetylcholine release from motor nerve terminals (presynaptic)
  2. Reducing end-plate sensitivity to acetylcholine (postsynaptic)
  3. Direct membrane stabilization
• Clinical implication: Patients receiving MgSO₄ infusions (e.g., eclamptic patients) require significantly reduced doses of NMBAs (succinylcholine and vecuronium/rocuronium). Neuromuscular monitoring is essential.
• Risk: Prolonged neuromuscular blockade if doses are not adjusted.

3. Dosing Summary

4. Monitoring and Toxicity

5. Contraindications and Precautions

• Renal failure (reduced excretion - can lead to dangerous accumulation)
• Myasthenia gravis (worsens neuromuscular block)
• Known hypersensitivity
• Caution with concurrent calcium channel blockers (additive hypotension)
• Adjust NMBA doses - always use neuromuscular monitoring

Summary