Magnesium in Anesthesia and Critical Care (ICU)

Fig. 16-12: The critical roles of Mg2+ - NMDA receptor antagonism, physiological Ca2+ antagonism, Mg-ATP complex formation, DNA/RNA processes, and osteoblast proliferation. (Barash Clinical Anesthesia, 9e)

1. Physiology and Basic Pharmacology

• NMDA receptor antagonism - blocks the voltage-gated Mg2+ channel of the NMDA receptor, explaining its analgesic and neuroprotective properties
• Physiological Ca2+ antagonist - competes with Ca2+ at membrane channels, reduces smooth muscle contractility and vascular tone
• Mg-ATP complex formation - cofactor for >300 enzymatic reactions including Na+/K+-ATPase
• Neuromuscular junction - inhibits presynaptic Ca2+ channels that trigger acetylcholine release; reduces postjunctional membrane excitability
• Cardiac membrane stabilization - slows SA node impulse formation and prolongs AV conduction

2. Clinical Indications in Anesthesia and ICU

A. Obstetric Anesthesia - Preeclampsia and Eclampsia

• IV route: 4-6 g loading dose over 15-20 min + 1-2 g/hr continuous infusion
• IM route (Pritchard regimen): 4-5 g IM, repeated every 4 hours

• MgSO4 profoundly potentiates nondepolarizing neuromuscular blocking drugs (NMBDs). After a 40 mg/kg MgSO4 dose, the ED50 of vecuronium is reduced by 25%, onset time is nearly halved, and recovery time nearly doubled
• Neostigmine-induced reversal is attenuated in patients on magnesium
• The mechanism involves dual action: prejunctional - blocks Ca2+ channels at nerve terminals reducing ACh release; postjunctional - decreases membrane excitability
• Succinylcholine interaction is controversial - some reports suggest antagonism of succinylcholine block; prolonged weakness can also occur
• Sugammadex is preferred over neostigmine for reversal in patients receiving MgSO4, given attenuated neostigmine response
• Objective neuromuscular monitoring is mandatory - dose NMBDs carefully; do not rely on clinical assessment alone
• The combination of MgSO4 with calcium channel blockers (especially nifedipine) can produce excessive neuromuscular blockade and severe hypotension
• Miller's Anesthesia 10e, p. 3310-3311; Barash Clinical Anesthesia 9e, block 14, 19

B. Multimodal Analgesia (NMDA Antagonism)

"Recent meta-analyses indicate that the perioperative administration of IV magnesium may also be an effective adjunct in the treatment of perioperative pain. IV magnesium infusion decreases pain scores, is opioid sparing in the first 24 hours following surgery and is devoid of any serious adverse effects."

• The combination of MgSO4 + low-dose ketamine has shown additive benefit in spinal surgery: magnesium bolus 50 mg/kg + maintenance 8 mg/kg/hr reduced postoperative morphine consumption and improved sleep/satisfaction scores
• Particularly useful in opioid-tolerant patients by suppressing neuropathic pain and central sensitization
• Recent meta-analyses (PMID 38848810, 2024) confirm analgesic efficacy and opioid-sparing effects in general abdominal surgery

• Loading dose: 30-50 mg/kg IV
• Maintenance: 6-20 mg/kg/hr

C. Antiarrhythmic Effects - Torsades de Pointes

• Necessary for transport of Na+, Ca2+, and K+ across cell membranes
• Slows SA node impulse formation and prolongs conduction through myocardial tissue
• Intravenous formulation required - oral magnesium has no role in acute arrhythmia management

• Pulseless: 1-2 g IV over 5-20 minutes
• With pulse: 1-2 g IV over 5-60 minutes

• Lippincott Pharmacology, block 2; Braunwald's Heart Disease, block 8

D. Bronchospasm / Severe Asthma / COPD Exacerbation

• Inhibiting release of bronchoconstrictors (histamine and acetylcholine)
• Calcium antagonism - relaxes bronchial smooth muscle
• Anti-inflammatory properties

E. Pheochromocytoma Resection

• MgSO4 causes vascular smooth muscle relaxation with arteriolar vasodilation
• Directly inhibits catecholamine release from the adrenal medulla
• Used perioperatively to blunt hypertensive crises and hemodynamic surges during tumor manipulation

• 40-60 mg/kg loading dose prior to intubation + 1-2 g/hr maintenance infusion
• Barash Clinical Anesthesia 9e, block 5

F. Emergence Agitation

G. Uterine Relaxation (Tocolysis)

• Used as a tocolytic for preterm labor
• Loading dose: 6 g IV + maintenance: 2 g/hr
• Also used for uterine relaxation during ex utero intrapartum treatment (EXIT) procedures and open fetal surgery - a magnesium sulfate bolus can be given upfront at uterine closure to reduce tone and improve operative field

H. Neuroprotection in Obstetrics

• Fetal and neonatal neuroprotection in premature births
• Can exacerbate hypotension and potentiate NMB in this context - requires careful monitoring

3. Hypomagnesemia in the ICU

• Nasogastric suctioning, diarrhea, vomiting, high-output fistulae
• Diuretic use (loop and thiazide)
• Chronic PPI use (inhibits TRPM6/7 channels reducing Mg2+ GI absorption)
• Chronic alcohol dependence (multifactorial: renal wasting + GI losses + reduced PTH)
• Pancreatitis, short bowel syndrome

• 1-2 g/hr IV over 3-6 hours, then 0.5-1 g/hr as needed

4. Hypermagnesemia and Toxicity

• Check DTRs regularly (loss = first sign of toxicity)
• Urine output >25-30 mL/hr (magnesium is renally cleared)
• Respiratory rate >12/min
• Serum levels every 4-6 hours if high-dose infusion

5. Neuromuscular Blocking Drug Interactions - Summary

6. Regional Anesthesia Adjuvant

• Dosing: 150-600 mg MgSO4 added to the primary local anesthetic
• Mechanism: NMDA receptor antagonism at the dorsal horn level

7. Dosing Reference Summary

Recent Evidence (2024-2026)

• PMID 38848810 (2024, Systematic Review): Perioperative IV magnesium in abdominal surgery reduces pain and opioid consumption in the first 24 hours
• PMID 38749290 (2024, Meta-Analysis): IV MgSO4 reduces emergence agitation in adult general anesthesia patients
• PMID 39089452 (2024, Meta-Analysis): MgSO4 reduces laryngospasm incidence in pediatric general anesthesia
• PMID 40606621 (2025, Systematic Review): IV MgSO4 effective in spinal surgery for analgesia and hemodynamic stability
• PMID 40637351 (2025, Network Meta-Analysis): IV magnesium ranks highly among bronchodilators in pediatric critical asthma

1. Magnesium potentiates ALL nondepolarizing NMBDs - always use objective monitoring and reduced doses
2. The first clinical sign of toxicity is loss of DTR - monitor before checking levels
3. Hypomagnesemia is present in up to 65% of ICU patients - check and correct before troubleshooting refractory hypocalcemia/hypokalemia
4. Sugammadex is preferred over neostigmine for reversal in patients on magnesium infusions
5. Magnesium is the drug of first choice for torsades de pointes - IV only