Magnesium sulfate

Mechanism

Mechanism of Action

Magnesium sulfate (MgSO₄) is an inorganic salt that acts as a physiological calcium antagonist in cellular membrane channels, in neuronal N-methyl-D-aspartate acid (NMDA) receptors, and in neuromuscular junctions.

Molecular Physiology of Neuroprotection and Anticonvulsant

1. Anticonvulsant Mechanism (Eclampsia Prophylaxis): The magnesium ion (Mg²⁺) selectively blocks the cortical NMDA receptor channel in a voltage-dependent manner, preventing the massive excitatory influx of calcium and limiting the propagation of repetitive membrane depolarizations. In addition, it dilates the cerebral vasculature selectively, reducing the generalized vasospasm of preeclampsia.
2. Fetal Neuroprotection Mechanism: Magnesium blocks L-type calcium channels and NMDA receptors at the fetal neuronal level in the phase of asphyxia or intrauterine hypoxia. This mitigates the cascade of excitotoxic necrotic and inflammatory cell death induced by extracellular glutamate accumulation.
3. Minor Tocolytic Mechanism: Competitively blocks calcium channels in the uterine myocyte, decreasing cell contractility.

Pharmacokinetics

Pharmacokinetics

• Routes of administration: Intravenous (continuous precision loading and maintenance bolus infusion regulated by infusion pump) and deep intramuscular (alternative route in resource-limited settings, Pritchard protocol).
• Distribution: Apparent volume of distribution of approx. 0.25 L/kg. Normal plasma magnesium is free or labile bound to albumin (30%). It freely crosses the placental barrier, reaching equilibrium concentrations in the amniotic fluid and plasma of the fetus within 2 to 4 hours after starting the maternal infusion.
• Metabolism: Does not undergo hepatic cellular metabolism. It is a pure inorganic trace element.
• Excretion: Elimination exclusively through the kidneys through glomerular filtration and passive proximal tubular reabsorption. Renal clearance of magnesium is directly proportional to the glomerular filtration rate (Cl_Cr).
• Half-life (t_1/2): Approximately 3 to 4 hours in patients with preserved renal function.

Indicators and dose

Indications

• Prevention and treatment of seizures in patients with preeclampsia with severity criteria or established active eclampsia.
• Fetal neuroprotection: Prevention of cerebral palsy and gross motor neurodevelopment disorders in fetuses at risk of imminent preterm birth less than 32 weeks of gestation.

Precision Dosing (Clinical Protocols)

1. Zuspan Protocol (Pure Intravenous Route - Choice)

• Loading dose: 4 g IV (diluted in 100 mL of 0.9% physiological saline) slowly infused via continuous infusion pump over an interval of 15 to 20 minutes.
• Maintenance dose: Continuous infusion of 1 to 2 g/hour (prepared as 20 g of magnesium sulfate in 500 mL of saline, scheduled at a rate of 25 to 50 mL/hour). It should be maintained for a minimum of 24 hours after delivery or since the last seizure.

Security

Contraindications

Absolute

• Myasthenia gravis (magnesium inhibits the presynaptic release of acetylcholine in the motor plate, which can precipitate lethal severe myasthenic crisis or respiratory paralysis).
• Pre-existing heart block (without pacemaker) or severe myocardial damage.
• End-end anuric renal failure (critical risk of accelerated lethal systemic accumulation).

Relative

• Moderate renal failure (requires a mandatory 50% reduction in the maintenance dose and hypervigilance of magnesium).
• Concomitant use with calcium channel blockers (nifedipine).

Adverse Effects (ADR) and Pathophysiology of Toxicity

Magnesium toxicity is directly proportional to the plasma concentration of Mg²⁺ (optimal therapeutic range: 4.8 to 8.4 mg/dL or 2 to 3.5 mmol/L):

Interactions

Dangerous additive synergistic effect with aminoglycosides and neuromuscular blockers (suxamethonium, vecuronium), potentially prolonging muscle paralysis irreversibly. Potentiation of cardiac toxicity with the simultaneous use of digoxin.

2. Mandatory Clinical Monitoring (Golden Rule in Obstetrics)

Every 1 to 2 hours, the nursing medical staff should evaluate the presence and status of the following three clinical parameters:

1. Presence of symmetrical patellar (patellar) reflex (first warning sign of toxicity).
2. Maternal respiratory rate: Should be 12-14 breaths per minute.
3. Hourly diuresis: It should be 30 mL/hour (accumulation is imminent if oliguria occurs due to transient renal failure).

Emergency Antidote to Magnesium Toxicity

In case of clinical suspicion of toxicity or evidence of abolition of patellar reflexes with respiratory depression, the magnesium sulfate infusion should be immediately suspended and 10% Calcium Gluconate: 1 g IV (1 ampoule of 10 mL) administered slowly intravenously over a period of 3 to 5 minutes. Calcium acts as a competitive direct ionic antagonist of magnesium at the receptor, immediately restoring neuromuscular transmission.

Pregnancy and Breastfeeding

FDA Category A. Widely validated in pregnancy as the gold standard anticonvulsant of choice over phenytoin or diazepam (MAGPIE studies). Its prolonged use for more than 5 to 7 prenatal days may be associated with transient alterations in bone mineralization and fetal decalcification, so its administration for long-term tocolysis is strictly discouraged. Compatible with breastfeeding safely.