What is the proper administration protocol for magnesium sulphate (MgSO4)?

Magnesium Sulfate Administration Protocol

Magnesium sulfate administration varies significantly by clinical indication, with specific loading doses, infusion rates, and monitoring requirements that must be followed to prevent toxicity while achieving therapeutic effect.

Administration by Clinical Indication

Preeclampsia/Eclampsia

• Loading dose: Administer 4-6 g IV over 20-30 minutes 1, 2
• Maintenance infusion: 1-2 g/hour by continuous IV infusion 1, 2
• Alternative regimen: 4 g IV loading dose followed by simultaneous IM administration of 5 g in each buttock (10 g total), then 5 g IM every 4 hours in alternating buttocks 2, 3
• Duration: Continue for 24 hours postpartum, though some evidence suggests stopping after 8 g predelivery may be reasonable in select populations 4
• The International Society for the Study of Hypertension in Pregnancy recommends using the dosing regimens from the Eclampsia and MAGPIE trials, which demonstrated approximately 50% reduction in seizure risk 4
• Research demonstrates that 1 g/hour maintenance is as effective as 2 g/hour with fewer side effects, though serum levels are lower 5

Severe Refractory Asthma

• Dose: 2 g IV magnesium sulfate over 20 minutes 6, 1
• Concentration: Must be diluted to 20% or less before administration 1, 2
• Patient selection: Reserved only for the most severe exacerbations with severe respiratory distress who have failed initial bronchodilator therapy 6
• Do NOT use for mild or moderate asthma exacerbations as it shows no benefit 6

Torsades de Pointes

• First-line dose: 1-2 g IV over 15 minutes, regardless of serum magnesium level 6, 1
• Repeat dosing: If episodes persist, repeat 2 g infusions may be necessary 6

Hypomagnesemia

• Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses 2
• Severe deficiency: Up to 250 mg/kg (approximately 2 mEq/kg) IM over 4 hours, or 5 g (40 mEq) added to 1 liter of IV fluid infused over 3 hours 2
• Acute hypomagnesemia: 1-2 g IV over 15 minutes, followed by maintenance infusion of 1 g/hour for 24 hours if needed 1

Perioperative Use (VATS)

• Intraoperative regimen: 50 mg/kg IV bolus over 10 minutes after intubation, followed by continuous infusion of 50 mg/kg/hour until end of surgery 4
• This regimen reduced postoperative opioid requirements and improved pulmonary function 4

Critical Safety Monitoring

Essential Monitoring Parameters

• Patellar reflex: Must be present before each dose; reflexes diminish at magnesium levels >4 mEq/L and may be absent at 10 mEq/L 2, 7
• Respiratory rate: Must be ≥16 breaths/minute 2
• Urine output: Maintain ≥100 mL during the 4 hours preceding each dose 2
• Serum magnesium levels: Target therapeutic range is 1.8-3.0 mmol/L (approximately 3-6 mg/100 mL or 2.5-5 mEq/L) for eclampsia control 7, 8

Toxicity Thresholds

• Loss of patellar reflex: 3.5-5 mmol/L 7
• Respiratory paralysis: 5-6.5 mmol/L 7
• Altered cardiac conduction: >7.5 mmol/L 7
• Cardiac arrest: >12.5 mmol/L 7

Critical Precautions

Immediate Availability Requirements

• Calcium gluconate or calcium chloride must be immediately available as an antidote to counteract magnesium toxicity 1, 2

Contraindications and Cautions

• Renal impairment: Use with extreme caution as magnesium is exclusively renally excreted; reduce dosage and do not exceed 20 g in 48 hours in severe renal impairment 2
• Pregnancy duration: Continuous maternal administration beyond 5-7 days can cause fetal abnormalities including hypocalcemia, skeletal demineralization, and osteopenia 2
• Digitalized patients: Administer with extreme caution due to risk of serious cardiac conduction changes and heart block 2
• Neuromuscular blocking agents: Concomitant use can cause excessive neuromuscular blockade 2

Administration Rate Limits

• Maximum IV injection rate: Generally should not exceed 150 mg/minute (1.5 mL of 10% solution), except in severe eclampsia with active seizures 2
• Concentration for IV infusion: Must be diluted to 20% or less 2
• IM administration in children: Dilute 50% solution to 20% or less before injection 2

Common Side Effects

• Flushing, hypotension, and bradycardia are common with IV administration 1
• Side effects are more frequent with higher maintenance doses (2 g/hour vs 1 g/hour) 5
• If flushing and sweating occur, administer with increased caution 2

Drug Interactions

• CNS depressants: Reduce dosage of barbiturates, narcotics, hypnotics, and anesthetics due to additive CNS depression 2
• Calcium: Antagonizes CNS depression and peripheral transmission defects caused by magnesium 2