What is the typical dose and administration of MgSO4 (magnesium sulfate) for a pregnant woman with a risk of eclampsia or for a patient with severe asthma?

Magnesium Sulfate Dosing and Administration

For Severe Pre-eclampsia and Eclampsia

The standard regimen is a 4-6g IV loading dose over 20-30 minutes, followed by 1-2g/hour continuous infusion for 24 hours postpartum. 1, 2, 3

Loading Dose Options

IV-Only Protocol (Preferred when IV access available):

• 4-5g IV diluted in 250mL of 5% dextrose or 0.9% normal saline, infused over 20-30 minutes 2, 3
• Alternatively, dilute to 10-20% concentration and give 4g over 3-4 minutes 3

Pritchard Regimen (When IV pumps unavailable or limited IV access):

• Loading: 4g IV plus 10g IM (5g in each buttock) simultaneously 2, 3
• Maintenance: 5g IM every 4 hours in alternating buttocks 2, 3
• This regimen was validated in the landmark MAGPIE trial showing approximately 50% seizure risk reduction 2

Maintenance Infusion

• Continue 1-2g/hour IV for 24 hours postpartum as the guideline-recommended standard 2, 3
• Alternative approach: May stop after 8g predelivery in select populations, though this requires consideration of local postpartum eclampsia incidence 2
• Do not exceed 30-40g total daily dose 3

Critical Safety Monitoring

Clinical monitoring guides therapy, NOT routine serum levels: 1

• Check patellar reflexes (loss occurs at 3.5-5 mmol/L) 4
• Respiratory rate >12/min (paralysis at 5-6.5 mmol/L) 4
• Urine output ≥30 mL/hour (magnesium is renally excreted) 1
• Oxygen saturation >90% 1

Only check serum magnesium if: 1

• Renal impairment (elevated creatinine)
• Loss of reflexes
• Respiratory depression
• Oliguria develops

Therapeutic range: 1.8-3.0 mmol/L (4.3-7.2 mg/dL) 4

Absolute Contraindications

• Never combine with calcium channel blockers (especially IV/sublingual nifedipine) - causes severe myocardial depression 1
• Severe renal insufficiency: maximum 20g/48 hours with frequent level monitoring 3

Antidote

• Have calcium gluconate 1g IV immediately available to reverse toxicity 2
• Cardiac arrest occurs at >12.5 mmol/L 4

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For Severe Acute Asthma

Give 2g IV magnesium sulfate diluted to ≤20% concentration over 20 minutes as adjunct to standard therapy. 5, 2, 3

Specific Protocol

• Dilute 2g in 50mL normal saline 5
• Infuse over 20 minutes 2
• Greatest benefit when FEV1 <20% predicted 5

Evidence Base

• Multicenter RCT showed significantly higher FEV1% predicted at 4 hours when added to standard treatment in patients with FEV1 <30% predicted 5
• No major adverse effects noted 5
• Common side effects: flushing, hypotension, bradycardia 2

Clinical Context

• Use in severe refractory asthma unresponsive to β2-agonists and corticosteroids 2, 6
• Evidence supports use in severe asthma, eclampsia, and torsades de pointes specifically 6
• Rapid 2-minute infusion has been documented as safe and may obviate intubation in status asthmaticus 7

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Special Pregnancy Considerations

Fetal Neuroprotection

• Give magnesium sulfate when delivery anticipated before 32 weeks gestation 1
• Reduces cerebral palsy risk (RR 0.68,95% CI 0.54-0.87) without increasing mortality 1

Duration Limits

• Do not continue beyond 5-7 days in pregnancy - causes fetal abnormalities 3
• Standard 24-hour postpartum course is safe 2, 3

Resource-Limited Settings

• Can be administered by trained midwives/nursing staff 1
• Emergency loading: 10g IM (5g each buttock) before transfer is better than no treatment 2