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

Magnesium Sulfate Dosing Guidelines

For Preeclampsia/Eclampsia

For pregnant women with eclampsia or severe preeclampsia, administer a loading dose of 4-6g IV magnesium sulfate over 20-30 minutes, followed by a maintenance infusion of 1-2g/hour, continuing for 24 hours postpartum. 1, 2, 3

Loading Dose Options

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

• Give 4-5g IV over 20-30 minutes, diluted in 250mL of 5% dextrose or 0.9% normal saline 3
• Alternatively, dilute 50% solution to 10-20% concentration and inject 40mL (10% solution) or 20mL (20% solution) over 3-4 minutes 3

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

• Total loading dose: 14g administered as 4g IV plus 10g IM (5g in each buttock simultaneously) 2, 3
• Maintenance: 5g IM every 4 hours in alternating buttocks for 24 hours 2, 3
• This regimen was validated in the landmark MAGPIE trial demonstrating approximately 50% reduction in seizure risk 1, 2

Maintenance Therapy

• Standard approach: Continue 1-2g/hour IV infusion for 24 hours postpartum 1, 2, 3
• Alternative approach: May discontinue after administering at least 8g predelivery in select populations, though this requires consideration of local postpartum eclampsia incidence 1, 2
• The 24-hour postpartum duration remains the guideline-recommended standard because eclampsia can occur postpartum 2

Monitoring Requirements

Essential safety monitoring includes: 1, 3, 4

• Deep tendon reflexes (loss occurs at 3.5-5 mmol/L) 4
• Respiratory rate (paralysis occurs at 5-6.5 mmol/L) 4
• Urine output (maintain >30 mL/hour) 1
• Serum magnesium levels (therapeutic range: 1.8-3.0 mmol/L or 4.8-7.2 mg/dL) 4

Critical toxicity thresholds: 4

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

Important Precautions

• Have calcium gluconate immediately available to counteract magnesium toxicity 2
• In severe renal insufficiency, maximum dosage is 20g/48 hours with frequent serum magnesium monitoring 3
• Do not exceed 30-40g total daily dose 3
• Do not continue beyond 5-7 days as continuous maternal administration can cause fetal abnormalities 3
• Limit total fluid intake to 60-80 mL/hour to avoid pulmonary edema risk 1

---

For Severe Asthma Exacerbations

For patients with severe asthma exacerbations remaining severe after 1 hour of intensive conventional treatment, administer 2g IV magnesium sulfate over 20 minutes as adjunctive therapy. 2, 5, 3

Indications for Use

• Life-threatening exacerbations (FEV1 <20% predicted) 5
• Severe exacerbations (FEV1 or PEF <40% predicted) that remain severe after 1 hour of intensive treatment with inhaled beta-agonists, anticholinergics, and systemic corticosteroids 5
• Greatest benefit occurs in patients with FEV1 <20% predicted 5

Administration Protocol

• Dose: 2g IV administered over 20 minutes 2, 5, 3
• Dilution: Must be diluted to 20% or less concentration prior to administration 3
• Timing: Administer after initial treatment with inhaled short-acting beta-agonists, anticholinergics, and systemic corticosteroids 5

Evidence Base

• A Cochrane meta-analysis demonstrated that IV magnesium sulfate improves pulmonary function and reduces hospital admissions by approximately 7 per 100 patients treated, particularly in patients with the most severe exacerbations 5
• Magnesium causes relaxation of bronchial smooth muscle independent of serum magnesium level 5
• Multiple major guidelines (American Academy of Allergy, Asthma, and Immunology, American Heart Association, British Thoracic Society) support its use for severe exacerbations 5

Safety Profile

• Minor side effects include flushing, hypotension, bradycardia, and light-headedness 2, 5
• Must be used as adjunctive therapy, not as replacement for standard treatments 5

Repeat Dosing Considerations

• Repeat magnesium sulfate is appropriate if the patient has a severe exacerbation (FEV1 or PEF <40% predicted) that remains severe after 1 hour of intensive conventional treatment 5
• Reassess patients at 60-90 minutes after initiation of therapy, including subjective response, physical findings, and FEV1 or PEF results 5

Nebulized Magnesium (Less Effective)

• Inhaled magnesium sulfate is less effective than IV administration for acute asthma 5
• May be used as adjunct to standard therapy with 3mL of 260 mmol/L solution every 20-60 minutes via nebulizer, but should not be first-line therapy 5