What is the appropriate dosage and administration of magnesium sulfate for an adult patient, possibly pregnant or with a history of severe asthma or neurologic disorders?

Magnesium Sulfate Dosing and Administration in Adults

For severe asthma exacerbations in adults, administer 2g IV magnesium sulfate over 20 minutes as a single dose; for severe pre-eclampsia/eclampsia, give an initial 4-5g IV loading dose followed by 1-2g/hour continuous infusion or 4-5g IM every 4 hours; for neurologic seizures (epilepsy, glomerulonephritis, hypothyroidism), give 1g IM or IV. 1

Severe Asthma Exacerbations

Indications for Use

• Administer IV magnesium sulfate to patients with life-threatening asthma exacerbations or those whose exacerbations remain severe after 1 hour of intensive conventional treatment (inhaled β2-agonists, anticholinergics, and systemic corticosteroids). 2
• The American Academy of Allergy, Asthma, and Immunology specifically recommends magnesium for patients with FEV1 or PEF <40% predicted after initial bronchodilator therapy. 2
• The British Thoracic Society notes the greatest benefit occurs in patients with FEV1 <20% predicted. 2

Standard Dosing Protocol

• The standard adult dose is 2g IV administered over 20 minutes as a single dose, which has been validated in multiple high-quality trials and reduces hospital admissions by approximately 7 per 100 patients treated. 2, 3, 1
• The FDA label specifies the rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of a 10% concentration). 1
• Solutions for IV infusion must be diluted to a concentration of 20% or less prior to administration using 5% Dextrose or 0.9% Sodium Chloride. 1

Safety Monitoring

• Monitor blood pressure during administration as rapid infusion may cause hypotension and bradycardia. 3
• Side effects are generally minor, including flushing and light-headedness. 2
• Magnesium should be used as an adjunct to standard therapy, not as a replacement. 2

Important Caveat

• Magnesium may reduce the antibiotic activity of streptomycin, tetracycline, and tobramycin when given together. 1

Severe Pre-eclampsia and Eclampsia

Indications

• Administer magnesium sulfate antenatally to women with severe pre-eclampsia and at least one clinical sign of seriousness to reduce the risk of eclampsia. 4
• This recommendation applies to both in-hospital and pre-hospital/emergency settings. 4

Loading Dose Options

• Initial IV loading dose: 4-5g in 250 mL of 5% Dextrose or 0.9% Sodium Chloride infused over 15-20 minutes. 1
• Alternatively, dilute the 50% solution to 10% or 20% concentration and inject 40 mL of 10% solution (or 20 mL of 20% solution) IV over 3-4 minutes. 1
• Simultaneous IM option: Up to 10g total (5g or 10 mL of undiluted 50% solution in each buttock) can be given simultaneously with IV loading dose. 1

Maintenance Dosing

• After initial IV loading dose, administer 1-2g/hour by continuous IV infusion until paroxysms cease. 1
• Alternative IM maintenance: 4-5g (8-10 mL of 50% solution) injected IM into alternate buttocks every 4 hours as needed, depending on presence of patellar reflex and adequate respiratory function. 1
• Target serum magnesium level is 6 mg/100 mL for optimal seizure control. 1

Maximum Dosing and Duration Limits

• Total initial dose is 10-14g of magnesium sulfate. 1
• Total daily (24-hour) dose should not exceed 30-40g. 1
• Critical warning: Continuous maternal administration beyond 5-7 days can cause fetal abnormalities. 1
• In severe renal insufficiency, maximum dosage is 20g/48 hours with frequent serum magnesium monitoring. 1

Neonatal Considerations

• Be aware that increasing maternal serum magnesium concentrations before birth are associated with neonatal hypotonia, lower Apgar scores, increased intubation rates, and special care nursery admissions. 5

Neurologic Seizure Disorders

Dosing for Epilepsy, Glomerulonephritis, or Hypothyroidism

• The usual adult dose is 1g administered IM or IV for controlling seizures associated with these conditions. 1

Other Indications (Less Common)

Magnesium Deficiency

• Mild deficiency: 1g (8.12 mEq) IM every 6 hours for 4 doses. 1
• Severe hypomagnesemia: Up to 250 mg/kg (0.5 mL of 50% solution) IM within 4 hours, or 5g (40 mEq) added to 1 liter IV fluid infused over 3 hours. 1

Barium Poisoning

• 1-2g IV to counteract muscle-stimulating effects. 1

Cerebral Edema

• 2.5g (25 mL of 10% solution) IV. 1

Paroxysmal Atrial Tachycardia

• 3-4g (30-40 mL of 10% solution) IV over 30 seconds with extreme caution, only if simpler measures have failed and no myocardial damage is present. 1

Critical Monitoring Parameters

• Monitor patellar reflexes before each IM dose - absence indicates magnesium toxicity. 1
• Ensure adequate respiratory function (respiratory rate >16/min). 1
• Have calcium chloride available to reverse potential magnesium toxicity. 6
• In pregnancy, therapeutic levels are achieved with serum concentrations of 4-6 mg/dL, while severe toxicity occurs at 12-15 mg/dL. 7

Administration Technique

• IM administration of undiluted 50% solution is appropriate for adults and provides therapeutic levels in 60 minutes. 1
• IV administration provides therapeutic levels almost immediately. 1
• For children, dilute to 20% or less concentration before IM injection. 1
• Inspect parenteral products visually for particulate matter and discoloration before administration. 1