What is the recommended dose of intravenous (IV) magnesium (Mg)

Recommended IV Magnesium Dosing Guidelines

The standard dose of intravenous magnesium sulfate is 2 g administered over 20 minutes for severe refractory asthma, while for torsades de pointes, 2 g can be infused as first-line therapy regardless of serum magnesium level. 1

Dosing by Clinical Indication

Asthma Exacerbations

• For severe refractory asthma exacerbations, administer 2 g IV magnesium sulfate over 20 minutes 1
• Only recommended for patients with the most severe exacerbations, particularly those with FEV1 <30% predicted who have failed to respond to initial bronchodilator therapy 1
• Not recommended for mild or moderate asthma exacerbations as it shows no benefit in these populations 1

Cardiac Arrhythmias

• For torsades de pointes, administer 2 g IV magnesium sulfate as first-line therapy regardless of serum magnesium level 1
• If episodes of torsades persist, repeat infusions of 2 g may be necessary 1
• For paroxysmal atrial tachycardia, 3-4 g (30-40 mL of a 10% solution) administered IV over 30 seconds, but only if simpler measures have failed and there is no evidence of myocardial damage 2

Magnesium Deficiency

• For mild hypomagnesemia: 1 g (8.12 mEq) IM every 6 hours for four doses 2
• For severe hypomagnesemia: up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours, or 5 g (40 mEq) added to 1 L of IV fluid for slow infusion over 3 hours 2
• For maintenance in hospitalized patients: 2 g IV at least twice daily to maintain total serum magnesium above 2 mg/dL 3

Pre-eclampsia/Eclampsia

• Initial loading dose: 4-5 g IV in 250 mL of compatible IV fluid 2
• Maintenance: 1-2 g/hour by continuous IV infusion 2
• Alternative regimen: 4 g IV loading dose followed by 5 g IM every 4 hours in alternating buttocks 4
• Total daily dose should not exceed 30-40 g in 24 hours 2
• In severe renal insufficiency, maximum dosage is 20 g/48 hours with frequent serum magnesium monitoring 2

Administration Guidelines

Dilution and Rate

• IV solutions must be diluted to a concentration of 20% or less prior to administration 2
• Common diluents include 5% Dextrose Injection and 0.9% Sodium Chloride Injection 2
• Standard infusion rate should not exceed 150 mg/minute (1.5 mL of a 10% solution) except in severe eclampsia with seizures 2

Monitoring

• Monitor deep tendon reflexes, respiratory rate, urine output, and serum magnesium concentrations 2, 4
• Loss of patellar reflex occurs at plasma concentrations between 3.5-5 mmol/L, indicating impending toxicity 4
• Respiratory depression occurs at 5-6.5 mmol/L 4
• Cardiac conduction abnormalities occur at >7.5 mmol/L 4
• Cardiac arrest can occur at concentrations >12.5 mmol/L 4

Special Considerations

• In patients with renal insufficiency, reduce dosage and monitor serum magnesium levels more frequently 2
• Continuous use of magnesium sulfate in pregnancy beyond 5-7 days can cause fetal abnormalities 2
• For patients with heart failure and low serum magnesium, IV magnesium may decrease the frequency of ventricular arrhythmias, particularly in those with frequent PVCs (≥300/hr) 5

Common Pitfalls

• Failure to dilute concentrated solutions before IV administration can lead to serious adverse effects 2
• Inadequate monitoring of respiratory status, reflexes, and serum magnesium levels during administration 4
• Administering too infrequently - serum magnesium levels typically fall below 2.0 mg/dL within 12 hours after a 2 g dose 3
• Using magnesium in mild asthma exacerbations where no benefit has been demonstrated 1

Remember that magnesium administration must be carefully adjusted according to individual requirements and response, and should be discontinued as soon as the desired effect is obtained 2.