From the Guidelines
Magnesium sulfate drips are typically administered at a rate of 1-2 grams per hour for maintenance therapy, following an initial loading dose of 4-6 grams given over 20-30 minutes, as recommended for seizure prophylaxis in preeclampsia 1. The absorption of intravenous magnesium sulfate is immediate as it bypasses the digestive system and enters directly into the bloodstream.
Key Considerations
- For seizure prophylaxis in preeclampsia, a common regimen includes a 4-6 gram loading dose followed by 1-2 grams per hour continuous infusion.
- The rate should be adjusted based on serum magnesium levels, which should be monitored every 4-6 hours, aiming for therapeutic levels of 4-7 mg/dL (2-3.5 mmol/L) 1.
- Renal function must be assessed before and during therapy as magnesium is primarily excreted by the kidneys, and dose adjustments are necessary for patients with renal impairment.
- Magnesium works by blocking calcium channels, resulting in smooth muscle relaxation and central nervous system depression, which explains its effectiveness in preventing seizures and treating conditions like preeclampsia and certain arrhythmias.
Clinical Context
- The use of magnesium sulfate in cardiac arrest is not recommended unless torsades de pointes is present, as stated in the 2010 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care 1.
- However, in the context of preeclampsia, magnesium sulfate is a recommended treatment for seizure prophylaxis, as emphasized by the Society for Maternal-Fetal Medicine Special Statement in 2022 1.
From the FDA Drug Label
In severe pre-eclampsia or eclampsia, the total initial dose is 10 to 14 g of magnesium sulfate. Intravenously, a dose of 4 to 5 g in 250 mL of 5% Dextrose Injection, USP or 0. 9% Sodium Chloride Injection, USP may be infused. Alternatively, after the initial IV dose, some clinicians administer 1 to 2 g/hour by constant IV infusion.
The rate of magnesium sulfate (MgSO4) drip is 1 to 2 g/hour by constant IV infusion. The absorption of magnesium sulfate is not directly addressed in the provided drug label. 2
From the Research
Magnesium Sulfate Drip Rate
- The rate of magnesium sulfate (MgSO4) drip varies, with some studies using a maintenance infusion of 1 gram per hour 3, 4, 5, while others use 1.5 grams per hour 3 or 2 grams per hour 3, 6.
- A study comparing 1 gram per hour and 2 grams per hour infusion rates found that the higher dose was more effective in achieving therapeutic levels of magnesium in overweight mothers with preeclampsia 3.
Absorption of Magnesium Sulfate
- After administration, about 40% of plasma magnesium is protein bound, and the unbound magnesium ion diffuses into the extravascular-extracellular space, into bone, and across the placenta and fetal membranes and into the fetus and amniotic fluid 4.
- Magnesium is almost exclusively excreted in the urine, with 90% of the dose excreted during the first 24 hours after an intravenous infusion of MgSO4 4.
- The pharmacokinetic profile of MgSO4 after intravenous administration can be described by a 2-compartment model with a rapid distribution phase, followed by a relative slow beta phase of elimination 4.
Therapeutic Levels of Magnesium
- The therapeutic level of magnesium is generally considered to be between 4.8 and 8.4 mg/dL 3, although some studies suggest that levels between 1.8 and 3.0 mmol/L may be effective for treating eclamptic convulsions 4.
- A study found that magnesium levels were inversely related to body mass index (BMI), with lower levels detected in patients with high BMIs 6.