Magnesium

This is a review of how magnesium is used in the obstetric population, typical dosing, contraindications, signs of overdose, and its antidote.

There is a significant amount of data to support the use of magnesium sulfate as seizure prophylaxis for pregnant patients who have preeclampsia with severe features and eclampsia - about a 50% risk reduction. There is no consensus for the use of magnesium in patients with gestational HTN or preeclampsia without severe features.

Data about ideal dosage is not as concrete. Typical dosing involves a 4-6g bolus over 20 minutes followed by a maintenance dose of 1-2 g/hour that continues until 24 hours postpartum. Patient’s undergoing a c-section should have the magnesium infusion continued intraoperatively. Therapeutic range for serum magnesium is 4.8-9.6 mg/dL (4-8 mEq/L).

Once a patient is started on a magnesium infusion, there are two major markers to follow: deep tendon reflexes and urine output.

Supratherapeutic levels of magnesium can lead to potentially dangerous side effects. As serum levels rise, patients will first lose their patellar reflexes. Then you may see respiratory paralysis followed by cardiac arrest. Since magnesium is excreted by the kidneys, a drop in urine output could result in an accumulation of serum magnesium.

Contraindications to magnesium include: myasthenia gravis, hypocalcemia, moderate-to-severe renal failure, cardiac ischemia, heart block, or myocarditis.

The antidote for a magnesium overdose is 10mL of calcium gluconate 10% solution over 3 minutes (a quick bolus can cause acute hypertension). Furosemide can also be useful to accelerate the rate of urinary excretion.