magnesium sulfate (parenteral)

General

High Alert Medication: This medication bears a heightened risk of causing significant patient harm when it is used in error.

Pronunciation:
mag-nee-zhum sul-fate

Ther. Class.
mineral and electrolyte replacements/supplements

Pharm. Class.
minerals electrolytes

Indications

• Treatment/prevention of hypomagnesemia.
• Prevention and treatment of seizures associated with severe eclampsia or pre-eclampsia.

Unlabeled Use(s):

• Treatment of torsades de pointes.
• Adjunctive treatment for bronchodilation in moderate to severe acute asthma.

Action

• Essential for the activity of many enzymes.
• Plays an important role in neurotransmission and muscular excitability.

Therapeutic Effect(s):

• Replacement in deficiency states.
• Resolution of eclampsia.

Pharmacokinetics

Absorption: IV administration results in complete bioavailability; well absorbed from IM sites.

Distribution: Widely distributed. Crosses the placenta and is present in breast milk.

Metabolism and Excretion: Excreted primarily by the kidneys.

Half-life: Unknown.

TIME/ACTION PROFILE (anticonvulsant effect)

Contraindication/Precautions

Contraindicated in:

• Hypermagnesemia;
• Hypocalcemia;
• Anuria;
• Heart block.

Use Cautiously in:

• Renal impairment;
• OB:  Avoid using for more than 5–7 days for preterm labor (may ↑ risk of hypocalcemia and bone changes in newborn); avoid continuous use during active labor or within 2 hr of delivery due to potential for magnesium toxicity in newborn
• Geri:   Older adults may require ↓ dosage due to age-related ↓ in renal function.

Adverse Reactions/Side Effects

CV: arrhythmias, bradycardia, hypotension

Derm: flushing, sweating

GI: diarrhea

Metabolic: hypothermia

MS: muscle weakness

Neuro: drowsiness

Resp: ↓ respiratory rate

* CAPITALS indicate life-threatening.
Underline indicate most frequent.

Interactions

Drug-Drug

May potentiate  calcium channel blockers  and  neuromuscular blocking agents.

Route/Dosage

Treatment of Deficiency

IM IV (Adults): Severe deficiency– 8–12 g/day in divided doses;  mild deficiency– 1 g every 6 hr for 4 doses or 250 mg/kg over 4 hr.

IM IV (Children >1 mo): 25–50 mg/kg/dose every 4–6 hr for 3–4 doses, maximum single dose: 2 g.

IV Neonates: 25–50 mg/kg/dose every 8–12 hr for 2–3 doses.

Seizures Associated with Eclampsia/Pre-Eclampsia

IV (Adults): 4–6 g loading dose over 15–30 min at onset of labor or induction/cesarean delivery, followed by 1–2 g/hr continuous infusion for ≥24 hours after delivery (max infusion rate = 3 g/hr). If seizure occurs while receiving magnesium, an additional bolus of 2–4 g may be administered over ≥5 minutes. Max dose = 40 g/24 hr.

IM (Adults): 10 g loading dose administered as 5 g in each buttock at onset of labor or induction/cesarean delivery, followed by 5 g every 4 hours for ≥24 hours after delivery.

Torsades de Pointes

IV (Infants and Children): 25–50 mg/kg/dose, maximum dose: 2 g.

Bronchodilation

IV (Adults): 2 g single dose.

IV Children: 25 mg/kg/dose, maximum dose: 2 g.

Parenteral Nutrition

IV (Adults): 4–24 mEq/day.

IV Children: 0.25–0.5 mEq/kg/day.

Availability (generic available)

Solution for injection (8.1 mEq Mg/g): 500 mg/mL (50%)

Premixed infusion: 1 g/100 mL, 2 g/50 mL, 4 g/50 mL, 4 g/100 mL, 20 g/500 mL, 40 g/1000 mL

Assessment

• Hypomagnesemia/Anticonvulsant: Monitor pulse, BP, respirations, and ECG frequently during administration of parenteral magnesium sulfate. Respirations should be at least 16/min before each dose.
• Monitor neurologic status before and throughout therapy. Institute seizure precautions. Patellar reflex (knee jerk) should be tested before each parenteral dose of magnesium sulfate. If response is absent, no additional doses should be administered until positive response is obtained.
• Monitor newborn for hypotension, hyporeflexia, and respiratory depression if mother has received magnesium sulfate.
• Monitor intake and output ratios. Urine output should be maintained at a level of at least 100 mL/4 hr.

Lab Test Considerations:

Monitor serum magnesium levels and renal function periodically during administration of parenteral magnesium sulfate.

Implementation

• High Alert: Accidental overdose of IV magnesium has resulted in serious patient harm and death. Have second practitioner independently double-check original order, dose calculations, and infusion pump settings. Do not confuse milligram (mg), gram (g), or milliequivalent (mEq) doses.
• IM IM route should only be used when unable to establish venous access. Administer deep IM into gluteal sites. Administer subsequent injections in alternate sides. Dilute to a concentration of 200 mg/mL prior to injection.

Patient/Family Teaching

• Explain purpose of medication to patient and family.
• Rep:   Monitor vital signs, oxygen saturation, respiration, deep tendon reflexes, level of consciousness, fetal heart rate, maternal uterine activity, and renal function when used during pregnancy. Monitor magnesium concentrations every 4 hr in patients with renal dysfunction (every 2 hr if serum magnesium is elevated). Breast milk concentrations are increased for only ~24 hr after end of therapy. When used for prevention of seizures, magnesium sulfate is considered compatible with breastfeeding.

Evaluation/Desired Outcomes

• Normal serum magnesium concentrations.
• Control of seizures associated with toxemias of pregnancy.