Magnesium Sulfate

A to Z Drug Facts

 Action Magnesium has CNS depressant effect; prevents/controls seizures by blocking neuromuscular transmission and decreasing amount of acetylcholine liberated at end plate by motor nerve impulse. Orally it attracts/retains water in intestinal lumen, thereby increasing intraluminal pressure and inducing urge to defecate.

 Indications

Parenteral: Seizure prevention and control in severe pre-eclampsia or eclampsia without deleterious CNS depression in mother, fetus or neonate, and in convulsions associated with hypomagnesemia. Unlabeled use(s): Control of hypertension, encephalopathy and convulsions in children with acute nephritis; inhibition of premature labor; treatment of life-threatening ventricular arrhythmias; prevention and treatment of nutritional magnesium deficiency. Oral: Laxative.

 Contraindications Toxemia of pregnancy during 2 hr preceding delivery; MI; myocardial damage; heartblock.

 Route/Dosage

Severe Hypomagnesemia

ADULTS & OLDER CHILDREN: IM 1–5 g (2–10 ml of 50% solution)/day in divided doses until correction of serum magnesium. ADULTS & OLDER CHILDREN: IV Must be given very carefully due to risk of cardiac arrest. IV 1–4 g diluted to 10% or 20% solution at rate not to exceed 1.5 ml (of 10% solution) r its equivalent per minute.

Milder Hypomagnesemia and Electrolyte Supplement

ADULTS: IM 1–2 g of 50% solution every 6 hrs as needed. CHILDREN: IM 20–40 mg/kg of 20% solution.

Eclampsia

ADULTS: IM 1–2 g (as 25% or 50% solution) followed by IM 1 g q 30 min until relaxation is obtained. IV infusion may be given as IV 4–5 g in 250 ml D5W not to exceed 3 ml/min.

Anticonvulsant

ADULTS: IM 1–5 g as 25% or 50% solution. May be repeated up to 6 times/day prn. CHILDREN: IM 20–40 mg/kg as 20% solution.

Laxative

Usually one-time dose. ADULTS: PO 10–15 g. CHILDREN: PO 5–10 g.

 Interactions

Neuromuscular blocking agents: Potentiation of neuromuscular blockade. Nitrofurantoin: Decreased absorption of nitrofurantoin (oral magnesium). Penicillamine: Reduced penicillamine effects (oral magnesium). Tetracyclines: Decreased absorption of tetracyclines (oral magnesium). INCOMPATIBILITIES: Amphotericin B, calcium salts, clindamycin, dobutamine, doxycycline, hydrocortisone sodium succinate, nafcillin, sodium bicarbonate, tetracycline, thiopental, vasopressin.

 Lab Test Interferences None well documented.

 Adverse Reactions

CV: Cardiac depression; circulatory collapse; hypotension; cardiac arrest. CNS: CNS depression; depressed reflexes; muscle weakness; flaccid paralysis. META: Hypocalcemia. RESP: Respiratory paralysis. OTHER: Flushing; sweating; hypothermia; hypomagnesemia.

 Precautions

Pregnancy: Category A. Lactation: Excreted in breast milk during parenteral use. Eclampsia: Use IV form only for immediate control of life-threatening convulsions. Renal impairment: Use with caution; renal insufficiency may lead to magnesium intoxication.

 Administration/Storage

• When giving by IV route, use infusion pump. Deliver in separate line and do not mix with other IV drugs unless compatibility has been established.
• Administer loading dose followed by drip titrated to maintain therapeutic serum level with minimal side effects.
• When giving by IM route, use deep, slow injection. Rotate sites to prevent tissue irritation.
• Administer oral drug early in day on empty stomach.
• Dilute oral drug in glass of water containing ice chips or flavored with lemon or orange juice. Follow with full glass of water.

 Assessment/Interventions

• Obtain patient history, including any drug history and any known allergies. Note kidney impairment and hydration status.
• Ensure that baseline ECG, calcium, phosphorus, magnesium, BUN and creatinine levels have been obtained before beginning parenteral magnesium administration and monitor regularly. Serum magnesium levels should be obtained q 6 hr or when toxicity is suspected and as necessary when using for therapeutic benefit (eg, eclampsia, hypomagnesia). Notify physician if deviations from normal laboratory values occur.
• Obtain baseline vital signs, patellar reflex and neurologic assessment. Continue to assess throughout parenteral magnesium administration.
• Assess vital signs q 5–15 min while infusing IV loading dose, then q ½–1 hr.
• Assess bowel sounds, abdominal distention and bowel patterns when using oral magnesium as laxative.
• If drug is being given for prevention of preterm labor, assess fetal heart rate and uterine contractions prior to first dose and continuously during administration.
• If patient experiences symptoms of parenteral magnesium toxicity (including sweating, flushing, hypotension, respiratory depression, diminished reflexes, oliguria or depressed CNS function), notify physician and discontinue drug.
• Maintain safety precautions such as keeping bed in low position with side rails up and instructing patient not to rise without assistance after receiving parenteral magnesium.
• Maintain strict, hourly I&O and keep patient well hydrated. Urine output should be at least 100 ml q 4 hr when giving parenteral magnesium.
• Keep 10% calcium chloride at bedside for adult patients calcium gluconate for neonates and pediatric patients).
• Keep resuscitation equipment readily available.
• If drug is being given for pre-eclampsia, eclampsia or convulsions, maintain seizure precautions. Provide a quiet, nonstimulating environment.
• If drug was administered during labor, prepare for neonatal resuscitation. Monitor neonate for magnesium toxicity and administer IV calcium if hypermagnesemia is present. Observe for uterine atony following delivery.
• If used as laxative, utilize additional measures to prevent constipation including increased dietary fiber and fluids.

 Patient/Family Education

• Instruct patient to take oral drug with full glass of water and emphasize importance of maintaining sufficient fluid intake.
• If drug is prescribed for home laxative use, advise patient about correct administration, side effects and other measures to prevent constipation.
• Caution that drug is for short-term laxative use only. Explain that prolonged use can lead to dehydration and electrolyte imbalance.
• After parenteral administration, instruct patient to report any of these symptoms to physician: tremors, tetany, muscle cramps, thirst, sedation, confusion, muscle weakness, inability to urinate.

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Copyright © 2003 Facts and Comparisons
David S. Tatro
A to Z Drug Facts