magnesium sulfate

Available forms
Available by prescription only
Injection for I.V. use: 4% (4, 20, and 40 g); 8% (4 g)
Parenteral injection: 10%, 12.5%, 50%
Parenteral injection for I.V. use: 1% (1 and 10 g) in D₅W; 2% (10 and 20 g) in D₅W

Indications and dosages
 Hypomagnesemic seizures. Adults: 1 g I.V. or I.M. Or, 1 to 2 g (as 10% solution) I.V. over 15 minutes; then 1 g I.M. q 4 to 6 hours, based on patient’s response and magnesium blood levels.
 Seizures secondary to hypomagnesemia in acute nephritis. Children: 0.2 ml/kg of 50% solution I.M. q 4 to 6 hours, p.r.n., or 100 to 200 mg/kg as a 1% to 3% solution I.V. given slowly over 1 hour with the first half of dose in the first 15 to 20 minutes. Also, 0.1 to 0.2 ml/kg of 20% solution (20 to 40 mg/kg) I.M. Adjust dosage according to magnesium blood levels and seizure response.
 Life-threatening arrhythmias. Adults: For patient with sustained ventricular tachycardia or torsades de pointes, give 1 to 6 g I.V. over several minutes followed by 3- to 20-mg/minute I.V. infusion for 5 to 48 hours depending on patient response and serum magnesium levels. For patient with paroxysmal atrial tachycardia, give 3 to 4 g I.V. over 30 seconds.
 Prevention or control of seizures in preeclampsia or eclampsia. Adults: Initially, 4 g I.V. in 250 ml D₅W and 4 to 5 g deep I.M. into each buttock (using undiluted 50% magnesium sulfate injection); then 4 to 5 g deep I.M. into alternate buttock q 4 hours, p.r.n. Or, 4 g I.V. as a loading dose followed by 1 to 3 g hourly as an I.V. infusion. Maximum daily dose is 30 to 40 g. Or, 8 to 15 g depending on weight of patient (8 g for 45-kg [100-lb] patient and 15 g for 90-kg [198-lb] patient); 4 g of magnesium sulfate (as magnesium sulfate injection or magnesium sulfate in D₅W) is given I.V. and the remaining dose is given I.M. using undiluted 50% magnesium sulfate injection. Dosage over next 24 hours based on serum level and urinary excretion of magnesium following initial dose. Later doses should be sufficient to replace magnesium excreted in urine, about 65% of the initial dose given I.M. q 6 hours.
≡ Dosage adjustment. For patients with severe renal insufficiency, maximum dose is 20 g in 48 hours.
 Barium poisoning, asthma. Adults: 1 to 2 g I.V.
 Management of preterm labor ◇. Adults: 4 to 6 g I.V. over 20 minutes as a loading dose, followed by maintenance infusions of 2 to 4 g/hour for 12 to 24 hours as tolerated after contractions subside.
 Mild hypomagnesemia. Adults: 1 to 3 g I.M. q 6 hours for four doses. Or, 5 g in 1 L of D₅W or dextrose 5% in normal saline solution I.V. over 3 hours.
 Reduction of CV morbidity and mortality caused by acute MI. Adults: 2 g I.V. over 5 to 15 minutes, followed by infusion of 18 g over 24 hours (12.5 mg/minute). Start therapy as soon as possible, and no longer than 6 hours after MI.

Pharmacodynamics
Anticonvulsant action: Magnesium sulfate has CNS and respiratory depressant effects. It acts peripherally, causing vasodilation; moderate doses cause flushing and sweating, whereas high doses cause hypotension. It prevents or controls seizures by blocking neuromuscular transmission.
 Drug is sometimes used in pregnant women to prevent or control preeclamptic or eclamptic seizures; it also is used to treat hypomagnesemic seizures in adults, and in children with acute nephritis.

Pharmacokinetics
Absorption: Given I.V. and I.M.
Distribution: Distributed widely throughout the body.
Metabolism: None.
Excretion: Excreted unchanged in urine; some appears in breast milk.

Contraindications and precautions
Parenteral administration of drug contraindicated in patients with heart block or myocardial damage. Use cautiously in patients with impaired renal function and in women in labor. Don’t give to patients with toxemia of pregnancy during the 2 hours preceding delivery.

Interactions
Drug-drug. Antidepressants, antipsychotics, anxiolytics, barbiturates, general anesthetics, hypnotics, narcotics: May increase CNS depressant effects. Reduced dosages may be required.
Cardiac glycosides: Changes in cardiac conduction in digitalized patients may lead to heart block if I.V. calcium is administered. Avoid use together, if possible.
Succinylcholine, tubocurarine: Potentiates and prolongs neuromuscular blocking action of these drugs. Use cautiously.
Drug-lifestyle. Alcohol use: Increases CNS depressant effects. Discourage alcohol use.

Adverse reactions
CNS: drowsiness, depressed reflexes, flaccid paralysis.
CV: hypotension, flushing, circulatory collapse, depressed cardiac function.
Metabolic: hypocalcemia.
Respiratory: respiratory paralysis.
Skin: diaphoresis.
Other: hypothermia.

Effects on lab test results
• May increase magnesium levels. May decrease calcium levels.

Overdose and treatment
Signs and symptoms of overdose with magnesium sulfate include a sharp decrease in blood pressure, respiratory paralysis, ECG changes (increased PR, QRS, and QT intervals), heart block, and asystole.
 Treatment requires artificial ventilation and I.V. calcium salt to reverse respiratory depression and heart block. Usual dose is 5 to 10 mEq of calcium (10 to 20 ml of a 10% calcium gluconate solution).

Special considerations
 ALERT Read label closely to ensure proper dosage and concentration.
• Effective anticonvulsant serum levels are 2.5 to 7.5 mEq/L.
• Inject I.V. bolus slowly to avoid respiratory or cardiac arrest.
• Administer by constant infusion pump if possible; maximum infusion rate is 150 mg/minute. Rapid drip causes feeling of heat.
• Discontinue drug as soon as needed effect is achieved.
• Level of magnesium sulfate for I.V. administration shouldn’t exceed 20% and rate shouldn’t exceed 150 mg/minute (1.5 ml of a 10% concentration or equivalent). For I.M. administration in adults, levels of 25% or 50% are generally used; in infants and children, levels shouldn’t exceed 20% (200 mg/ml).
• Respiratory rate must be 16 breaths per minute or more before each dose. Keep I.V. calcium gluconate 20% on hand.
• To calculate grams of magnesium in a percentage of solution: X% = X g/100 ml (for example, 25% = 25 g/100 ml = 250 mg/ml).
• Monitor serum magnesium level and clinical status to avoid overdose.
• When giving repeated doses, test knee jerk reflex before each dose; if absent, discontinue magnesium. Use of drug beyond this point risks respiratory center failure.
• When using drug as a tocolytic agent, monitor patient for magnesium toxicity and monitor I.V. infusion to avoid circulatory overload.
• After use in toxemic women within 24 hours before delivery, newborn requires observation for signs of magnesium toxicity, including neuromuscular and respiratory depression.
Breast-feeding patients
• Drug appears in breast milk. In patients with normal renal function, all magnesium sulfate is excreted within 24 hours of discontinuing drug. Recommend an alternative to breast-feeding during therapy.
Pediatric patients
• Drug isn’t indicated for use in children.

Patient education
• Inform patient about short-term need for drug, and address any questions or concerns.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use