Magnesium Dosing Information for Health Care Workers

For the treatment of status asthmaticus†:

Intravenous dosage (magnesium sulfate):
Adults: One hundred thirty-five asthmatic patients aged 18 to 65 years of age with forced expiratory volume in one second (FEV1) less than 75% predicted were randomized to receive 2 g IV magnesium sulfate or placebo in addition to standardized emergency department procedure for acute asthma. Patients were further categorized as severe (baseline FEV1 < 25% predicted on presentation) or moderate (baseline FEV1 25 to 75% predicted on presentation). Intravenous magnesium sulfate decreased admission rates and significantly improved FEV1 at 120 min and 240 min in the severe magnesium-treated group but not in the moderate or placebo groups.1773

Children: Thirty-one children (mean age 11 yrs) with peak expiratory flow rates (PEFR) < 60% of predicted after receiving 3 nebulized doses of a beta-agonist were randomized to either IV magnesium or placebo. The magnesium-treated group’s PEFR improved at 110 minutes greater with magnesium than with placebo. Four of the 15 magnesium recipients were discharged while none of 16 placebo infusion recipients were discharged. The dose of magnesium was 25 mg/kg administered as an IV infusion in this study.1335

For the inhibition of uterine contraction in premature labor†:

Intravenous dosage (magnesium sulfate):
Adults: Initial doses of 4—6 g infused IV over 20—30 minutes as a loading dose, followed by maintenance IV infusions of 2—4 g/hour, delivered via controlled infusion pump device, until contractions cease. Continue infusion at the lowest effective dose for 12—24 hours as tolerated after cessation of contractions. To avoid circulatory fluid overload, the amount of IV fluids administered and the rate of administration should be observed. Deep tendon reflexes, respirations, urinary output, and serum magnesium concentrations should be monitored. Average magnesium concentrations associated with cessation of contractions were 5—8 mg/dl in clinical studies.
Oral dosage (magnesium chloride, magnesium gluconate, or magnesium oxide) for maintenance of uterine contraction stabilization:
Adults: 648—1200 mg PO of elemental magnesium per day in divided doses. Example regimens: magnesium chloride (128 mg elemental magnesium) PO every 4 hours; magnesium oxide (200 mg elemental magnesium) PO every 4 hours.

For use as a laxative to treat constipation or as bowel preparation prior to surgery or radiologic, proctoscopic, or sigmoidoscopic procedures

Oral dosage (magnesium oxide):
Adults: 2—4 g PO at bedtime with a full glass of water.
Oral dosage (magnesium sulfate):
Adults and children >= 12 years of age: The usual dosage is 10—30 g PO as a single dose or in divided doses.
Children 6—11 years of age: The usual dosage is 5—10 g PO as a single dose or in divided doses.
Children 2—5 years of age: The usual dosage is 2.5—5 g PO as a single dose or in divided doses.

For the treatment of sustained ventricular arrhythmias and/or torsade de pointes associated with magnesium depletion, antiarrhythmic agent toxicity, cardiac disease, or cardiac glycoside-induced arrhythmias due to either digoxin toxicity† or digitoxin toxicity†:

Intravenous dosage (magnesium sulfate):
Adults: Doses of 2—6 g IV administered over several minutes have been used. In a series of 12 patients with torsade de pointes due to various causes, a 2 g IV bolus was used and a response was achieved in 9 patients.342 This may be followed by an IV infusion at a rate of approximately 3—20 mg/minute for 5—48 hours depending on patient response and serum magnesium concentrations. Infusion rates of 30—50 mg/min for short periods (e.g., 90 minutes) have also been used.343

For the treatment of recurrent or refractory ventricular fibrillation† or ventricular tachycardia† during cardiopulmonary resuscitation†:

Intravenous dosage (magnesium sulfate):
Adults: 1—2 g diluted in 100 ml D5W and administered IV over 1—2 minutes.

For the treatment of dyspepsia:

Oral dosage (magnesium oxide):
Adults: 140 mg PO 3—4 times per day (capsules) or 400—840 mg PO per day (tablets).

Patients with renal impairment:
Dosage should be modified depending on clinical response and degree of renal impairment, but no quantitative recommendations are available.

Intermittent hemodialysis:
Dosage should be modified depending on clinical response, degree of renal impairment, and frequency of HD; no quantitative recommendations are available.

Patients with hepatic impairment:
Specific guidelines for dosage adjustments in hepatic impairment are not available; it appears that no dosage adjustments are needed.

Therapeutic Drug Monitoring:
•Desired serum concentrations: serum magnesium 1.4—2 mEq/L; may vary depending upon laboratory standards.
•Laboratory monitoring: serum magnesium, serum potassium, serum creatinine.

†non-FDA-approved indication

Contraindications

•AV block
•cardiac disease
•dehydration
•GI obstruction
•hypermagnesemia
•hypermagnesemia
•ileus
•renal impairment

Parenteral magnesium should be avoided in patients with AV block because it can exacerbate this condition. If parenteral treatment with magnesium salts is necessary, magnesium should be infused at a slower rate and magnesium levels monitored closely. Cardiac disease in the form of myocardial damage generally is considered a contraindication of magnesium salts; there is increasing evidence, however, that magnesium sulfate is useful during the acute phase of a myocardial infarction to prevent arrhythmias. Do not use magnesium salts in patients with preexisting hypermagnesemia.

Magnesium salts should be used with caution in patients with renal impairment. Magnesium salts are renally eliminated, so patients with renal impairment have an increased risk of developing magnesium toxicity from decreased excretion of magnesium. In patients with severe renal dysfunction, no more than 20 grams (162 mEq) of magnesium should be administered within a 48-hour period. Parenteral magnesium should be avoided in patients with a creatinine clearance of less than 20 mL/minute. Up to 30% of an orally administered dose is absorbed systemically.

When used as laxatives, oral magnesium salts should be avoided in patients with GI obstruction or ileus.

Use oral magnesium salts cautiously in dehydrated patients. Repeated administration of oral magnesium salts may lead to severe dehydration due to fluid losses via the gastrointestinal tract.

Parenteral magnesium sulfate is classified pregnancy category A and oral magnesium sulfate and other magnesium salts are classified pregnancy category B. Parenteral magnesium sulfate readily crosses the placenta and rapidly attains fetal serum concentrations that approximate those in the mother. Magnesium’s effects in the neonate may be similar to those in the mother and may include hypotonia, drowsiness, and respiratory depression. Bony abnormalities and congenital rickets have been reported in neonates born to mothers treated with parenteral magnesium sulfate for prolonged periods of time (4 to 13 weeks’ duration).

Magnesium sulfate is distributed into breast milk. Milk concentrations are approximately twice those in maternal serum. Problems with other magnesium salts have not been demonstrated with normal daily recommended amounts.

Drug Interactions

antacids
antidepressants
barbiturates
benzodiazepines
calcium salts
cardiac glycosides
cellulose sodium phosphate
diuretics
edetate disodium
ethanol
etidronate
general anesthetics
H1-blockers
laxatives
local anesthetics
neuromuscular blockers
nifedipine
opiate agonists
phenothiazines
polystyrene
quinolones
tetracyclines
vitamin D analogs

Parenteral magnesium sulfate can enhance the neuromuscular blocking effects of neuromuscular blockers such as d-tubocurarine, and succinylcholine. Caution should be exercised when using these agents concurrently.

Because of the CNS-depressant effects of magnesium sulfate, additive central-depressant effects can occur following concurrent administration with barbiturates, opiate agonists, H1-blockers, antidepressants, benzodiazepines, general anesthetics, local anesthetics, and phenothiazines.

Concurrent use of oral magnesium salts with sodium polystyrene sulfonate (Kayexalate®) is not recommended. Sodium polystyrene sulfonate may bind with magnesium salts administered orally; however, the risk of binding with oral magnesium salts may be less with rectal administration of sodium polystyrene sulfonate.

Excessive ethanol or glucose intake has been found to increase urinary excretion of magnesium. Avoid high intakes of ethanol and glucose while taking magnesium salts.

Concurrent use of intravenous calcium salts with parenteral magnesium sulfate can neutralize the effects of parenteral MgSO4; however, calcium gluconate and calcium gluceptate are used clinically to antagonize the toxic effects of hypermagnesemia. Simultaneous use of parenteral magnesium sulfate and intravenous calcium salts is also used in patients with post-parathyroidectomy “hungry bones” syndrome or tetany associated with hypocalcemia and hypomagnesemia. Oral calcium-containing medications may increase serum calcium or magnesium concentrations in susceptible patients, primarily patients with renal insufficiency.

Administration of oral magnesium salts with cellulose sodium phosphate or edetate disodium (EDTA) may result in binding of magnesium. Do not administer oral magnesium salts within 1 hour of cellulose sodium phosphate or edetate disodium.

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