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This Manual has been designed for use in the NICU at London Health Sciences Centre (LHSC), London, Ontario, Canada, and represents clinical practice at this institution. The information contained within the Manual may not be applicable to other centres. If users of this Manual are not familiar with a drug, it is recommended that the official monograph be consulted before it is prescribed and administered. Any user of this information is advised that the contributors, Editor and LHSC are not responsible for any errors or omissions, and / or any consequences arising from the use of the information in this Manual.

Magnesium Sulfate
(MgSO4)

Indication

  • the treatment of hypomagnesemia (the predominant clinical signs and symptoms of hypomagnesemia are neurological (neuromuscular excitability, clonic twitching and tremors)

Clinical Pharmacology

  • after potassium, magnesium (Mg) is the second most abundant intracellular cation
  • magnesium is an important regulator of cell processes and has been identified as a cofactor in more than 300 enzymatic reactions involving energy metabolism and protein and nucleic acid synthesis
  • total body content
    • 60% in the bone (a slow turnover pool)
    • 39% is distributed equally between muscle and non-muscular tissue in the intra-cellular space
    • 1 to 2 % is found extra-cellularly (1/3 protein bound; 2/3 free, ionized) in the circulation; it is this free, ionized fraction that is available for biochemical processes as well as filtration by the kidney

  • magnesium is necessary for both the secretion and function of parathyroid hormone (PTH); PTH is important in regulating the serum calcium concentration; thus, hypomagnesemia is frequently associated with hypocalcemia
  • hypokalemia is also associated with hypomagnesemia (due to magnesium's effect on the renal tubules)
  • normal (SJHC standard) serum concentrations of magnesium are 0.66 to 1.15 mmol/L
  • the kidney is the primary regulator of extracellular magnesium concentration

Side Effects

Effects are related to the serum magnesium level

> 1.2 mmol/L:CNS depression, blocked peripheral neuromuscular transmission leading to anticonvulsant effect
> 2 mmol/L:Depressed deep tendon reflexes, flushing, hypotension, sweating
> 4.8 mmol/L:Respiratory paralysis, complete heart block, deep tendon reflexes may disappear
  • hypotension may develop with rapid IV administration and/or high doses
  • cardiac arrhythmias, respiratory/CNS depression are also possible during the infusion
  • flushing and sweating may occur with moderate doses ( a result of magnesium's peripherally acting vasodilatory effect)
  • diarrhea, abdominal cramps, muscle weakness and hypothermia may also occur
  • both the CNS depression and peripheral neuromuscular blockade produced by hypermagnesemia can be reversed with an infusion of calcium

Overdosage Treatment

  • magnesium intoxication is manifested by a sharp drop in blood pressure and respiratory paralysis
  • treat with IV calcium gluconate (same dose as for cardiac arrest)

Monitoring

  • continuously monitor heart rate, blood pressure and respiratory rate during the infusion
  • monitor serum Mg levels to avoid overdosage
  • also monitor serum Ca levels which may become depressed following the administration of Mg
  • renal function
  • deep tendon reflexes

Contraindications

seriously impaired renal function
myocardial damage
heart block
colostomy or ileostomy
intestinal obstruction, impaction or
perforation
appendicitis
abdominal pain

Precautions

  • use with caution in patients with impaired renal function (accumulation may lead to hypermagnesemia) and in patients who are receiving digoxin

Drug Interactions

  • CNS depressants (eg morphine, phenobarbital, midazolam) : additive effects
  • Neuromuscular blocking agents (eg vecuronium) : additive effects
  • Cardiac glycosides (eg. digoxin): may alter cardiac conduction leading to heart block
  • Aminoglycosides : increased risk of respiratory arrest

Dosage and Administration

Conversion Factor:

500 mg MgSO4 = 2 mmol Mg = 4mEq Mg
2 mmol/mL Mg = 50% MgSO4solution

Hypomagnesemia:

  • 25 to 100 mg/kg MgSO4, (equivalent to 0.05 to 0.2 mL/kg of a 50% solution) slow IV over 2 to 4 hours (or IM)
  • in severe situations 1/2 of the dose may be infused over the first 15 to 20 minutes (see Side Effects)

    reduce dose in renal impairment

  • maximum concentration for IV or IM dose is 200 mg/mL magnesium sulphate (therefore, solution from the vial MUST BE DILUTED before administration)
  • may repeat q 6-12h for 2 to 4 doses (until symptoms resolve and serum Mg is within normal limits (0.66-1.15 mmol/L); monitor serum Mg levels and test knee jerk reflexes prior to each dose

Seizures and Hypertension: (** Children's Dose **)

  • 25 to 100 mg/kg MgSO4 slow IV over 2 to 4 hours (or IM)
         (note that this is a child's dose)

Stability in IV Solutions:

  • stable in dextrose and saline solutions
  • compatible with both amino acid / dextrose component of TPN as well as with lipid component

Available:

  • 10 mL vial of magnesium sulphate 50%
  • each mL contains 500 mg magnesium sulphate (=2 mmol Mg=4mEq Mg)

500 mg MgSO4 = 2 mmol Mg = 4mEq Mg

References:

  1. McEvoy G K (ed): AHFS Drug Information, American Society of Hospital Pharmacists, 1999.
  2. Taketomo CK, Hodding JH and Kraus DM: Pediatric Dosage Handbook: 3rd Ed., Lexi-Comp Inc., Cleveland, 1996.
  3. Krogh CME et al (ed): Compendium of Pharmaceuticals and Specialties, Canadian Pharmaceutical Association, 2000.
  4. Zenk KE, Sills JH and Koeppel RM: Neonatal Medications and Nutrition - A Comprehensive Guide, NICU INK, Santa Rosa, Ca, 1999.
  5. Phelps SJ and Hak EB : Guidelines for Administration of Intravenous Medications to Pediatric Patients, American Society of Health-System Pharmacists, Bethesda,MD, 5th Ed, 1996.
  6. 6. Perlman M, Kirpalani HM and Moore AM : residents Handbook of Neonatology, The Hospital for Sick Children, BC Decker Inc., 1999

Updated: March 2003

New : September 2000
Christopher Reynaert, BScPhm, contributed to this monograph.


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