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Magnesium Sulphate

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Dose and administration

Initial Dose

  1. 200mg/kg dose.
  2. Dilute to 8% concentrate in D5W. Infuse IV for 30 minutes. DO NOT exceed 150 mg/minute.

Continuous IV infusion

  1. 20 - 50mg/kg/hour. Dilute to 8% concentration in D5W.
  2. Usual dilution 4 grams magnesium sulphate to make 50ml with D5W = 80mg/ml
    0.25ml x weight = 20mg/kg/hour

Indications

Contraindications and precautions

  • Patients with heart block or myocardial damage.
  • CAUTION in patients with impaired renal function and/or electrolyte imbalance.

Clinical pharmacology

At high serum concentrations Mg is a potent vasodilator, muscle relaxant and sedative. Magnesium is the second most common intracellular cation. One half of body Mg is in bone, one-fourth is in muscle and one-fourth is in soft tissue. About 25% to 30% of total plasma Mg is bound to protein, 10% to 15% circulates in complex form and 55% to 60% is ionised.

Readily crosses the placenta and is distributed in mothers milk, however breastfeeding is not contraindicated. In the newborn Mg absorption occurs in the small intestine: 55% to 75% of ingested Mg normally is absorbed. The main route of Mg loss is through the kidneys. Serum magnesium concentrations are maintained within a narrow range. At the three major target organs for hormonal control of Mg homeostasis (bone, intestine and kidney) the close inter-relationship between Mg and Ca is evident.

An elimination half life of 43.2 hours has been reported in newborn infants whose mothers received magnesium sulphate. The elimination rate is the same for both preterm and term infants.

Possible adverse effects

  • ECG changes (prolongation of the atrio-ventricular conduction time, sinoatrial block and atrio-ventricular block).
  • Circulatory collapse, hypotension.
  • Gastrointestinal disturbances (diarrhoea, abdominal distension, absence of bowel sounds).
  • Urinary retention.
  • CNS depression (central sedation, muscle relaxation, hyporeflexia and decreased excitability).
  • Calcium and potassium disturbances.
  • Respiratory depression.

Special considerations

  • Anticipate change in calcium and phosphorus balance.
  • Drug interaction has been reported between magnesium sulphate and gentamicin (respiratory arrest).
  • Monitor serum magnesium and calcium levels.
  • Antidote for hypermagnesaemia is calcium gluconate.

Management of Magnesium administration

Description

Clear, colourless solution 49.3%, (493mg/ml in 5 ml ampoules).

Prescription

Slow IV infusion

Individual doses are charted on stat page of drug chart giving:

  • amount of drug to be added.
  • base fluid type and volume.
  • mg/kg/dose.
  • time to be infused.
  • total amount to be infused.

Continuous Infusions

Charted on fluid chart giving:

  • rate in ml/hour
  • dose in mg/kg/hour

Also charted on drug chart under continuous infusions giving:

  • amount of drug to be added
  • base fluid type and volume
  • mg/kg/ml

Administration

Continuous Infusion

  • Administered by a nurse with Neonatal IV Drug Certification.
  • Dilute prior to use to make an 8% concentration in D5W (80 mg/ml).
    • Draw up 8ml of 49.3% magnesium sulphate
    • Add to 42ml D5W
    • 50ml = 4000 mg in 50ml
                = 80 mg in 1ml
  • Filter prior to administration through a 5 micron filter.
  • Compatible with NS, D5W.
  • Incompatible with calcium and sodium bicarbonate.
  • Do NOT mix with other drugs, IV solutions, blood or blood products. Use a separate line for administration of magnesium sulphate.
  • Administer via a syringe pump.
  • Change fluid and tubing every 24 hours.

Observation and documentation

  • Monitor for adverse reactions.
  • Monitor blood pressure frequently. (Continuously if possible).
  • Continuous cardiorespiratory monitoring.
  • Document vital signs hourly and PRN.
  • Monitor fluid balance.
  • Observe for and document seizure activity.
  • Have resuscitation equipment and ventilatory support available.

Storage

  • Unopened - store at room temperature <25°C.
  • Discard ampoule after opening.
  • Do NOT refrigerate.
  • Continuous Infusion: Diluted solution stable at room temperature for 24 hours.

Selected references

  1. Pawlak RP, Herfert LAT. Drug administration in the NICU, 2nd Ed 1991. Neonatal Network p158-159.
  2. Abu-Osba YK, Golal O, Manasra K, Rejjal A. Treatment of severe persistent pulmonary hypertension of the newborn with magnesium sulphate. Arch Dis Child 1992 Jan 67 (1 SPECN); 31-35.
  3. Yeh TF Ed. Drug therapy in the neonate and small infant. Year book medical publishers Inc, Chicago, p266-273.
  4. Briggs GG, Freeman RK, Yaffe SJ. Drugs in pregnancy and lactation. A reference guide to fetal and neonatal risk. Williams and Wilkins 2nd Ed 1986; p256-257.

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Document Control

  • Date last published: 26 September 1996
  • Document type: Drug Dosage Guideline
  • Services responsible: ADHB Pharmacy, Neonatology
  • Editor: Sarah Bellhouse
  • Review frequency: 2 years