MAGNESIUM SULPHATE

Magnesium

Reviewed by Dorothy Cooper
September 1996
Administration Newborn Drug Protocol Index Newborn Services Home Page

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

  1. Seizures refactory to other anticonvulsant therapy.
  2. Hypomagnesaemia.
  3. Severe persistent pulmonary hypertension of the newborn unresponsive to other vasodilation management.

Contraindications and Precautions

  1. Patients with heart block or myocardial damage.
  2. 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

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

Special Considerations

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