Magnesium sulphate is the anticonvulsant of choice for pre-eclampsia prophylaxis and treatment.
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Indications for use
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- Magnesium Sulphate is used in pregnancy for women with pre-eclampsia for whom there is concern about the risk of eclampsia
- Firstline management of an eclamptic seizure.
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Presentation
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10mL and 50mL vials = 493 mg/mL (49.3% treat as ~ 50% solution) - use undiluted
(493 mg/mL = 2 mmol/mL of Mg2++ and 2mmol/ML SO4= )
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Route of administration
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IV infusion: Magnesium Sulphate is delivered by controlled infusion via a syringe pump.
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- The infusion should be connected via a multi-flow adapter to a peripheral line of Normal Saline 0.9% and monitored by an infusion pump. This is to reduce localized irritation and monitor fluid balance.
- The magnesium line should be labeled clearly and not used to inject other drugs.
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Note: MgSO4 infusions should only be administered in Birth Suites, Theatre or High Dependency Unit.
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Dose
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Prevention of seizures in pre-eclampsia
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Loading dose: using a 10mL vial of magnesium sulphate prepare 4gram (i.e. 8mL) of magnesium sulphate 50% in a 10mL syringe, configure the pump to accept the 10mL syringe and set the pump to 32mL an hour for 15 minutes.
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Maintenance rate: once the loading dose has been completed, re-set the pump to administer the maintenance rate of 50mL of 50% magnesium sulphate in a 50mL syringe with the pump set at 2mL/hour (1gram/hr) or as ordered.
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Treatment of eclamptic seizures
| The priorities are to terminate the seizure and prevent maternal and fetal hypoxia.
| Loading dose: prepare 4gram (i.e. 8mL) of magnesium sulphate 50% in a 10mL syringe, administer over 5 minutes (rate of 96mL/hr) to 10 minutes (rate of 48mL/hr), then continue maintenance rate (refer below).
| In urgent circumstances this dosage may be given as a slow IV push over 5 minutes. Follow with a 5mL normal saline flush.
| Note: the rapid infusion of magnesium in this setting requires ECG monitoring and the presence of an anaesthetist.
| Maintenance rate: 1gram/hr (i.e. 2mL/hr) or as prescribed by a medical officer until at least 24 hours post birth/delivery.
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Magnesium level monitoring
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Measurement of magnesium levels will facilitate management where there are signs of toxicity or in the presence of renal impairment.
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Serum magnesium concentrations should be checked every 6 hours in the antepartum and intrapartum phase (therapeutic level of magnesium sulphate: 1.7- 3.5 mmol/L).
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Magnesium is excreted by the kidneys and regular monitoring of serum levels should be conducted in women with oliguria (urine output <100mL over 4 hours) or urea >10mmol/L and those with renal impairment.
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| | | | | | | ECG changes (P-Q interval prolongation, widen QRS complex)
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| | reduction in deep tendon reflexes
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| | loss of deep tendon reflexes
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| | sinoatrial and atrioventricular blockade. Respiratory paralysis and CNS depression
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Note: If serum magnesium level is >3.5mmol/L, cease infusion and consult with obstetrician.
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Side effects
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- hypotension secondary to reductions in systemic vascular resistance
- facial flushing
- visual disturbances
- flushing at injection site
- chest pain
- nasal stuffiness
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NOTE: caution should be taken with any additional intravenous fluid administration to manage hypotension due to the potential risk of pulmonary oedema.
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The following may also occur:
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- ECG changes
- circulatory collapse
- gastro-intestinal upset
- urinary retention
- magnesium toxicity
- tissue necrosis at the injection site.
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Contraindications and precautions
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- Magnesium sulphate should be administered with caution in women being treated with cardiac glycosides/digitalis.
- Concurrent use of magnesium sulphate and CNS depressants may result in an enhanced CNS depressant effect.
- Heart block may occur if calcium gluconate is require to treat magnesium sulphate toxicity.
- As magnesium is excreted in urine, patients with impaired renal function or electrolyte imbalance should be managed cautiously and magnesium levels should be monitored closely because of the increased risk of magnesium toxicity.
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Monitoring
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Resuscitation and ventilator support should be available during and after administration of magnesium sulphate.
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During administration of the loading dose:
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- 5 minutely blood pressure and pulse (x 4 readings)
- observe for the development of side effects
- check patellar reflexes after completion of the loading dose.
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During maintenance infusion:
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- ½ (half) hourly blood pressure, pulse and respiratory rate (pre-treatment respiratory rate should be ≥ 16 per minute). Note: post birth these observations may be undertaken hourly if the woman’s condition is stable.
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- 1 hourly patellar reflexes
- 1 hourly urine measures, 4 hourly testing of urinary protein
- 2 hourly temperature
- continuous electronic fetal monitoring from 26 weeks gestation until clinical review/discussion by medical staff. Between 24- 26 weeks gestation, individualised management with regard to fetal monitoring will be considered
- maintain strict fluid balance chart.
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Record patellar reflexes as:
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Request magnesium level and review management if:
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- respiratory rate < 12 breaths/minute
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- urine output < 100mLs in 4 hours
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- loss of patellar reflexes
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Antidote for magnesium toxicity
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The following clinical signs of magnesium toxicity must be reviewed by a consultant obstetrician/anaesthetist:
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- urine output <100mL in 4 hours
- absent patellar reflexes
- respiratory depression.
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The antidote for magnesium toxicity is: calcium gluconate (10mL of 10% solution over 10 minutes) by slow intravenous injection. The patient requires ECG monitoring during and after administration because of the potential for cardiac arrhythmias.
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Resuscitation and ventilator support should be available during and after administration of both magnesium sulphate and calcium gluconate.
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Cease magnesium infusion in the following emergencies:
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- Respiratory arrest: and CALL ADULT CODE BLUE
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- Cardiac arrest: and CALL ADULT CODE BLUE
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Ceasing a magnesium infusion
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Magnesium infusion to continue for a minimum of 24 hours post birth/delivery (continue hourly clinical observation for 4 hours following the discontinuation of magnesium infusion). Cease according to medical orders.
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Postpartum magnesium levels may be adequately assessed clinically (reflexes, respiratory rate) unless there is renal impairment/oliguria when serum levels should be continued 6 hourly.
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References
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Refer to: References: Magnesium Sulphate.
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Also refer to the Women's CPG: Pre Eclampsia: Management.
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Revised and updated: 9 April 2009
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Royal Women's Hospital Clinical Practice Guidelines (CPGs) are intended to provide guidance to health care professionals, based on a thorough evaluation of research evidence, on the practical assessment and management of specific clinical issues or situations. The guidelines allow some flexibility on the part of the health care professional based on the needs of the specific patient for whom they are caring.
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