Magnesium Sulphate

ADMINISTRATION ROUTES:

IV

ALTERNATIVE NAMES:

Magnesium Sulphate injection BP 49.3%

ICU INDICATIONS:

1. Hypomagnesaemia
2. Atrial arrhythmias, Torsades de Pointes and ventricular ectopy
3. Eclampsia
4. Asthma

PRESENTATION AND ADMINISTRATION:

IV:

Injection 49.3% in 5 mL solution contains 10 mmol of magnesium sulphate (Each mL contains 493 mg magnesium sulfate heptahydrate)

Store at room temperature

May be administered by direct IV injection only if:

• concentration injected does not exceed 20% AND
• rate of infusion does not exceed 150 mg/min (0.75 mL/min of 20% solution)

A 20% solution can be made by diluting 5 mL of 49.3% solution with at least 12.5 mL of compatible IV fluid.

In an emergency, to treat Torsade de Pointes, 10 mmol can be administered by direct IV injection over 1 - 2 mins (preferably via a central line).

The usual means of administration in ICU is by intermittent infusion. The required dose should be added to 50 - 500 mL of compatible IV fluid and mixed thoroughly before being infused over 20 - 60 mins at a rate no greater than 150 mg/min.

Compatible with the following IV fluids:

Normal saline, Glucose and sodium chloride, 5% & 10% Dextrose, Hartmann's

DOSAGE:

IV:

Hypomagnesaemia, atrial arrhythmias, ventricular ectopy:

10 - 20 mmol of magnesium sulphate over 20 - 60 mins

Eclampsia:

Loading dose:

20 mmol of magnesium sulphate in 100 mLs Normal saline over 20 mins

Maintenance infusion:

Add 40 mmol of magnesium sulphate to 500 mL Normal saline

For patients weighing <55 kg, begin infusion at 50 mL/hr (approximately 1 gm/hr)

For patients weighing ≥55 kg, begin infusion at 75 mL/hr (approximately 1.5 gm/hr)

Adjust to target serum magnesium concentration of 2.0 - 3.0 mmol/L

Torsades de Pointes:

10 mmol magnesium sulphate over 1-2 mins then 20 mmol over 6 hours

*Severe asthma:

Bolus:

5-10 mmol magnesium sulphate given over 20 mins

Continuous infusion:

Add 100 mmol magnesium sulphate to 100 mL of compatible IV fluid and commence at 5 mL/hr (5 mmol/hr)

DOSAGE IN RENAL FAILURE AND RENAL REPLACEMENT THERAPY:

Specific recommendations for dosage in renal failure are not available; however, patients with renal failure are at increased risk of magnesium toxicity (particularly when infusions are used) and dose reduction may be required.

DOSAGE IN PAEDIATRICS:

IV:

Hypomagnesaemia:

0.2 mL/kg

Asthma:

Bolus:

Dilute 0.1 mL/kg (approx 50 mg/kg) of magnesium sulphate with 20 mLs Normal saline & give over 20 mins

Maximum dose 5 mLs (2.5 g) of magnesium sulphate

CLINICAL PHARMACOLOGY:

Magnesium is the second most plentiful cation of the intracellular fluids. It is essential for the activity of many enzyme systems and plays an important role with regard to neurochemical transmission and muscular excitability.

CONTRAINDICATIONS:

1. Heart block (unless pacing wires or internal pacemaker are present)

WARNINGS:

Hypermagnesaemia

The principal hazard in parenteral magnesium therapy is the production of abnormally high levels of magnesium in the plasma. The most immediate danger to life is respiratory depression. Calcium chloride or calcium gluconate provide an effective antidote to life threatening hypermagnesaemia.

Toxicity in the newborn

When Magnesium Sulphate, is administered intravenously by a continuous infusion for longer than 24 hours before delivery, the possibility of the baby's showing signs of neuromuscular or respiratory depression of the newborn should be considered, since foetal toxicity can occur. A baby with hypermagnesemia my require resuscitation and assisted ventilation.

PRECAUTIONS:

General:

As magnesium is excreted almost entirely by the kidneys, it should be given very cautiously in the presence of serious impairment of renal function

Laboratory Tests:

Patients with eclampsia treated with magnesium by infusion should have serum magnesium levels measured 6 hourly until stability is achieved. The target serum magnesium concentration in eclampsia is 2.0-3.0 mmol/L

Drug/Laboratory Test Interactions:

None known

IMPORTANT DRUG INTERACTIONS IN ICU:

When barbiturates, narcotics, hypnotics (or systemic anaesthetics), or other central nervous system depressants are to be given in conjunction with magnesium, their dosage should be adjusted with caution because of the additive central nervous system depressant effects of magnesium.

Magnesium may also potentiate the action of both depolarising and non-depolarising neuromuscular blocking agents resuling in a reduced onset time and prolonged duration of action. Muscle relaxant doses should be reduced accordingly by up to 50%.

Loop diuretics increase renal excretion of magnesium.

ADVERSE REACTIONS:

The most clinically signficant adverse reactions are related to high plasma levels of magnesium and include flushing, sweating, hypotension, circulatory collapse, and cardiac and central nervous system depression. Respiratory depression is the most life-threatening effect.

Adverse reactions roughly correlate with serum magnesium levels thus: