Core pharmacology

Onset Peak Duration

onset / duration immediate / 30 mins

Half-life

not known

Chemical Pharmaceutics

Inorganic Sulphate

Physiological effects

CVS:
▪ Peripheraly causes vasodilation and hypotension in high doses
▪ Slows rate of SA node impulse formation and prolongs SA conduction time.
▪ Prolonged PR interval and AV node effective refractory period.

RESP:
▪ Bronchodilation
• ↓ing cytosolic Ca2+ concentrations.
• ↓’d Mg levels → ↑ airway hyper-responsiveness
▪ Attenuates hypoxic pulmonary vasoconstriction

CNS:
▪ ↑s effect of CNS depressants and NMBs
▪ CNS depressand and exhibits anticonvulsant properties.
▪ Inhibition of catecholamine released from adrenergic nerve terminal and the adrenal medulla

OTHER
▪ Pain on IM injection
▪ Osmotic laxative
▪ Renal vasodilator and diuretic effects.
▪ ↓ed uterine tone and contractility
• Competes with Ca2+ at:
1. Membrane L-type VG Ca2+ channels → ↓ [Ca2+] entry
2. Low-affinity Ca2+ binding sites on SR membrane → ↓ [Ca2+] induced [Ca2+] release
• ↓ IC [Ca2+]

▪ ↑d placental perfusion
▪ ↑d clotting time of whole blood.
▪ ↓ed thrombin induced platelet aggregation.

Volume of distribution

Vd 8L/kg

Main Action

Unclear.
Possible explanation: Antagonises Ca2+ activity in uterine muscle cells → relaxation

Class Group

Electrolyte and mineral replacement agent with anticonvulsant and antiarrhythmic uses.

Introduction

Magnesium sulfate heptahydrate is an injectable magnesium salt. A 50% preparation contains 500 mg/mL, equivalent to 2 mmol of magnesium ions per mL.

Indications Uses

Treatment of acute hypomagnesaemia, prevention of hypomagnesaemia during parenteral nutrition, prevention and treatment of eclamptic seizures, and emergency treatment of selected arrhythmias such as torsades de pointes.

Mechanism of action

Magnesium is an essential cation involved in membrane stability, neuromuscular transmission and many enzyme reactions. Pharmacologic magnesium depresses neuromuscular transmission and can stabilise excitable membranes.

Absorption

For the concentrated injectable product, intravenous administration bypasses an absorption phase. Intramuscular administration is also permitted.

Protein binding

30% (Alb)

Metabolism

No conventional hepatic metabolism. Magnesium sulfate dissociates into magnesium and sulfate ions, with magnesium handled physiologically and eliminated mainly by the kidneys.

Route And Dose

May be administered intravenously or intramuscularly. For torsades de pointes, current Australian product information lists 2 g by slow intravenous infusion over about 20 minutes.

Presentation

Clear, colourless 50% magnesium sulfate heptahydrate concentrated injection. Current Australian products contain 500 mg/mL, equivalent to 2 mmol magnesium per mL.

Adverse Effects Toxicity

Toxicity: • Somnolence, areflexia, AV and intraventricular conduction disorder • Progressive muscular weakness • Cardiac arrest • Reversal of toxicity with Calcium administration

Excretion

Eliminated predominantly by the kidneys. Accumulation and toxicity are more likely in renal impairment.