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An acronym for Major Adverse Cardiac Event e.g. myocardial infarction, congestive cardiac failure, cardiac arrest. See Cardiac risk for non-cardiac surgery.


Magnesium is the second-most abundant cation in the body after potassium. The normal blood level of magnesium is 0.75–1.0 mmol/L (1.5–2 mEq/L). About 50% of the body's magnesium is in bone and the rest is in muscle and other tissues. Less than 1% of the body's magnesium is in blood.

Effects of hypomagnesaemia

  1. Ventricular dysrhythmias.

  2. May lead to intracellular potassium depletion.

  3. Neuromuscular excitability, tetany, nystagmus and delirium.

Effects of hypermagnesaemia

  1. somnolence

  2. absent deep tendon reflexes, muscle paralysis, apnoea, respiratory arrest

  3. heart block, cardiac arrest



Magnesium sulfate (MgSO4) is presented in 5 mL ampoules containing 2.465 g of magnesium sulfate, which equates to 10 mmol or 20 mEq of magnesium ions. This is a 50% w/v solution. Magnesium therapy has many pharmacologically useful effects. These include:

  1. Antiarrhythmic effects—prolongs AV nodal conduction and suppresses conduction in accessory pathways.

  2. Reduction of catecholamine release.

  3. Antagonist at alpha-adrenergic receptors.

  4. Potent arteriolar vasodilator.

  5. Lung effects. MgSO4 promotes bronchodilation and improved lung function in patients with asthma. It reduces lung hyperinflation and improves respiratory muscle strength in stable COPD.

  6. Potentiates the effects of non-depolarising neuromuscular blocking drugs (NDNMBDs) and general anaesthetic drugs.

  7. Analgesic-sparing effect.1

  8. Uterine tocolytic drug.

In addition to hypomagnesaemia, indications for magnesium include:

  • Cardiac rhythm disturbances

  • Obstetric and asthmatic indications

  • Anaesthetic indications, including anaesthesia for phaeochromocytoma

  • Miscellaneous indications

  • anaesthetic

  • phaeochromocytoma

  • miscellaneous.

Magnesium deficiency

Magnesium sulfate 5 g in 1 L N/S over 3 h.

Antiarrhythmic indications

Magnesium is used as adjunctive therapy for certain types of arrhythmias and as a first-line drug for torsades de points. See Torsades de pointes. Examples include:

  1. Atrial fibrillation and multifocal atrial tachycardia.

  2. Prevention of AF after cardiac surgery.

  3. Refractory SVT, VT and VF. See Cardiac arrest.

  4. Ventricular arrhythmias during acute myocardial infarction.

Obstetric indications

  1. Preeclampsia/eclampsia. Magnesium therapy is used to prevent seizures and may assist in lowering blood pressure by reducing SVR. For dose see Preeclampsia/eclampsia.

  2. Tocolytic drug for treatment of preterm labour.


Magnesium is useful for treating asthma resistant to first-line drugs. See Asthma.

Anaesthetic indications

  1. Intraoperative magnesium as an adjuvant to postoperative analgesia due to antagonist effects on N-methyl-D-aspartate (NMDA) receptors.2 It is useful for opioid minimisation anaesthesia for indications such as bariatric surgery. See Bariatric surgery.

  2. Reduction of ...

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