DESCRIPTION

Magnesium sulfate is used to treat torsades de pointes and hydrofluoric acid toxicity.

FORMS AND USES

Each 20 ml solution of magnesium sulfate for intravenous infusion contains 2 g of magnesium sulfate (16.2 mEq magnesium), with sodium hydroxide or sulfuric acid added to adjust pH.

MECHANISM OF ACTION

• In treating polymorphic ventricular tachycardia (torsade de pointes), magnesium prolongs the PR interval, the atrioventricular nodal effective refractory period, and the sinoatrial conduction time, thereby preventing the early depolarization of the ventricle, which would otherwise cause ventricular tachycardia.
• In treating hydrofluoric acid burns to the skin, each magnesium ion binds two ions of fluoride, forming a salt (MgF₂); this action may prevent complexation of fluoride and calcium, which can cause burns and cell death.
• In the blood, fluoride ions induce life-threatening hypocalcemia by binding calcium ions (CaF₂); magnesium complexes these fluoride ions and thereby may prevent systemic hypocalcemia and the resultant ventricular dysrhythmias.

DRUG AND DISEASE INTERACTIONS

• The concomitant use of magnesium and aminoglycoside can produce weakness and paralysis.
• Magnesium sulfate potentiates the effects of neuromuscular blocking agents.

PREGNANCY AND LACTATION

• US FDA Pregnancy Category B. Animal studies indicate no fetal risk, and there are no controlled human studies, or animal studies show an adverse fetal effect but well-controlled studies in pregnant women do not.
• High-dose magnesium therapy has been used for many years in obstetrics with an excellent safety record.

Section Outline:

• DESCRIPTION
• FORMS AND USES
• MECHANISM OF ACTION
• DRUG AND DISEASE INTERACTIONS
• PREGNANCY AND LACTATION

• Polymorphic ventricular tachycardia (torsade de pointes) and hemodynamically stable dysrhythmias are indications for the use of magnesium sulfate; unstable ventricular dysrhythmias should be treated according to Advanced Cardiac Life Support guidelines.
• Hydrofluoric acid exposure to the skin is treated symptomatically by topical application or local infiltration of magnesium sulfate, which is the second-line agent after calcium gluconate gel.
• In the setting of hydrogen fluoride ingestion, laboratory-determined hypocalcemia or any degree of its clinical symptoms should be immediately treated with calcium chloride or magnesium sulfate to prevent ventricular dysrhythmia.
• A history of a large ingestion may indicate use of magnesium infusion to prevent hypocalcemia. (Immediate consultation with a poison control center or medical toxicologist is recommended.)

CONTRAINDICATIONS

• The dose should be reduced in renal insufficiency; magnesium should not be used in an anuric or dialysis-dependent patient unless dialysis is available.
• History of anaphylaxis to magnesium sulfate contraindicates use.

ADVERSE EFFECTS

• Weakness in or loss of deep tendon reflexes may progress to respiratory insufficiency if large doses are needed.
• Vasodilation and hypotension may develop.
• Other overdose effects include nausea, vomiting, flushing, CNS depression, bradycardia, QRS interval widening, and QT interval prolongation.

Section Outline:

• CONTRAINDICATIONS
• ADVERSE EFFECTS

POLYMORPHIC VENTRICULAR TACHYCARDIA (TORSADE DE POINTES)

• An initial bolus of 2 to 4 g in adults, or 25 to 50 mg/kg in children, may be administered by intravenous push.
• The dose may be repeated in 10 to 15 minutes.
• A continuous infusion of 1 to 2 g/h should be started, titrated to antidysrhythmic effect.
• The patient should be monitored frequently for hyporeflexia and respiratory insufficiency.
• In reports of severe cases of refractory torsade de pointes, such large doses of magnesium were required to control the dysrhythmia that endotracheal intubation and mechanical ventilation were needed.

HYDROFLUORIC ACID EXPOSURE TO THE SKIN

• Magnesium salts have been used to treat dermal exposure to hydrofluoric acid, but there is less human experience than with calcium gluconate.
• Magnesium gluconate or acetate (10%) injected subcutaneously or intradermally has been effective in animal models.
• Extemporaneous preparations of magnesium gluconate gel were less effective than calcium gluconate gels in animal studies.

HYDROFLUORIC ACID EXPOSURE VIA INGESTION

• Due to the potentially catastrophic deterioration of patients who have ingested hydrogen fluoride, pretreatment with magnesium is a reasonable therapeutic option.
• After suicidal ingestion by an adult, an initial intravenous bolus of 2 to 4 g may be administered.
• Maintenance infusion is 1 to 2 g/h, although higher rates may be needed if ECG evidence of hypocalcemia develops or persists.
• The infusion should be continued for 1 to 2 hours and then should be tapered slowly while monitoring ECG for evidence of QT interval prolongation.
• Serum calcium levels (levels of ionized calcium, if available) should be followed as well.

Section Outline:

• POLYMORPHIC VENTRICULAR TACHYCARDIA (TORSADE DE POINTES)
• HYDROFLUORIC ACID EXPOSURE TO THE SKIN
• HYDROFLUORIC ACID EXPOSURE VIA INGESTION

• Renal function and urine output must be monitored in patients receiving high-dose magnesium.
• Frequent clinical monitoring of magnesium toxicity (decreased reflexes) should be used instead of serum magnesium levels.
• In severe cases, magnesium toxicity must be tolerated to reach the dose that suppresses ventricular dysrhythmia.

Toxic effect of other metals.

See Also: SECTION II, Ventricular Dysrhythmia chapter; and SECTION IV, Hydrofluoric Acid chapter.

RECOMMENDED READING

Vukmir RB. Torsade de pointes: a review. Am J Emerg Med 1991;9:250-255.

Author: Katherine M. Hurlbut

Reviewer: Luke Yip