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  1. Pharmacology

    1. Magnesium is the fourth most common cation in the body and the second most abundant intracellular cation after potassium. Magnesium plays an essential role as an enzymatic cofactor in a number of biochemical pathways, including energy production from adenosine triphosphate (ATP).

    2. Magnesium is a cofactor and has a direct effect on the Na+/K+-ATPase pump in both cardiac and nerve tissues. It may facilitate the influx of K+ and stabilize myocardial membrane potentials leading to correction of dispersed ventricular repolarizations. Further, magnesium has some calcium-blocking activity and may indirectly antagonize digoxin at the myocardial Na+/K+-ATPase pump.

    3. Magnesium modifies skeletal and smooth-muscle contractility. Infusions can cause vasodilation, hypotension, and bronchodilation. It can reduce or abolish seizures of toxemia.

    4. Magnesium is primarily an intracellular ion, and only 1% is in the extracellular fluid. A low serum Mg level (<1.2 mg/dL) may indicate a net body deficit of 5,000 mg or more.

    5. Hypomagnesemia can be associated with a number of acute or chronic disease processes (malabsorption, pancreatitis, diabetic ketoacidosis). It may result from chronic diuretic use, cisplatin administration, or alcoholism. It is a potentially serious, life-threatening consequence of hydrofluoric acid and ammonium bifluoride poisoning.

  2. Indications

    1. Replacement therapy for patients with hypomagnesemia.

    2. Torsade de pointes ventricular tachycardia (Table I–7).

    3. Other arrhythmias suspected to be related to hypomagnesemia. Magnesium may be helpful in selected patients with cardiac glycoside toxicity but is not a substitute for digoxin-specific Fab fragments.

    4. Prevention of torsade de pointes ventricular tachycardia in cases of medication or toxin-induced QTc prolongation.

    5. Barium ingestions. Magnesium sulfate can be used orally to convert soluble barium to insoluble, nonabsorbable barium sulfate if given early.

    6. Magnesium may have a role in the treatment of cardiac arrhythmias associated with aluminum and zinc phosphide intoxications.

  3. Contraindications

    1. Magnesium should be administered cautiously in patients with renal impairment to avoid the potential for serious hypermagnesemia.

    2. Heart block and bradycardia.

  4. Adverse effects

    1. Flushing, sweating, hypothermia.

    2. Depression of deep tendon reflexes, flaccid paralysis, respiratory paralysis.

    3. Depression of cardiac function, hypotension, bradycardia, general circulatory collapse (in particular with rapid administration).

    4. Gastrointestinal upset and diarrhea with oral administration.

    5. Use in pregnancy. FDA Category A. Magnesium sulfate is used commonly as an agent for premature labor (Introduction).

  5. Drug or laboratory interactions

    1. General CNS depressants. Additive effects may occur when CNS depressants are combined with magnesium infusions.

    2. Neuromuscular blocking agents. Concomitant administration of magnesium with neuromuscular blocking agents may enhance and prolong their effect. Dose adjustment may be needed to avoid prolonged respiratory depression.

  6. Dosage and method of administration (adults and children)

    1. Magnesium can be given orally, IV, or by IM injection. When it is given parenterally, the IV route is preferred and the sulfate salt generally is used.

    2. Magnesium dosing is highly empiric and guided by both the clinical response and the estimated total body deficit of Mg based on serum levels.

    3. Adults: Give 1 g (8.12 mEq) every 6 hours IV for ...

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