Magnesium
Magnesium (Mg) is a divalent cation that is required for a variety of enzymatic reactions involving protein synthesis and carbohydrate metabolism. It is also an essential ion for proper neuromuscular functioning. Oral magnesium is widely available in over-the-counter antacids (eg, Maalox and Mylanta) and cathartics (milk of magnesia and magnesium citrate and sulfate). IV magnesium is used to treat toxemia of pregnancy, polymorphous ventricular tachycardia, refractory ventricular arrhythmias, and severe bronchospasm.
Mechanism of toxicity
Elevated serum magnesium concentrations act as a CNS depressant and block neuromuscular transmission by inhibiting acetylcholine release at the motor end-plate. Hypermagnesemia amplifies the response to neuromuscular blockers.
Magnesium also competitively antagonizes calcium at calcium channels, thus impeding calcium flux and impairing both muscle contraction and electric conduction.
Pharmacokinetics. The usual body content of magnesium is approximately 1700–2200 mEq in a 70-kg person; it is stored primarily in bone and intracellular fluids. The oral bioavailability ranges from 20–40% depending on the salt form. Although best modeled with two-compartment pharmacokinetics, the average volume of distribution is about 0.5 L/Kg, and the elimination half-life averages 4–5 hours in healthy adults. Normal kidney function is essential for clearance because 97% of ingested magnesium is eliminated in the urine.
Toxic dose. Although most acute or chronic overexposures do not result in hypermagnesemia, poisoning has been reported after IV overdose or massive oral or rectal overdose. Patients with renal insufficiency (creatinine clearance <30 mL/min) are at higher risk with standard doses because of impaired clearance.
Commonly available antacids (Maalox, Mylanta, and others) contain 12.5–37.5 mEq of magnesium per 15 mL (1 tablespoon), milk of magnesia contains about 40 mEq/15 mL, and magnesium sulfate (in Epsom salts and IV preparations) contains 8 mEq/g.
Ingestion of 200 g of magnesium sulfate caused coma in a young woman with normal renal function. Pediatric deaths have been reported after the use of Epsom salt enemas.
Clinical presentation. Orally administered magnesium causes diarrhea, usually within 3 hours. Repeated or excessive doses of magnesium-containing cathartics can cause serious fluid and electrolyte abnormalities. Moderate toxicity may cause nausea, vomiting, muscle weakness, and cutaneous flushing. Higher levels can cause cardiac conduction abnormalities, hypotension, and severe muscle weakness and lethargy. Very high levels can cause coma, respiratory arrest, and asystole (Table II–33
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