Magnesium Questions and Explanations

Author: Eddie Chen, DO

When do we treat hypomagnesemia?

Please keep in mind that only 0.3% of total body magnesium is found in the serum. Approximately 53% is in the bones, 27% in the muscle, and 19% is in the soft tissues. 90% of the intracellular magnesium is bound to organic matrices. The serum magnesium are 62% ionized, 33% protein bound (mostly albumin), and 5% complexed into anions such as citrate and phosphate. Thus, serum magnesium is not an accurate predictor of the body’s total magnesium stores, however, it does have a place in acute situations or for monitoring during magnesium replacement therapy or diuretic therapy.

If the patient is asymptomatic, there is no need to treat in most cases if the magnesium level is not below 1.0 in a normal individual. If the patient has certain disease states that causes a magnesium deficient state, such as malnutrition (alcoholism, prolonged fasting, TPN, short gut syndrome, malabsorption, fistulas), Bartter’s and Getelman’s syndromes, burns, pancreatitis, chronic diarrhea, SIADH, primary hyperaldosteroneism, post parathyroidectomy, medications(ampho, cisplantin, aminoglycosides, dig, foscarnet, cyclosporine, pentamidine, diuretics), DKA, and NG drainage, treatment is considered. Oral sustained-release preparations are recommended. Over 50% of IV magnesium is immediately excreted in the urine. Uptake by cells is slow and repletion requires sustained correction of the hypomagnesiumia. In our formulary , MagTabs ( 84 mgs of elemental Mg or 7 mEq) are only available. 6-8 tablets in divided doses per a day are required for severe depletion, while 2-4 tablets are required in divided doses a day for mild, asymptomatic disease. Other available preparations are Slow-Mag (64mg and 5.3 mEq of elemental Mg) and Mag-ox 400mg ( 240 mg and 20 mEq of elemental Mg)

For symptomatic patients, who are experiencing cardiac arrythmias, neuromuscular irritability with positive Chvostek and Trousseau signs, tremors, fasciculations, tetany, treat with IV and PO sustained release preparations if possible. Magnesium Sulphate 1 gm is equilvalent to 98 mg or 8 mEq of elemental Mg.

Suggestive sliding scale treatment with Magnesium Sulfate in a normal functioning kidney

The Relationship between magnesium and calcium are well documented. Absorption appears to be inter-related. Parathyroid Hormone (PTH) secretion is a common link. Hypomagnesemia appears to impair the hypocalcemic release of PTH. Magnesium is also required for the sensitivity of the target tissues to PTH and Vitamind D metabolities. PTH release also inhances magnesium reabsoption in the kidney, gut and release from bone.

Magnesium also acts as a non-competitive inhibitor of the inositol 1,4,5-triphosphate (IP3) gated calcium channel and of IP3 binding for intracellular metabolism.

References:
Fawcett et al. Magnesium: physiology and pharmacology. Br J Anaesth 1999; 83: 302-20.

Created: 01/2005
Updated: 07/10/2007