Magnesium

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Normal values: 1.5 to 2.5 mEq/L (0.7 to 1.2 mmol/L)

Physiology:

Magnesium has a widespread physiological role in neuromuscular functions and enzymatic systems. About 60% is in bone, most of the remainder is within cells. Because only about 1% is in the extracellular fluid, serum levels may not be predictive of deficiency. Factors affecting calcium homeostasis also affect magnesium homeostasis. Magnesium movement generally follows that of phosphate and is the opposite of calcium.

Empiric daily requirement: 7 to 15 mEq/day

For positive balance: 15 to 45 mEq/day

Hypomagnesemia: levels below 1.5 mEq/L

Causes:

•	Decreased intake due to malabsorption or malnutrition or prolonged administration of magnesium-deficient IV or TPN solution.

•	Decreased absorption may occur with lower GI tract disease, short bowel syndrome or pancreatic insufficiency.

•	Increased GI output: vomiting, diarrhea, fistulas, and nasogastric suctioning.

•	Increased renal output: diuretics, hypoparathyroidism.

•	Chronic alcoholism.

•	Medications: diuretics, laxatives, aminoglycosides and cisplatin may cause magnesium depletion.

Assessment:

Magnesium depletion usually is associated with neuromuscular symptoms such as weakness, tremor, tetany and increased reflexes. Magnesium also affects the CNS, symptoms include disorientation, convulsions, mood changes, disorientation, agitation, hallucinations and coma. Cardiac dysrhythmias (ventricular tachycardia and fibrillation) are the most serious effects of magnesium deficiency.

Labs::

Serum magnesium below 1.5 mEq/L.

Correction:

Magnesium sulfate (IV or IM) is the drug of choice. The usual IV dose is 5 grams in 5% dextrose in water to be infused over 3 hours. Prior to IV administration the respiration rate should be at least 16 and urine output 100 ml over the preceding 4 hours. During administration monitor vital signs, respiratory rate and urine output.

TPN correction:

Increase the magnesium content of the TPN solution to a maximum of 12 mEq per liter. The total daily dosage of magnesium should not exceed 36 mEq.

Hypermagnesemia: Levels above 2.5 mEq/L

Causes:

•	Decreased excretion due to renal disease.

•	Excess intake from IV administration, antacids or laxatives.

•	Hepatitis.

•	Addison's disease.

Assessment:

Magnesium excess is associated with neuromuscular signs and symptoms (muscle weakness, loss of deep tendon reflexes). Bradycardia, drowsiness and decreased clotting mechanisms are associated with magnesium excess. At levels over 15 mEq/L respiratory paralysis and complete heart block occur.

Labs:

Serum magnesium levels above 2.5 mEq/L, hypocalcemia.

Correction:

Administer diuretics and 0.45% sodium chloride to help excrete the excess magnesium. For severe hypermagnesemia give calcium gluconate intravenously to antagonize the action of magnesium, however this is only a temporary symptomatic measure which does not correct the underlying magnesium excess.

TPN correction:

Discontinue the current TPN infusion if it contains magnesium and begin an infusion of D10NS at the current TPN infusion rate. Reorder a TPN solution without magnesium and continue to hold magnesium from the TPN solution and all other intravenous fluids until the serum magnesium returns to normal.