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Magnesium depletion has been described as "the most underdiagnosed electrolyte abnormality in current medical practice"
Important for electrically excitable tissues and smooth muscle cells, Mg is mostly located in bone, muscle and soft tissue. Because only 1% is located in blood, your patient can be Mg depleted with normal serum levels.
65% of ICU patients are magnesium depleted (and may not be hypomagnesemic). Because labs are unreliable, consider predisposing causes, such as diuretics, antibiotics (aminoglycosides, amphotericin), digitalis, diarrhea, chronic alcohol abuse, diabetes and acute MI (80% of AMI patients will have magnesium depletion in the first 48 hours).
Mg depletion is typically accompanied by depletion of other electrolytes (K, Phos, Ca), and can cause arrhythmias (especially torsades) and promote digitalis cardiotoxicity.
Hypermagnesemia is less common, and can be caused by hemolysis, renal insufficiency, DKA, adrenal insufficiency and lithium toxicity. Clinical findings include hyporeflexia, prolonged AV conduction, heart block and cardiac arrest. Treatment includes fluid and furosemide, calcium gluconate and dialysis.
References
Marino P. The ICU Book. 3rd ed. Lippincott Williams & Wilkins, 2007:625-638.