Lets talk about a common case of vanishing Magnesium levels in blood – A patient admitted with dehydration & hypokalemia had serum Magnesium levels at 1.2, after an IV Magnesium Sulfate bolus (2 grams over 4 hours), levels were rechecked 6 hours later , came back as 2.0, next morning it was back to to 1.4 . Where did all that Magnesium vanish !! Serum Magnesium levels, like most humans, do not like to be pushed hard- An IV Magnesium bolus can raise serum Mag levels pretty good over a few hours – but is a temporary affair. The rapid bump in blood magnesium levels makes kidneys pee out the excess spike. The remaining magnesium then slowly seeps into tissues, especially bones, until a new equilibrium between blood and tissues is achieved.
So the point of this post is this: After Intravenous Magnesium infusion is given as a bolus, a level recheck done too soon can give you a false sense of success due a temporary bump. The slower the infusion, the more sustained the rise in magnesium level will be. Raising levels fast urgently and rechecking to see if goal levels are maintained can be important for situations like Torsades or other arrhythmias with very low Mag levels, Eclampsia, etc. – but in other less urgent situations, unlike potassium levels, Magnesium levels should ideally be rechecked 2 days post-infusion, after the renal-flush and body-equilibrium is done, to give you a more true picture.
I usually order an intravenous Magnesium Sulfate 2 to 4 grams IV push over 4 to 6 hours and start the patient on oral Magnesium supplement twice daily from the next day, and recheck Mag levels after 2 days!
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