While I think repleting potassium is pretty self-explanatory (both PO and IV forms replete about 0.1 mEq in a dose dependent form, so 40 mEq could replete 0.4 of potassium), magnesium is a little more mystifying. That is, until I found a very helpful article from CliniCalc. You should read it, but here are the major points:
- In acute situations (on the wards/inpatient side) you should give IV repletion. PO repletion is better for chronic repletion, and no matter WHAT the dose of PO magnesium is, it will only replete about 0.1 mg/dL.
- IV magnesium will replete by 0.1 mg/dL per 1 gram given (so if you give 4 grams of IV magnesium, expect 0.4 mg/dL repletion).
- Although IV repletion is better, it also takes a long time to run, so if you have a patient who has many other IV medications and there aren’t enough lines, you might need to just go with PO repletion instead.