Should we really be replacing potassium and magnesium all the time? Eh, the evidence isn't great but we usually do it anyways to avoid getting paged constantly and "just in case". It's probably most useful in patients with severe systolic heart failure undergoing aggressive diuresis since they would be at highest risk for arrhythmias. Here's some background:
Common clinical practice suggests replacing K > 4 and Mg > 2, especially in patients at risk for cardiac arrhythmias.
2000 Practice Guidelines = JAMA Article
However, evidence for replacing to these goals is very poor.
Background = Potassium Repletion: Why So Common in Hospitalized Heart Failure Patients?
Journal Article = O'Sullivan et al (JAMA Dec 2019)
Evidence shows that extremes (K < 3.5 or > 4.5) are associated with worse outcomes. Not just a random K of 3.6 or 3.8. So we probably are over-replacing electrolytes in patients with very mildly decreased potassium and no risk factors for arrhythmia.
Journal Article = Patel et al (Eur Heart J Acute Cardiovascular Care May 2017)
Potassium
Give 10 mEq for every 0.1 you want the potassium to go up.
Example: K 3.6 and you want it to be 4.0 = give 40 mEq IV or PO potassium (PO preferred)
Example: K 3.2 and you want it to be 4.0 = give 40 mEq IV potassium + 40 mEq PO potassium
If patient is severely hypokalemic (K < 3.0), they will need more than 10 mEq per 0.1 increase.
* If patient has AKI or CKD, give LESS potassium to avoid hyperkalemia (probably 50% or less of what you would normally give)!
Per /u/renegaderaptor, K is also absorbed very readily through the gut, and PO K is much less expensive than IV K. So unless the pt is critically ill, or the labs are critically low, PO K > IV K in most cases. It’s also best to give in divided doses orally if you go over ~40 mEq (so PO 30 & 30 four hrs later would be good for a person with K 3.4). Also, the tab is sustained release (that’s what SR is), which means it won’t act rapidly and won’t be reflected in the labs for several (I think 12ish?) hours. If you’re going to be checking a K soon after, or want rapid (but not emergent) onset, do oral solution.
Options:
KCl SR tablet = downside is it is a very large pill and can be hard for patients to swallow
KCl oral liquid = tastes horrible, some patients prefer though
KCl IV = burning is a very common side effect which is intolerable for some patients. You can try giving with lidocaine (may not be best practice but some places do it) or piggyback onto maintenance fluids to reduce this effect. Also each 10 mEq equals 50 mL which may be a lot of unwanted volume for patients.
K-Lyte (potassium bicarb and potassium citrate) effervescent tablets = good option to try if patient can't tolerate the regular tablet, comes in 25 mEq and 50 mEq forms
Peripherally you can only give 10 mEq/hr by IV.
If you have central access, you can give up to 20 mEq/hr by IV.
* One reason for refractory hypokalemia = low magnesium. Check their magnesium levels and replace it before giving potassium if this is the case!
Magnesium normally inhibits K+ secreting channels in the collecting duct, so if you don't have enough Mg you will start spilling potassium.
Side effects of hypokalemia = muscle weakness, paralysis, arrhythmias
Magnesium
Each 1 gm IV magnesium will raise serum level by 0.1.
400 gm PO magnesium (the standard dose) is equivalent to about 2 gm IV.
Options
MgSO4 IV = pretty much the go to method for magnesium replacement. Just consider how much fluid is in each dose.
Magnesium oxide tablets = causes diarrhea frequently which is why it is not preferred. Give if needed though. Common doses you can try are 400 mg x 1, 400 mg daily for 2 days, 400 mg BID for 2 days, etc.
Side effects of hypomagnesemia = muscle cramps (Charley horse), numbness/tingling, arrhythmias, hypokalemia, hypocalcemia (induces resistance to PTH)
Calcium
Remember to check if the patient is actually hypocalcemic or not by correcting for albumin.
Corrected Ca = (4 - albumin) * 0.8 + Ca
We also sometimes check serum ionized Ca levels which bypasses the need for correction entirely
Options
Calcium chloride IV = preferred in patients with cardiac arrest, higher concentration (3x more) than calcium gluconate. Generally only to be given in patients with CENTRAL ACCESS given the risk of tissue necrosis.
Calcium gluconate IV = preferred in non-cardiac arrest patients, lower risk of tissue necrosis if fluid extravasates. In general, most studies show calcium gluconate raises serum calcium the same as calcium chloride but with lower risk, just a few studies in critically ill patients showed that calcium chloride was a little faster. Typical dose = 1-2 gm.
Calcium carbonate (Tums) PO = 500mg to 4g/day in 1-3 divided doses
Calcium citrate PO = 200mg to 1g/day in single or divided doses
Nice algorithm for treatment on UpToDate = link
Basically, if calcium <7.5 or ionized <0.8, give IV. Otherwise try PO if patient can tolerate.
* One reason for refractory hypocalcemia = low magnesium. Check magnesium levels and replace!
Side effects of hypocalcemia = perioral numbness (first sign), Trousseau's sign (involuntary hand/arm contraction with checking BP), Chvostek's sign (facial twitching with light touch), bronchospasm, muscle numbness/spasms/tingling, seizures, altered mental status, arrhythmias (prolonged QT common)
Phosphate
For this one, we use mmol instead of mEq to calculate how much to give.
Normal range at UC Davis = 2.4-5.0 mg/dL
UpToDate says to weight base dose all this stuff but it seems a little too complicated. Usually what I do is choose between IV sodium phosphate (if their potassium is normal) or potassium phosphate (if potassium is low). Keep in mind that these IV formulations will leave the patients attached to an IV pole for quite a while which is a downside. You can try scheduled K-phos tablets instead if the hypophosphatemia isn't too bad.
2-2.5 = 15 mmol sodium or potassium phosphate IV over 4 hours
<2.0 = 30 mmol sodium or potassium phosphate IV over 6 hours
<1.5 = 45 mmol sodium or potassium phosphate IV over 8 hours
Options
Potassium + sodium phosphate (K-Phos) tablet PO = use if just mild hypophosphatemia ( > 2.0), side effect is diarrhea if you use too much so if you need high doses just give IV. Just give 1-2 tablets PO TID with meals for a day or two idk.
Sodium phosphate IV = probably use this one more than IV potassium phosphate because there's no potassium
Potassium phosphate IV = 21 mEq potassium per 15 mmol phosphate
Side effects of hypophosphatemia = basically all the same stuff as above, muscle weakness and cramps, bone pain
