Relationship of Magnesium & Potassium

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I'm having trouble understanding the relationship between potassium, and magnesium, mostly related to the bowels. I know that hypomagnesemia causes increased membrane excitability, and hypokalemia causes cells to be less responsive to normal stimuli. My confusion, is that constipation/decreased motility accompanies both imbalances. It seems to make sense in terms of hypokalemia, because of the decrease in excitability of cells. But, if magnesium normally inhibits nerve transmission, it doesn't make sense to me why having low magnesium levels wouldn't cause increased intestinal motility instead of decreased.

I've tried finding the answer in every resource available to me, with no luck. Any help is appreciated!

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Hypokalemia causes cell membranes to be more excitable-- that's why we hate it when people with bad hearts have hypokalemia and we're always giving them KCl after furosemide.

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Oh! My med-surg book is really off then! It says "Low serum potassium levels increase the difference in the amount of potassium between the fluid inside the cells, and the fluid outside the cells. This increased difference reduces the excitability of cells. As a result, the cell membranes of all excitable tissues, such as nerve and muscle, are less responsive to normal stimuli." That's from the Ignatavicius med surg book. (See why I am so confused) I understand the effects of potassium on the heart, my question is about the changes in intestinal motility caused by hypokalemia and hypomagnesemia.

But if my book is wrong, then it makes complete sense! lol

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Specializes in Pediatric Hematology/Oncology.

I've thought a lot about this and I've looked far and wide (include my old pathophys text book) for a definitive answer but it's hard to find. Like AliNajaCat said, myocardial cells are more excitable in the presence of reduced potassium. It seems, however, that smooth muscle cells are less excitable when serum potassium is low. So, when potassium is low, magnesium may very likely also be low, especially if the reason for low potassium was something like diarrhea - the potassium and magnesium all get sent out together. Constipation itself seems to be a function more of low potassium and low magnesium is incidentally linked by excretion and retention of the kidneys (and, also calcium as that and magnesium are cation homies, like potassium and sodium are cation homies).

Hypokalemia: Practice Essentials, Background, Pathophysiology

Hypomagnesemia: Background, Pathophysiology, Etiology

The pathophysiology of potassium and magnesium disturbances. A cardiac perspective. - PubMed - NCBI

This was a really interesting question, by the way. I'm surprised how far down that rabbit hole a seemingly simple question took me. I hope that helped.

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Specializes in Cardiac (adult), CC, Peds, MH/Substance.

What Kitty said agrees with your book.

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Specializes in Cardiac/Tele.

For the heart muscle portion of this... Hypokalemia (and/or hypomagnesemia) prolongs the QTc -- in less wonky terms, it increases the absolute rest period of the heart muscles; the time during which they cannot contract again. In hypokalemia and/or hypomagnesemia, it takes longer for the ventricles to get ready for the next beat; by the time they are ready to beat again in a patient with low K+/Mg+, the next ventricular contraction may already have fired, and then you are in danger of going into a rhythm like vtach or Torsades de pointes. It's called an R-on-T phenomenon, and can quickly degrade into a life threatening situation. If someone has low potassium and I need to give them an unrelated drug that also prolongs the QTc, like zofran or haldol or many others, I need to get an EKG and replace potassium or risk having to use my CPR skills on them. Taking it back a step, if I'm giving a patient something that lowers their K+ or Mg+, like some diuretics or some laxatives (or they have diarrhea), and I'm giving them other meds that lengthen QTc I definitely want to know their K/Mg levels first. I deal with this almost daily -- I'm giving lasix and not replacing it, and a potassium level hasn't been drawn in three days and the last one was borderline low, AND you want me to give more lasix? I call the doc and ask for another lab draw, which they are generally happy to have the nurse questioning! We all love patient safety ;)

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

https://allnurses.com/nursing-student-assistance/med-surg-fluid-804058.html

table of commonly used iv solutions.doc - most commonly used iv solutions; includes tonicity, ph, the ingredients of the solutions, its uses and complications

icufaqs

http://www.merckmanuals.com/professi...scitation.html

There are 6 major electrolytes. Sodium, potassium, calcium, chloride, magnesium and phosphorus. It is primarily potassium, calcium and sodium that will cause problems when they are out of whack.

  • hypokalemia
  • hyperkalemia
  • hypocalcemia
  • hypercalcemia
  • hyponatremia
  • hypernatremia

When there is a sodium imbalance quite often there will be changes in mental status - confusion, delirium, etc. Often seen with traumatic brain injury where diabetes insipidus (pathological voiding of large amounts of dilute urine) and its opposite SIADH (syndrome of inappropriate antidiuretic hormone - minimal urine output but very concentrated) may occur. Sodium imbalances are also seen with dehydration in some patients (elderly, burn victims, many others) and the blood levels will go up. Very rarely, sodium levels in the blood will go down because of consuming large quantities of fluids.

The other main electrolyte imbalance seen is when potassium is out of whack, and its most serious consequence is cardiac problems that can be life-threatening (you will see T wave changes: depression with hypokalemia, elevation with hyperkalemia, among other changes in the EKG like QRS interval changes).

For these 2 main electrolyte imbalances remember: Sodium equals mentation, and Potassium equals cardiac.

Sodium does affect fluid. In fact, they say sodium always follows water. There are a lot of people with edema related hypernatremia; and a lot of dehydration related to sodium and chloride losses. Potassium tends to affect the heart and in the clinical area you will see dramatic instances of people with hypokalemia and hyperkalemia. Calcium affects the muscles and is not as commonly seen clinically because it is detected because of lab testing.

  • Sodium - body water balance
  • Potassium - contraction of skeletal and smooth muscle and nerve impulse conduction
  • Calcium - formation and structure of bones and teeth, cell structure and function, cell membrane permeability and impulse transmission, the contraction of all muscle types and is necessary in the blood clotting process
  • Chloride - important in the digestive acids; closely linked to sodium
  • Magnesium - affects nerve and muscle action by affecting calcium usage, activates enzymes involved in carbohydrate and protein metabolism, helps in the transport of sodium and potassium across cell membranes, and influences the levels of sodium, potassium, calcium and some body hormones (parathyroid hormone)
  • Phosphorus - formation and structure of bones and teeth, this electrolyte is needed in the following activities: utilization of B vitamins, acid base homeostasis, bone formation, nerve and muscle activity, cell division, the transmission of hereditary traits, metabolism of carbohydrates, proteins and fats
  • http://www.elmhurst.edu/~chm/vchembook/253fluidkidneys.html

Third-spacing: Where has all the fluid gone?

c-electrolyte-surface-of.gif

table of commonly used iv solutions.doc

IV fluids: Do you know what's hanging and why? | Modern Medicine

http://ww2.rch.org.au/emplibrary/clinicalguide/IVFLUIDCHART.pdf

http://www.med.mun.ca/getdoc/6f26f870-6c78-4a73-80f0-98200858aafd/IntravenousFluids.aspx

c-electrolyte-surface-of.gif

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