Sunday, July 3, 2011

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Hypokalemia - Potassium replacement calculation


 DEFINITION 
Hypokalemia is defined as a serum potassium level of less than 3.5 mmol/L.

Normal level= 3.5-5.5 mmol/L.
It is encountered in >20% of patients.

Patients are usually asymptomatic but severe arrhythmias and rhabdomyolysis can occur. Non-specific complaints include easy fatiguability and skeletal muscle weakness.

The preferred method of replacement is via the oral route but at times this is not possible. The article below will give you an idea about how to calculate the amount of KCl to be given I.V.






1) Potassium deficit in mmol is calculated as given below:


Kdeficit (mmol) = (Knormal lower limit - Kmeasured) x kg body weight x 0.4


2) Daily potassium requirement is around 1 mmol/Kg body weight.
3) 13.4 mmol of potassium found in 1 g KCl. (molecular weight KCl = 39.1 + 35.5 = 74.6)




Suppose we get an asymptomatic patient of  70 Kg with a serum potassium level of 3.0 mmol/L and he is on nil by mouth but having an adequate diuresis, we proceed this way. 


1) Deficit of potassium in mmol = (3.5 - 3.0) x 70 x 0.4 = 14 mmol
2) Daily potassium requirement = 1 x 70 = 70 mmol
3) Total requirement = 14+70 = 84 mmol = (84/13.4) = 6.3 g KCl 


Therefore we can give the patient 1.0 g KCl (around 13 mmol of K+) in 500 mL Normal Saline (N/S) solution to run 4 hourly and reassess the serum potassium level after 1 day. So, in around 24 hours, we have given the patient around (24/4 = 6 pints of N/S solution, total KCl administered = 6.0 g i.e. around 80 mmol K+). Now if on the next day, he is still nil by mouth and there is still some degree of hypokalemia, we can repeat the above calculations to find his new total requirement and adjust the dosage accordingly.

But, if we have a symptomatic patient or someone with changes on the EKG, then better give 20 mmol of K+ in 50 mL of N/S via syringe pump over an hour through a central line and then recheck the serum potassium 1-2 hours after completion of the infusion. In extreme cases we can even go for up to 40 mmol of K+ per hour.


1) Never give potassium I.M or rapid I.V push
2) Never give more than 1.5 g KCl or 20 mmol of K+ over 1 hour without any continuous ECG monitor.
3) Do not just add the KCl solution to the hanging I.V fluid bag. Fully invert it around 10 times to ensure proper mixing.
4) 1 tab of Slow K gives around 8 mmol potassium.
5) 10 mL of KCl syrup = 20 mmol of K+.
6) Peripheral veins are damaged by a potassium concentration greater than 30 mmol/L i.e. 1.1 g KCl/ 500 mL I.V infusion solution. For higher concentrations, central lines are preferred.
7) Hypokalemia is associated with hypomagnesemia and the severity of the hypokalemia correlates with a similar degree of hypomagnesemia. Magnesium replacement should usually accompany potassium repletion. Unless the patient receives at least 0.5 g/hr of magnesium sulfate along with potassium replacement, potassium will not move intracellularly and the patient will lose potassium through excretion.
8) Correction of large potassium deficits may require several days. Oral and intravenous replacement can occur simultaneously.
9) Monitoring the plasma potassium level as an index of total body potassium is like evaluating the size of an iceberg by its tip since only 2% of total body potassium is extracellular. Thus repeated measurements of the serum potassium should be done. In an averaged-size adult with a normal serum K+ of  4 mEq/L, a total body K+ deficit of 200–400 mEq is required to produce a decrease in plasma K+ of 1 mEq/L.
10) Please leave a comment stating how useful the calculation turned out to be in your clinical practice (if ever you used it).

Further readings:
Alcoholic liver disease - complete review
Hypokalemia- ecg changes

Last reviewed on : 1 September 2015

35 comments:

  1. great blog...it helps a lots...thank u =)

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  2. thanks a lot, it really helps

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  3. Thank you for the information, really helped

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  4. make it in meq/l please,usually we dont adv pottasium in grams.

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    Replies
    1. 1 meq K = 74.5 mg KCl

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    2. Dr. Teena GuptaMay 7, 2012 at 6:54 PM

      1 meq k+ = 1mmol
      so in above eg. 84 mmol = 84 meq
      ie; nearly 2 amp. of kcl20 in 5% dex or NS @ 0.5cc/hr

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    3. Dr. Teena GuptaMay 7, 2012 at 7:37 PM

      to be cont...

      ie; nearly 2 amp. of kcl20 in 5% dex or NS @ 0.5cc/hr
      ie in 4-6 hrs

      2 such therapies to correct 84 meq loss

      becoz one can't add >10meq in 100 cc
      ie in 5oocc not more than 2 amp of kcl20

      hope you understood if yes kindly reply

      :)

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    4. superb explaination...anyway just wanna know what's ur references??

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    5. Thanx...just saved a patient

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  5. In our Hospital Setting we used this kind of formula in dealing with Potassium deficit patients:

    Potassium Deficit= {[(Desired K - Actual K)(Weight in Kg)(350)] / 3 Days} +/- 50

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  6. This is very helpful. Thank you for posting. :)

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  7. It's been of helpful as it is well ellaborated.

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  8. I give some time in severe cases 10 time the formula it work better than without any problems

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  9. keep up with the good work.i am grateful

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  10. glad i found this site. very helpful. thanks.

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  11. would like to know why we need times 0.4 for the deficit of potassium?

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  12. 1 tsf or 5 ml KCl is equivalent to 540 mg KCl. So giving 2 tsf or 10 ml KCl would equal to 1080 mg KCl. Devide it with 74.6 and we get 14.47

    So 10 ml KCl syrup= around 14 mmol K+

    Anyhow a very handy article. Many thanks.

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  13. Informative and helpful article.
    Thank you so much.

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  14. Informative and helpful article.
    Thank you so much.

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  15. For pt with hepatic failure can I give kcl in dextrose 10%

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  16. Thank you. Informative and applicable

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  17. Can someone calculate it for me? Patient: 48 kg, serum K 2.9

    I'm not sure I got it right. I'm a student.

    ReplyDelete
    Replies
    1. If the serum k is 2.9, use the following formula
      Deficit=(3.5-serum k)×weight×0.4
      This equals 11.52
      Now add 48 to it for the daily k requirement= 59.52
      So 59.52 know of k is required by this patient.

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  18. Very helpful information regarding calculation of the deficit and total dose needed for repletion. However, I've never seen a hospital that will allow you to do a 20mEq bolus in a 50ml syringe on a pump - way too risky! Nor would they make a 40mEq/500ml bag because they most likely have either 1 or 2 strengths of pre-made piggybacks that they purchase for K-repletion protocols. Most hospitals in the US limit the IVPB size to 10mEq/100ml due to safety concerns, unless a patient meets certain criteria such as fluid-restrictions, having a central line, is on cardiac monitoring, and/or in the ICU. If a patient meets some of these criteria, they may allow a 10mEq/50ml or 20mEq/100ml piggyback.

    Rather than following external/3rd-party advice on how to mix/administer the dose, please use these calculations to figure out how much needs to be given, then ask your hospital pharmacist about the facility policy regarding K boluses and how to best go about administering the dose. It's much safer, and will also save you the obligatory phone call from the pharmacist to educate you about their approved processes!
    Signed - a hospital pharmacist with 26yrs experience

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  19. May I know what book that you used as the references to that formula?
    need your answer soon.
    thx..

    ReplyDelete