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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

Comments

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

    ReplyDelete
  2. thanks a lot, it really helps

    ReplyDelete
  3. Thank you for the information, really helped

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

    ReplyDelete
    Replies
    1. 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

      Delete
    2. 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

      :)

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

      Delete
    4. Thanx...just saved a patient

      Delete
    5. Very nice to do it it's very helpful
      The old calculation was only (.4xwtx deficits ) but with adding the daily requirement it is very much accurate.

      Delete
  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

    ReplyDelete
    Replies
    1. This is a very useful and helpful information. Thank you

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

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

    ReplyDelete
  8. This comment has been removed by the author.

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

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

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

    ReplyDelete
  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.

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

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

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

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  16. Thank you. Informative and applicable

    ReplyDelete
  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.

      Delete
  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

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

    ReplyDelete
  20. Sir, your fornula for potassium deficit calculn seems to be at fault!! Potassium drficit is not an extracellular but an intracellular phenomenon... So actual deficit is much much more than thaat calculated by formula u have mentioned... This formula is for calculating Sodiun deficit and not potassium


    For every o.5 meq/l decrease in s. potassium below normal, there os a 100 meq/l potassium deficit...

    ReplyDelete
    Replies
    1. Reference -

      1. https://safetyandquality.gov.au/wp-content/uploads/2012/01/tools_royalhobart.pdf

      Delete
  21. Abhshek Savala i think the threshold for clinical illness is very much wide between intra cellular level in comparison with the intra vascular level but the threshold for clinical illness between intra vascular level is that we know not less than 3.5 .

    ReplyDelete
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  23. well for post operative patient (eg laprotomy) i havent seen anyone replace npo defecit for 3 to 4 days. only potassium defecit is replaced without replacing the daily requirements. how much are the potassium reserves in the body.

    ReplyDelete
  24. As we all know, K Cl is an intracellular element and barely reflects in serum level correctly. Awareness and daily supplements in IV infusion in surgical patients post operatively is d best way alongwith sodium and other fluid-water balance. Daily requirements should be added in IV infusion without waiting for deficit to develop. By then, it will be late to do so and very difficult. It's defficiency has vital damaging effects.

    ReplyDelete
  25. can you please explain how did you get 0.4 from the formula Kdeficit (mmol) = (Knormal lower limit - Kmeasured) x kg body weight x 0.4? and is there any reference for the formula?

    ReplyDelete
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  28. With 1 ampule kcl ,how much milliequivalent of potassium will increase

    ReplyDelete
    Replies
    1. 1amp is 10ml kcl ie 20 meq, now calculate.For corrections one has to know the deficit and body wt

      Delete
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