# I can't figure out this calculation!

by direw0lf, BSN (Member)

direw0lf has <1 years experience as a BSN.

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(it's not a school assignment I'm studying for nclex)

Q asks: prescribed is IV heparin 25,000 u in 500mL D5W.

After 6 hours at prescribed rate of 1300u/hr, the PTT is 44 sec. If it's 44 secs, then you increase infusion by 100 u/hr.

What rate in mL per hr should the RN set the IV infusion pump?

So...I'm having trouble just starting my calculation. Can anyone give me a hint please? I use ratio methods. TIA. (I got 2.8ml/hr on my first attempt...which I think is wrong)

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cayenne06 has 10 years experience as a MSN, CNM and specializes in Reproductive & Public Health.

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The first step when doing dose calculations for liquid meds is to figure out how much drug is in each mL of solution. It should make a lot more sense once you do that.

If you already did that and are still stuck let me know!

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direw0lf has <1 years experience as a BSN.

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Thanks! I did this:

1400u/1hour = 500ml/X

and got 2.8 ml/hr

do you know what I'm doing wrong?

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SopranoKris is a BSN, RN and specializes in Critical Care.

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OK, always start with what you're looking for. In this case, you want to know how many mL/hr to set the pump.

Look at the data you're given. You want mL at the top of your fraction and hours on the bottom. Which ratio are you given that has mL in the numerator? It's your heparin. Always start with the unit you need on top. You have 500 mL in every 25,000 units. Now, you initially set the IV pump to run at 1300 units per hour. Initially, your pump would be set at:

(500 mL / 25,000 units) x (1300 units/hour) = 26 mL/hr

Since you need to increase your dose by 100 units/hr, your equation now becomes:

(500 mL/ 25,000 units) x (1400 units/hour)

Edited by SopranoKris
clarification

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Pixie.RN has 11 years experience as a MSN, RN, EMT-P and specializes in EMS, ED, Trauma, CNE, CEN, CPEN, TCRN.

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I always figure out the concentration first. You know that you have 25,000 units in 500 ml, which is a concentration of 50 units per ml (25,000 divided by 500). If you want 1300 units per hour, you need to know how many ml it takes to make that 1300 units. If you know there is 50 units in each ml, you would divide 1300 by 50 to tell you how many ml you would require for 1300 units. 1300/50 = 26. And so forth. I am not good at cross multiplying (though I did learn it that way too), but I like to reason my way through math problems like this. Works better with my brain!

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direw0lf has <1 years experience as a BSN.

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Just a quick thank you for the helpful replies, very appreciated!

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SopranoKris is a BSN, RN and specializes in Critical Care.

3,066 Posts; 32,771 Profile Views

Just a quick thank you for the helpful replies, very appreciated!

YAY! You got the right answer. Good job :)