Narcotic infusion Calculation

[wanderlustnurse88, RN]

Hi,

I am going to be doing some volunteer nursing in Africa and they have sent me a medication calculation quiz to fill out and I am stuck on one question since I haven't had to do this type of med calculations in a long time. I was wondering if I could get some assistance with it.

IV morphine infusion (this is the standard morphine drip order)

Use 50mL syringe: add 1mg/kg morphine and total to 50ml with NaCl 0.9% (max 50mg/50mL).

Run at 10-40mcg/kg/hr=0.5-2ml/hr (run as per anesthesia order).

Question: Your last patient has come back from PACU with a morphine infusion. He weighs 42 kg, so PACU has mixed 42mg morphine in 50mL normal saline for syringe pump infusion. (See above for standard morphine drip orders)

a. What is the concentration of your infusion in mg/mL?

b. The doctor has ordered 20-60 mcg/kg/hour. What is your dose range in mL/hour?

c. You are allowed to give your patient a 1-3mL bolus every 15 minutes if needed for pain, with a 10mL maximum. How many milligrams of morphine would you be giving with each 3mL bolus?

For a) I got 0.84mg/mL but not sure if that is correct. Then for b and c I am lost on where to start. If I could get some assistance on what calculation to use to help answer these questions, that would be great. I really need to understand how to solve these questions, not just get the answers.

Thanks :)

[MunoRN, RN]

For the concentration where you know that you have 42mg in 50ml, you want to know how many mg in just one ml, so you would divide your 42mg by 50mls, this would be the dose in one ml, you're correct it's 0.84.

Making the concentration based on the weight sort of negates the need to then calculate the dosage range for the drip rate in terms of mcg/kg, since the range of 20mcg/kg/hr to 60mcg/kg/hr will always be the same mls per hour regardless of the dosage concentration.

But to calculate it, start with the low end of the range which for a 42 kg patient would be 20 (mcg) X 42 (kg), then take that and divide by 1000 to convert that to mg. If you've figured the concentration correctly then it should be easy at this point to see how many mls that is for that dose in mg. You could either then repeat that calculation substituting the upper end of the range (60) with the 20, Or, a shortcut would be to just multiply the lower end of the dose in mls by 3, since you know the upper end of the range is 3 times that of the lower range.

[sallyrnrrt, ADN, RN]

This is another example of high school math, ; ie ratios