HELP! What am I doing wrong with this calculation?

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For the life of me, I can't figure this out (dimensional analysis). Anyone out there with a clear head...I must be very tired!! Thanks.

A Dopamine drip (400mg in 250 cc of IV fluid) is infusing on your 80 kg patient at 20 cc/hour. How many mcg/kg/min are infusing for this patient?

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Do you have to use dimensional analysis? I worked it out a different way.

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Anything will work. Got to figure this one out. Thanks

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Specializes in Post Anesthesia.

I'm sure there are various "formula"to help you figure this out- plug in the numbers and out pops the answer. I prefer to just think it through. Your drip is in MG but you want your answer in mcg- so convert 400mg x1000= 400,000mcg in 250cc. Now you are in the same units. How many mcg is the patient getting: 400000 in 250cc= 1600mcg in each cc (400000/250). The patient is getting 20cc/hr so 1600mcgx20cc=32000mcg/hr. Wait- dopamine is titrated in mcg/kg/min. Convert to the hourly dose to the per minute dose: 32000/60(min in an hr)=533mcg/min. That is his total body dose- how much for each kg of weight. 533/80kg= 6.62 mcg for each kg of body weight ie: 6.62mcg/kg/min. You can work the steps in any order you wish- the answer is still the same. With each step in the calculation ask yourself what do I have and what do I want to have (ie: have a mg number but want mcg number) and take it one step at a time.

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Thank you so much. I really appreciate your help!

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In that case, I worked backwards from a mcg/kg/min to ml/hr problem using a ratio.

Step 1: You have the concentration and the mL/hr that the pt is receiving:

400:250 = x:20

250x = 8000

x = 32mg/hr

Step 2: Change mg/hr to mcg/hr

32mg = 32000mcg/hr

Step 3: Change mcg/hr to mcg/min

32000mcg/60 min = 533.33mcg/min

Step 4: Divide by kg

533.33mcg/80kg = 6.7mcg/kg/min

Answer = 6.7mcg/kg/min

To check:

6.7mcg x 80kg = 536mcg/min

536mcg/min x 60 min = 32160mcg/hr

32160mcg = 32.16mg/hr

Plug these numbers into the ratio:

400:250 = 32.16:x

400x = 8040

x = 20.1mL/hr, which rounds to 20mL/hr in the original problem

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Specializes in med/surg, telemetry, IV therapy, mgmt.

I worked this out by dimensional analysis for you on your post on the Nursing Student Assistance Forum.

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