Dosage Calculation ???

Daytonite, BSN, RN · Sep 13, 2007

a solution of 400 mg dopamine hcl in 250 ml of d5w is infusing at 20gtt/min. calculate the mcg/min dosage.

actually, yes this is solvable, but it is missing the drop factor of the iv tubing being used. you know the formula, dose desired divided by dose on hand? it does have another part to it if you carry it through: equals dose to give. what you have here is a problem that is playing around with plugging figures into the equation and leaving you to find the desired dose. desired dose is usually information that is given to you in a problem, not being asked for. fooled you, didn't they? make a picture to help yourself understand what i am saying.

dose desired divided by dose on hand = dose to give

plug given information into the formula

x

/

400 mg/2 ml = 20 gtt/min.

that's the problem you need to solve. it's going to require a conversion factor (mg to mcg) and how the heck you get from ml's to gtts? you are missing the drop factor (drops/milliliter) of the tubing being used. now, this may be a mistake on the part of the instructor, but maybe not. they may be testing your knowledge of iv tubing. there are 2 different kinds of tubings that can be used:

microdrip (or pediatric) tubing which delivers 60 gtts/ml
macrodrip tubing delivers at different rates based on the manufacturer specifications
- 10 gtts/ml
- 12 gtts/ml
- 15 gtts/ml (probably the most common, often called standard tubing)
- 20 gtts/ml

the answer to the question might be "there is not enough information to solve this problem"

the "a" answer, at least the brown noser answer, the "go to the head of the class answer", might be to use all the various drop factors to get the following solutions. take a deep breath and begin (once you get the first calculation, you just substitute [i'll be copying and pasting] the drop factor into the remaining calculations. remember, you want to end up with "mcg" in the numerator and "minutes" in the denominator, so you manipulate the ratios to yield those labels in their correct positions for you while maintaining relationships within their proper ratios.

microdrip tubing

(actually, knowing about dopamine, this is probably the tubing that was being used):

400 mg/250 ml

(dose on hand) x

20 gtts/1 minute

(dose to give)

x 1000 mcg/1 mg

(conversion factor to change mg to mcg)

x 1 ml/60 gtts

(drop factor of microdrip tubing)

= 533.333 mcg/minute, or

533 mcg/min

(rounded off)

macrodrip tubing (10 gtts/ml)

400 mg/250 ml

(dose on hand) x

20 gtts/1 minute

(dose to give)

x 1000 mcg/1 mg

(conversion factor to change mg to mcg)

x 1 ml/10 gtts

(drop factor of microdrip tubing)

=

3200 mcg/minute

(rounded off)

macrodrip tubing (12 gtts/ml)

400 mg/250 ml

(dose on hand) x

20 gtts/1 minute

(dose to give)

x 1000 mcg/1 mg

(conversion factor to change mg to mcg)

x 1 ml/12 gtts

(drop factor of microdrip tubing)

= 2666.665 mcg/minute, or

2667 mcg/minute

(rounded off)

macrodrip tubing (15 gtts/ml)

400 mg/250 ml

(dose on hand) x

20 gtts/1 minute

(dose to give)

x 1000 mcg/1 mg

(conversion factor to change mg to mcg)

x 1 ml/15 gtts

(drop factor of microdrip tubing)

= 2133.333 mcg/minute, or

2133 mcg/minute

(rounded off)

macrodrip tubing (20 gtts/ml)

400 mg/250 ml

(dose on hand) x

20 gtts/1 minute

(dose to give)

x 1000 mcg/1 mg

(conversion factor to change mg to mcg)

x 1 ml/20 gtts

(drop factor of microdrip tubing)

=

1600 mcg/minute

(rounded off)

fyi. . .dopamine is usually ordered by the doctor as mcg/kg/min. you can also calculate a possible weight on this patient if you know what that mcg/kg/min figure is! let's have some fun.

if the patient is getting 533 mcg/minute from a bag of 400 mg dopamine hcl in 250 ml of d5w that is infusing at 20gtt/min and you know that, let's say, the doctor has ordered

5 mcg/kg/minute

. you can calculate that this patient weighs:

533 mcg/1 minute x 1 kg/5 mcg

(dose desired)

x 1 minute/1

(part of the dose desired)

x 2.2 pounds/1 kg

(conversion factor to move from kg to pounds) = 29.315 pounds, or

234.52 pounds

. rules of etiquette of scientific notation require you to round this to a whole number, so

235 pounds

would be an acceptable answer by any science teacher; nursing instructors are another story.

you can shorten the same calculation by taking the

533 mcg

and dividing it by

5 mcg

to get

106.6

for this patient in kilograms. multiply the

13.324

by 2.2 (the conversion factor to change it to pounds) and viola! you get the same answer:

234.52 pounds

.

be aware that you can rework any med calc problems once you have all three elements of the dose desired divided by dose on hand equals the dose to give formula. all you have to do is replace one of the three figures with the dreaded "x" and solve for it. most problems don't ask you for the dose desired or the dose on hand, but a tricky instructor might do it to see if you know how to do the math! these kinds of questions are what the guys in premed used to call "ace-breakers", separate the thinkers from the rest of the pack and make the difference between an "a" and a lower grade.

erin1205 · Sep 13, 2007

Daytonite-thanks a million for your great explanation! The book gave the answer of 533.3 mcg/min, so they must have assumed the microdrip tubing. Thanks for talking me through it!

Daytonite, BSN, RN · Sep 13, 2007

Whoa! Is that hokey, or what? Go get your "A".