dosage calculations: help

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hello everyone,

it's my first time in this forum, which i must say is very interactive.

i'm a first year student nurse in france having difficulties with dosage calculations. my calculations book confuses me and we don't have a dosage calculations class.

so if anyone could help me solve these problems i'd be grateful.

exercise 1

order: 120mg of gentamicin im

supply: ampoules of 10mg, 40mg, 80mg of 2ml each

choose the ampoules to be used and the volume in ml to be injected

i answered as follows:

volume to inject choosing the more concentrated solution: 120mg x 2ml /80mg= 3ml,

now i will be short 1ml since there is 2ml by ampoule

so i'll adjust by adding 1ml from the 10mg/2ml solution

exercise 2

order: zofran 6mg in 20 minutes with 100ml of 0.9%nacl

supply: 8mg/4ml

what's the volume to be injected?

calculate the flow rate in gouttes/min and in ml/h

volume to inject: 6mg x 4ml/8mg = 3ml

flow rate in ml/h= total ml/total hours, here the total hours is 1/3(20minutes), so 100ml/1/3h = 300ml/h

flow rate in gttes/min= v ml/ t min x c (gttes/ml), here the gttes/ml will be 20, so 300ml/60min x 20gttes/ml = 100gttes/min:confused:

am i doing it right:confused:

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Specializes in Critical Care.
hello everyone,

it's my first time in this forum, which i must say is very interactive.

i'm a first year student nurse in france having difficulties with dosage calculations. my calculations book confuses me and we don't have a dosage calculations class.

so if anyone could help me solve these problems i'd be grateful.

exercise 1

order: 120mg of gentamicin im

supply: ampoules of 10mg, 40mg, 80mg of 2ml each

choose the ampoules to be used and the volume in ml to be injected

i answered as follows:

volume to inject choosing the more concentrated solution: 120mg x 2ml /80mg= 3ml,

now i will be short 1ml since there is 2ml by ampoule

so i'll adjust by adding 1ml from the 10mg/2ml solution

i'm not quite sure what you are doing here. you are correct that if you use ampules with a concentration of 80 mg/ 2ml, you'd need 3 ml. however, this means you need 1 ml from a second 80 mg/ 2 ml ampule. if you were to use the 10 mg / 2 ml ampule as you intended, you'd only be giving 85 mg total.

exercise 2

order: zofran 6mg in 20 minutes with 100ml of 0.9%nacl

supply: 8mg/4ml

what's the volume to be injected?

calculate the flow rate in gouttes/min and in ml/h

volume to inject: 6mg x 4ml/8mg = 3ml

flow rate in ml/h= total ml/total hours, here the total hours is 1/3(20minutes), so 100ml/1/3h = 300ml/h

flow rate in gttes/min= v ml/ t min x c (gttes/ml), here the gttes/ml will be 20, so 300ml/60min x 20gttes/ml = 100gttes/min:confused:

am i doing it right:confused:

you did this correctly. however, if you want to get technical, remember that you added 3 ml to the 100 ml bag, giving you a total volume of 103 ml, not 100 ml.

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

exercise 1

order: 120mg of gentamicin im

supply: ampoules of 10mg, 40mg, 80mg of 2ml each

choose the ampoules to be used and the volume in ml to be injected

you will choose the highest concentration because you want to inject the smallest amount of solution into the patent's muscle. you would use the 80 mg/2ml solution of the gentamycin.

120 mg
(dose desired)
/80 mg
(dose on hand)
x 2 ml
(volume the dose on hand comes in)
=
3 ml
(dose to give). since the 80 mg come in ampoules of 2 mls you will need to use 2 ampoules. you will take the entire 2 mls out of one ampoule and only 1 ml out of the second one and discard the 1 ml that is remaining since it will not be used. you do not mix and match ampoules because of the risk of making a mistake.

exercise 2

order: zofran 6mg in 20 minutes with 100ml of 0.9%nacl

supply: 8mg/4ml

what's the volume to be injected?

calculate the flow rate in gouttes/min and in ml/h

6 mg
(dose desired)
/8 mg
(dose on hand)
x 4 ml
(volume the dose on hand comes in)
=
3 ml
(dose to give), then,
100 ml
(the diluting solution)
+ 3 ml
(the amount of zofran)
=
103 ml
(total volume to be injected)

flow rate in ml/hour:
103 ml/20 minutes
(total volume to be injected in infusion time given in problem)
x 60 minutes/1 hour
(conversion factor)
=
309 ml/hour

drops/minutes cannot be calculated without knowing what the drop factor of the iv tubing being used. that wasn't stated in the problem.

there are medication calculation tutorials that you can go to and self learn how to do these kinds of problems listed on post #2 of this sticky thread on allnurses:

https://allnurses.com/general-nursing-student/nursing-math-thread-264395.html - the nursing math thread

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