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pharmaceutical calculations

Hey guys I am struggling (again) with an exercise. The problem says:

A baby weighing 3.4kg requires treatment with phenytoin at a dose of 20mg/kg. A 5-mL vial of phenytoin injection containing 50mg/mL is available for dilution before infusion. A suitable volume if phenytoin injection is diluted up to 50 mL with sodium chloride 0.9%

a) calculate the volume of phenytoin injection that must be used in the preparation of the infusion

b) The infusion rate must not exceed 3 mg/kg/minute. Calculate the maximum flow rate in mL/minute if these conditions are applied to the administration.

I am really struggling to do part b) . I do not know how to approach it.

Thank you very much for your help :smile:

Reply 1

Rx_Sarcasm

Haha, so what module is giving you all these calculations?

So I'm presuming you have got the answer to part a) so I won't bother going over the working, but I got 1.36 mL of phenytoin 50mg/mL injection is required, giving a total dose of 68mg.

For part b)...

The maximum infusion rate is 3mg/kg/minute, so for a 3.4kg baby, the maximum infusion rate would be 10.2mg/minute.

Your infusion bag contains 68mg of phenytoin in 50mL, and we're basically trying to find what volume of the infusion bag will contain 10.2mg, and give that volume every minute.

I usually do these types of calculations by one of two ways.

If you divide 68/50, this gives tells you how much phenytoin is in 1mL of the solution (1.36 mg/mL), so from this we can work out how many mL is required to get 10.2mg (10.2/1.36)

Alternatively you can divide 10.2/68 to get a ratio.
10.2 : 68
= 0.15 : 1
You then apply this ratio to the volume so 0.15 * 50

Either way gives you the answer 7.5mL/minute.

Again, hope my explanation makes a jot of sense (and hopefully I actually got the correct answer!) Also bear in mind that when they set these types of questions, they also usually want you to convert the answer to or from mL/hour as well so watch out for that!