Infusion Computation

[ione]

Hello, I was doing practice Q&A test for nclex and I encountered this problem

"Lidocaine is mixed 2 g in 500 ml D 5 W. The nurse prepared to start an infusion at 2 mg/h using a 60-drop tubing. Which of the following is the correct rate to start the infusion on a pump?" (Multiple choices given)

And the correct answer is: 30 mL

(Solution below)

30 ml is 2 mg/h. 1000 mg = 1 g. 2 g is 200 mg. 2000mg:500ml::2mg: x ml 2000 x = 1000 x = 0.5 ml/hr 60 drops = 1 ml. 60 drops X 0.5 ml. = 30 ml/hr.

I don't quite understand the solution. Can anyone please help?

[sharann, BSN, RN]

Don't feel badly. I don't either.

[P_RN, ADN, RN]

total # of milliliters (volume)

_____________________ x drip factor = gtt/min

total # of minutes (time)

http://www.accd.edu/sac/nursing/math/ivprob.html

[jemb]

they used the cross-multiplication method to calculate.

1000 mg is 1 gm, 2000 mg (not 200-- that was a typo) is 2 gm.

2000 mg is to 500 ml as 2 mg is to xml,

written as:

2000mg/500ml = 2mg/xml (write it out with 2000 over 500, etc.)

cross multiply 2000mg x xml and 2mg x 500nl.

result is 2000x = 1000,

therefore x = 1000/2000 or 1/2 or 0.5.

if 1ml is 60 gtts, then 0.5 ml would be 30 gtts.

did that help?

[ione]

thank you very much. i get it now. :kiss

[Quailfeather]

Are you sure that the question reads 2mg/hr? Lidocaine infusions usually start at 2mg/min. The 60 drop tubing reference is irrelevant if you are using an IV pump. The pump is calibrated to deliver in ml's per hour, regardless of the tubing size. In this example, assuming that they want to deliver 2mg/min (the usual lido infusion dose) you simply reduce your solution concentration (2000mg/500ml = 4mg/ml) then calculate how many ml's an hour to deliver 2mg/min (60min/1hr x 2mg/min x 1ml/4mg = 30ml/hr). I hope I haven't thoroughly confused you now.

[jemb]

Are you sure that the question reads 2mg/hr? Lidocaine infusions usually start at 2mg/min. The 60 drop tubing reference is irrelevant if you are using an IV pump. The pump is calibrated to deliver in ml's per hour, regardless of the tubing size. In this example, assuming that they want to deliver 2mg/min (the usual lido infusion dose) you simply reduce your solution concentration (2000mg/500ml = 4mg/ml) then calculate how many ml's an hour to deliver 2mg/min (60min/1hr x 2mg/min x 1ml/4mg = 30ml/hr). I hope I haven't thoroughly confused you now.

I think it was just a math problem to practice drug calculations (not actually based on any nursing/medication reality).

[Quailfeather]

I think it was just a math problem to practice drug calculations (not actually based on any nursing/medication reality).

That's what I thought, at first. But since infusion pumps are set to deliver ml's per hour, 30ml/hr of a 2gm in 500ml D5W = 120mg/hr, not 2mg/hr. That's why I wondered if the question really read 2mg/min (120mg/hr). Something just doesn't add up :uhoh21:

[Vanfnp]

I believe there was a typo and the nurse planned to start the infusion at

2 mg/minute. 2 mg x 60 min to an hour = 120mg/hour. 2gm is 2000 mg per 500 ml, a 4 to 1 ratio equalling 4 mg in each ml. To get 120 mg per hour, divide 4 into 120 to equal the hourly pump setting of 30ml (30x4 =120).

The drops per minute for the tubing was just thrown in as extra info, not needed for pump calculations. NCLEX style questions do that from time to time! Hope this helps...