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# I'm back with math help needed again please!

by direw0lf, BSN (Member)

direw0lf has <1 years experience as a BSN.

26 Likes; 10,329 Visitors; 1,023 Posts

I have trouble with the word problems...nothing seems to follow a formula in my brain.

Can you guys please tell me why sometimes we multiply and other times divide? This is for a test I need to take for work.

These were practice problems I was given:

Problem 1. A pt receiving an infusion of Lidocaine at 30mL/hr. The lidocaine concentration is 1g of Lidocane in 250 mL D5W. This infusion rate delivers what dose of lidocaine in mg per minute?

Ok first thing I know to do is change 1g to 1000 mg then divide that by 250 mL. I get 4mg/mL.

Then I get mixed up what to do with the 30mL/hr and 60 minutes.

My friend told me multiply 30 and divide by 60min/hr but wasn't able to explain why. Can someone explain why please? Because when converting hours to minutes we should multiply 60 not divide?

Problem 2. A pt who weights 318 lb ordered to receive dobutamine IV infusion at 8mg/kg/min. What is the infusion rate in mL/hr based on a concentration of 1 g of dobutamine in 250 mL D5W?

So again I know to convert 1g to 1,000,000 mcg then divide that by 250 mL to get 4,000 mcg/mL.

Then I know to convert lbs to 145 kg.

Then I don't understand whether to multiple or divide the rest. How do you know which to do next?

THANK YOU FOR ANY HELP

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18 Likes; 818 Visitors; 97 Posts

Okay I can't guarantee that this is all right because I've only had 1 health calculations class and haven't started the nursing program but this is how I learned to do it in my med calc class:

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18 Likes; 818 Visitors; 97 Posts

I think your confusion lies in that you're trying to do it in too many steps, I do mine in 1 or 2 large dimensional analysis problems. Just make sure that everything cancels out except for your end units and then there's almost no room for error.

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10,499 Visitors; 949 Posts

My friend told me multiply 30 and divide by 60min/hr but wasn't able to explain why. Can someone explain why please? Because when converting hours to minutes we should multiply 60 not divide?

Sounds like you are getting a wee bit confused with keeping your numerator and denominator separated.

you are correct that if you want to convert a number of hours into minutes then you multiply by 60. so 3 hours is how many minutes? 3 x 60 = 180 minutes.

In your example of 30mL/hr your friend told you to divide by 60 but couldn't explain why... think of it this way.. you are multiplying the 1 hour by 60 to get minutes not the 30 mL. If you think of this as an individual math problem then answer if you have a rate of 30mL/hr then how many mL/min would that deliver?

30mL/hr is the same as 30mL/60 min (multiply the 1hr by 60 to get 60 min). This would leave you to now divide 30 by 60 to answer the question. 0.5mL/minute

Now if you are giving 1/2 mL every minute and there are 4 mg in every mL -- how many mg is being given every minute? 2 mg

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10,499 Visitors; 949 Posts

For problem 2 - you have typed 8mg/kg/min, but I'm sure you did mean 8mcg/kg/min.

Don't get confused thinking this is medical math... it's just math.. same math you learned in grade school..if you need to break down each step of the problem. Exactly as you have already done by figuring out grams to micrograms and pounds to kilograms.

next figure out what the prescribed dose is.. 8mcg per kilogram per min... weight is 145 kg so how many mcg/min is this person going to receive? 8 x 145 = 1160 mcg/min

how many mL/min would deliver the 1160 mcg? there are 4000 mcg/ml. (remember to do simple math checks along the way.. if there are 4000 mcg in every mL and you only need 1160 mcg then you automatically know that any answer 1 or grater is incorrect.) you divide what you want by what you have.. 1160/4000 = 0.29 mL every minute will give you the dose of 1160 mcg/min.

if you are giving 0.29 mL every minute how many mL would that be every hour? 0.29 x 60 = 17.4 mL/hr

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1,135 Likes; 7 Followers; 21,301 Visitors; 2,696 Posts

Okay I can't guarantee that this is all right because I've only had 1 health calculations class and haven't started the nursing program but this is how I learned to do it in my med calc class:

Link is to a screenshot of my work through these problems. Hope this helps!

I skimmed the math; looks good.

One issue, which I think is because of a typo in the OP (but results in something that should always give us pause to ask a question)....

You got a final answer of 17,346 ml per hour. Of course that's 17+ LITERS of fluid PER HOUR.

I'm fairly sure OP meant to originally write 8 mcg/kg/min.

Final answer should be 17.34 ml/hr.

Again, you did great math and the typo has nothing to do with you/your math.

This is just a general encouragement/reminder that things should make logical sense as we go.

:)

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18 Likes; 818 Visitors; 97 Posts

I skimmed the math; looks good.

One issue, which I think is because of a typo in the OP (but results in something that should always give us pause to ask a question)....

You got a final answer of 17,346 ml per hour. Of course that's 17+ LITERS of fluid PER HOUR.

I'm fairly sure OP meant to originally write 8 mcg/kg/min.

Final answer should be 17.34 ml/hr.

Again, you did great math and the typo has nothing to do with you/your math.

This is just a general encouragement/reminder that things should make logical sense as we go.

:)

Yeah I thought that my answer was weird, but since I don't know anything really about meds I just went with it LOL

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1,135 Likes; 7 Followers; 21,301 Visitors; 2,696 Posts

I figured. :) It's all good.

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direw0lf has <1 years experience as a BSN.

26 Likes; 10,329 Visitors; 1,023 Posts

Big thank you to everyone!

Unfortunately we weren't taught dimensional analysis, my class was actually discouraged to use it but it was a pharmacologist who was teaching us. I did a practice right now to try and learn it but I think I better just go without it for now. I'm afraid of getting mixed up.

It was supposed to be 8 MCG yes!

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45 Likes; 1 Follower; 47,304 Visitors; 2,124 Posts

While there are formulae available that will work for most problems you will encounter problems that no formula works well. This is why, when I work with nursing students or new nurses, I stress dimensional analysis.

If you are interest in learning dimensional analysis you might the attachments in the first two posts in the following thread helpful: Master Your Drug Calculations BEFORE You Get to Nursing School.

Best wishes.

common pharmacology formulae.pdf

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schen12 has 23 years experience.

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Let me see if I can make it a bit clear.

For problem 1, you have gotten 4 mg/ml. Next, you multiply 4 mg/ml by 30 ml/hour - this result get you 120 mg/hour. Yes, at this step, you established the fact that in an hour, the patient received 120 mg Lidocaine IV. Last, let's divide 120 mg/hour by 60 minutes/hour - you get 2 mg/minute.

For problem 2, you have a good start. Let's move on by multiplying 145 kg body weight by 8 mcg/kg body weight/min- this gets us 1160 mcg/min. Thus, in each minutes, the patient should receive 1160 mcg dabutamine. The, let's figure out how much would he receive in an hour. We, then, multiply 1160 mcg/min by 60 minutes/hour - we get 69600 mcg/hour. Last, we can use the other number that you figured out initially, the concentration of dabutamine, 4000 mcg/ml. Let's divide 69600 mcg/hour by 4000 mcg/ml, and we get 17.4 ml/hour.

When I do drug calculation, I pay attention to conversion among different measuring units. Thus, I like to place my measuring unit in front of my eyes by writing down mg/ml or mcg/hour....etc. By doing so, I can easily cross off the repeated ones and see the new measuring unit forms in front of me. These measuring unites, mg/min, mg/ml, ml/hour.... explains to me what step I am in and what step to take to reach my goal.

If I can use problem 2 as an example to show what the measuring unit help me solving same problem in a different way:

After we established that the patient should receive 1160 mcg/min dabutamine, we can also figure out exactly how many ml that is! Thus, we divide 1160 mcg/min by 4000 mcg/ml, and we would get 0.29 ml/min. The patient gets 0.29 ml every minutes. Now, there are 60 minutes in an hour. Last, we multiply 0.29 ml/min by 60 min/hour, and we get 17.4 ml/hour.

I feel that you have already gotten the first few steps in both problems. You can do it. :)

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245 Likes; 1 Follower; 13,039 Visitors; 1,364 Posts

Dimensional analysis is the bomb. Makes it all faster. I learned it from Khan academy, when I took chem. Kind of forgot it again until med math, but relearned it fast.

It's worth it, imo. I can do it without using dimensional analysis but it takes longer. Also, at some point I'm usually like wait-- what was I doing again? Dimensional analysis gets rid of that issue.

I got the same answer as Schen12 for problem 1. Dimensional analysis goes like this:

30mL/60 min x 1000mg/250mL

mL cancel out because one is numerator and other is denominator.

You are left with

30/60 min x 1000mg/250

Which is what you want. You want mg as the numerator and min as the denominator. Because you are looking for mg/min.

So multiply 30 x 1000mg/ 60min x 250

That's 30,000mg / 15,000 min

Reduce to 2mg/1min.

Edited by FolksBtrippin