Need help with a dosage problem?

A nurse has an order to start magnesium sulfate on a preterm labor client. The order reads: Give a 4-g bolus over 15 minutes, then decrease the rate to 2g/hour. The nurse has 50 g of magnesium sulfate mixed in 1000 mL of lactated Ringer's on hand. What is the rate the nurse will set the pump rate to deliver the 2-g maintenance dose? Record your answer using a whole number.

19 Answers

Dose Ordered (2,000 mg) X (Volume 1,000 ML)

____________________

Dose Available (50,000 mg)

That gives you the correct answer, but you have to tack on hours. It works because we just need to know how much to give, we already know the time. That's technically correct, but since we're assuming the /hr part it's not the "mathematically" correct way to express it.

ml 1,000 ml 2,000 mg

--- = ---------- x -----------

hr 50,000 mg 1 hr

Both get you 40ml/hr, but the second method is more mathematically correct. However it doesn't matter, just remember to correctly express the answer in ml/hr.

I'm ironically terrible at math competencies, but here I am explaining factor analysis :)

Specializes in Pediatric ICU.

Well I got an answer. However, in order for us to help you, we would need you to tell us first which method you use in solving Med math problems; desired over have, etc.

Secondly, it would probably be better if you were to explain to us what you did to solve this problem, and then what you got for an answer. That way it would be easier for people on the forum to help you out. If we were to just tell you the answer and/or what we got for an answer that's not really helping anyone out much.

Specializes in OR, Nursing Professional Development.
No, 40 mL is not correct.

As with the OP, how did you get this answer?

You might want to check your first sentence... Im coming up with that answer too.

OP, first you need to figure out what information in the problem you need and what information is a distractor. But before we give you any more help, please show us how you are setting up your equation and what answer you are getting.

Well I took 50g = 50,000mg so 50mg/ml Then I thought how many mg are we looking to solve for 2000mg. if it is 50mg/ml then 40ml=2,000mg. 50*40=2000. So 40ml/hr.

This is how I got my answer.

Specializes in OR, Nursing Professional Development.
Well I took 50g = 50,000mg so 50mg/ml Then I thought how many mg are we looking to solve for 2000mg. if it is 50mg/ml than 40ml=2,000mg. 50*40=2000. So 40ml/hr.

This is how I got my answer.

That's one way to do it, but does include extra steps. The formula desired (2g)/ have (50g) x volume (1000mL) would have given you the same answer in fewer steps. D/H x V was always an acceptable formula in all of my nursing classes.

Noctor_Durse said:
Thank you for this info! Can I use this equation when I have drop factors or mg/kg conversions?

For drops/minute, you can use the following formula:

  • (total volume to infuse [mL] / total time to infuse [minute]) x drop factor

For me, it is easier if I determine mL/hour first, and then calculate drops per minute using the following:

  • (mL/hour / 60) x drop factor
Specializes in Pediatric ICU.

It depends on how specific they want your answer. If they don't want to count, or subtract that first 4g, then you can technically just base your answer off of the entire 50g in 1,000 mL. However, if they are giving you the previous information, the 4g, looking for you to subtract that from your would-be answer, then you need to make sure you calculate your answer with that in mind.

So in that case you want to find out much medication is in your bag per mL. In our case here just divide your total 1,000 mL by how much mag you have, which is 50g. This comes out to 20mL/1g (mag). Now they said previously that you were to administer 4g of mag. The 15 minutes of time is not really relevant in this case, unless they were too otherwise state that it was important, because after the 15 minutes you will just be administering per hour.

So we take your 4g and multiply that by your 20 mL (20mL/1g that we just got). This comes out to 80mL. Subtract this 80mL from your total 1,000mL of solution, this leaves us with 920mL.

Now your problem starts. Keep in mind that you are solving for mL's. mL's per hour, but mL's nonetheless. Your problem is now: you need to administer 2g of mag per hour. You have on hand: 46g of mag in 920mL solution.

Your equation = 920mL of solution divided by how much medication you have left on hand, which is 46g, times the amount of medication per hour you want, which is 2g. So your equation will look like:

920mL / 46g X 2g = 40mL per hour.

For drop factors you would use volume/time to infuse (minutes or hours) x drop factor.

Most problems can be solved using very similar equations. I would definitely suggest once you get into nursing school, or even before, if you can find a med math tutor, to definitely utilize that resource. Many nursing programs nowadays have Med math tutors. Good luck my friend. I hope that helped somewhat, and I hope it wasn't too drawn out. I was just trying to go step by step. That's how I learn anyways, and just was hoping that may help.

Specializes in OR, Nursing Professional Development.

Plutonium, you do realize that you've taken so many extra unnecessary steps here right? The simplest way to do this is either the D/H x V or figuring out how many mg/mL. Wether you have 50g in 1L or 46g in 920mL, the concentration is the same. Why take more steps to get an answer when there's a quicker way?

Specializes in Med-Tele; ED; ICU.
40ml?
40mL is not correct. 40 mL is a volume, not a rate. 40mL/hr is correct.

It's not semantics.

Specializes in Med-Tele; ED; ICU.

I did it in my head thusly...

50g in 1000mL = 5g in 100mL = 1g in 20mL = 2g in 40mL

40mL over 1hr = 40mL/hr

Specializes in Pediatric ICU.

Hahaha. Rose Queen. I actually spelled it out in detail so it seems long, but in actuality all I did was volume over what the dose you have on hand, times the desired dose. Super simple. I literally did it in my head. I can definitely see how it looks like it would be long though. Just trying to explain it in detail for someone who may be like me, or learn the way I learn, and need/want to know the exact way to solve something. Before someone explained it to me in detail like this, it just really didn't fully make sense to me. I'm slightly OCD when it comes to certain things like this though, and I need to know the why of things sometimes. Once I learned Med math in detail like this, and realized just how simple it is, I have been able to solve most Med math problems in my head. Although, again, I'm slightly OCD when it comes to certain things so I will still most times wrote them out.

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