Stuck on Med Calculation

[Garo]

Hi there,

I'm pretty comfortable with my dosage calcs, but this one has me a bit confused. Here it is:

You are to administer MgS04 IVPB at 1Gm. per hour. You are to add 20gm of MgS04 to 500mL D5LR. You have a 10mL vial of 50% MgS04 on hand.

a. How much will you add?

Ok, I know I will add 40ml, but the next part is where I get stuck

b. At what rate will you infuse the IVPB?

Next I figure I add the the 40 ml to the 500, resulting inthis equation:

ml/hr = 540ml/20gx 1g/hr= 27ml/hr

The reason I ask is that some in my class say you don't add the 40ml to the 500. This would result in a rate of 25ml/hr. Why would I not figure the 40 ml in the rate if that fluid is in fact is added to the bag?Am I missing something?

Thanks so much,

Garo

[laxxrickLPN]

This confuses me too...

If you have 10mL vial of 50% MgSO4, wouldn't you need to know the mg per mL?

[Garo]

The questions asks for the grams, not the mg.

Since it says "50%", that means there are 50 grams in 100 ml. Remember "%" means grams "per hundred 100 mL". If the answer asked for mg you would just do a 1g/1000mg conversion.

[John--RN]

Well this post is probably not gonna be much help other then to agree with you. I do not see how you could possibly ignore the extra 40ml of volume.

[EDRN-2010]

I posted a similar question. My friends are telling me anything less than 10% you do not add to the total...called the 10% rule. I don't see why it is wrong if you do it, even if it is ok not to add it if less than 10%, if you did add it the answer would be a more accurate calculation.

[vanlo001]

The difference between the two infusion rates is 25ml/hr if you don't add in the 40mls and 27ml/hr if you do account for the extra 40mls. 2cc/hr is not significant. Think about 2cc very small amount. Now if your talking about a little baby maybe it's significant.

[HM2VikingRN, RN]

I think this illustrates the importance of having another person double check your figures. As a psych nurse I don't work with this med (Or IVs for that matter).

50% solutions tend to raise red flags for me.

I would think the vial would have the medication grams per unit dose stated. If that is the case then it becomes much easier. You already know each vial contains 5 mg. So 4 vials would be added to the IV solution. (I also question the ivpb order as that implies the mag sulfate is running in followed by the IV solution instead of as an admixture.) 500 mls divided by 20 yields 25 mls/hr.

(Of course the med will be run through a pump which also adds a margin of safety...)

[vanlo001]

We often give magnesium as a bolus. There is a specific amount you can give per kg per hour. Generally mag runs in over four hours and a patient must be on a CRM monitor during the infusion. It is not added to the IVF because you do not want the patient to experience hypermagnesium so usually a bolus is given a level check the next day perhaps an oral supplement added after that. I've also seen some facilities remove the same amount they are adding. So if you are adding 40 ml youvwill remove 40 ml of the IVF and then add the med. Most important to me is to look up the meds each time.