Loading Dose Calculation Help

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This is actually my Chemistry for Health Sciences class but the prof wants to get us thinking critically early so here is a problem he gave us with regard to acetaminophen overdose:

The nurse was instructed to administer an IV with a loading dose of 150 mg/kg of acetylcysteine in 200 mL of D5W infused over 15 minutes. If the patient weighs 108 lbs, what is the resulting loading dose?

I got as far as converting the patient's weight to kg (49 kg) and after that, I'm lost...I have been scouring the net for 2 hours and can't seem to find a formula for this particular problem. Any help would be appreciated!

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Specializes in ICU.

You are right to convert the weight to kgs.

Now you need to think about what the question is asking...what is the loading dose...well with this question the loading dose would be in mg. The question has more information than you need. You do not need to know the the infusion time, as that does not affect the dose, and the 200ml is the set vloume amount that your dose is going to be in, so you do not need the 200ml.

So, you are left with: you need 150mg/kg, and you have 49 kg, and need to know Xmg.

Try thinking that through, let me know if you need more help, I don't to completely give the answer though, because you will need to learn the process for future problems.

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That was very helpful! The too much info part is what kept throwing me. I kept wondering why that info was necessary. So if the instrux are 150 mg/kg and I have 49 kg then I multiply 150 mg x 49 kg and get 7350 mg? That sounds like a lot.

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Specializes in ICU.

You got it!

If you think about it, the answer is going to be high. you have 150 mg per every kg...that adds up quickly.

the hardest part with most of those questions if finding out what you really need to know to answer the question.

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But wait! There's more! :specs: See, this was an article from Nursing magazine back in 2005. I'll type the para and then I'll tell you what the prof is asking from the article.

Administer an IV loading dose of 150 mg/kg of acetylcysteine in 200 mL of D5W, infused over 15 minutes. Follow this with a maintenance dose of 50mg/kg in 500 mL of D5W infused over 4 hours, then 100mg/kg in 1000 mL of D5W, infused over 16 hours.

The first question was the one you helped me solve above. The second question is:

Based on the information above (7350 mg) and the instructions the nurse received, what is the concentration of the maintenance dose that should be adminstered?

Am I missing something obvious again? Is the answer right there in the question?

Next question:

The maintenance dose is to be in 500mL of D5W over 4 hours. What is the volume per hour that should be adminstered?

And the last question is:

This is followed by 100 mg/kg in 1000 mL of D5W infused over 16 hours. What is the amount to be adminstered (how many mg) and what is the rate (mL/hour?)

Well the answer to the first part of this one is obvious, right? Multiply 40kg by 100 mg?

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Specializes in ICU.

Ok, so take it one step at a time.

The 7350mg is the loading dose.

For question 2 do you need the concentration of the loading or maintainence dose?

Question3

The volume per hour will be in ml/hr. So you have 500ml over 4 hours= x ml/hr

Question 4

You are right, the dose will be the weight 49kg*100mg = xmg

The concentration is figured out the same way as question 3. Your answer will be ml/hr. You have 1000ml over 16hr =x ml/hr

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For question 2, the concentration of the maintenance dose.

I think I was trying to make this exercise harder than it was. So for question 3 I got 500mL/4 hours = 125 mL/hr

question 4-4900 mg and part 2 was 1000mL/16 hours = 62.5 mL/hr

Right?

And last, can you verify if I got this other question right? Here it is. Answer follows:

Children have a maximum daily dosage of acetaminophen of 90mg/kg. What is the maximum amt of tablets (at 500 mg per tablet) that a child weighing 85 lbs can take in one day?

I converted the weight to kg - 37 kg. THen I did a conversion chart:

37 kg x 90 mg x 1 tablet/ 1 kg x 500 mg and got 6.7 tablets a day.

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Specializes in ICU.

You have it.

For question 2 there will be 2 answers then, since the maintainence dose is for 4 hrs and then for 16 hrs. 2450mg/500ml for 4 hrs followed by the 4900mg/1000ml for 16 hrs.

For the new question, the only problem is 85lbs=38.6kg, but you did it right. With 38.6kg, it would be 6.9 500mg tabs in a day--heads up though, you can't really give 6.9 tabs, so you would have to round down--either to 6.5 or 6--some instructors are picky with that. So for your answer you can write that in along the following lines--mathmatically, the answer is 6.9 tabs, but I would not give more than 6tabs as I can not give 6.9. Does that make sense?

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That was enormously helpful. I thank you. :nuke:

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Specializes in ICU.

your welcome, not a problem.

Just remember, take everything one step at a time, and figure out what the question is asking and realize a lot of time there is more information provided than what you need to answer the question.

Good luck with future problems, you are on the right track

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Specializes in med/surg, telemetry, IV therapy, mgmt.
the nurse was instructed to administer an iv with a loading dose of 150 mg/kg of acetylcysteine [mucomyst] in 200 ml of d5w infused over 15 minutes. if the patient weighs 108 lbs, what is the resulting loading dose?

this is for a chemistry class, right? so, your instructor is probably looking for you to use dimensional analysis to solve this problem. do the words dimensional analysis ring a bell at all? dimensional analysis is used in chemistry and physics all the time to solve equations.

in this particular problem, you are looking to end up with a number that will be expressed as mg/200ml of d5w. you set up a series of fractions to manipulate the labels on the numbers to end up with those labels. the only thing you need to find in this problem in how many mg you are going to need. i come up with a different answer because of rounding differences.

loading dose
:
150 mg
/
1 kg
(dose to give)
x
1 kg
/
2.2 lb
(conversion factor)
x
108 lbs
(patient weight)
= 7363.6363 mg
(all your other labels on the numbers cancel out)
=
7364 mg (rounded off)/200 ml of d5w

follow this with a maintenance dose of 50mg/kg in 500 ml of d5w infused over 4 hours, then 100mg/kg in 1000 ml of d5w, infused over 16 hours.

maintenance dose for next 4 hours
:
50 mg
/
1 kg
(dose to give)
x
1 kg
/
2.2 lb
(conversion factor)
x
108 lbs
(patient weight)
=
2454.5454 mg
(all your other labels on the numbers cancel out)
=
2455 mg in 500ml of d5w
(to infuse over 4 hours)

maintenance dose for subsequent 16 hours
:
100 mg
/
1 kg
(dose to give)
x
1 kg
/
2.2 lb
(conversion factor)
x
108 lbs
(patient weight)
=
4909.0909 mg
(all your other labels on the numbers cancel out)
=
4910 mg in 1000ml of d5w
(to infuse over the next 16 hour)

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