# Dosage Calculation trouble



## ClarkKent (Jul 26, 2011)

I am taking an online class for dosage calculations for nursing.  I am done reading the book and all of the homework.  On the last chapter called Advanced Adult Intravenous Calculations, she added in a question I have never seen before.  I worked it out but she is say I am wrong.  Once you turn in the homework online, she sends you the answers via e-mail.  She is “out of the office during the summer time”, and good luck getting ahold of here.  So here is the question

A patient’s IV order for lidocaine states that he should receive an IV bolus of 100 mg over 2 minutes using a supply of 2% lidocaine.  

What is the supply dosage of the lidocaine in mg/mL? __________ mg/mL
2% = 2g / 100mL = 2000mg / 100mL
Answer is 20 mg/mL (This part I have done)

How many milliliters will you administer for this order using the available supply? __________mL

20mg / mL = 100 mg / X	(I used a different way, but this is her answer, not mine)
20X=50
X=2.5
(This is the part I do not understand, going from 100 to 50 [units not listed on her answer page, remember this is her answer not mine])

Formula I have been using since the start of the class is

(Dosage Ordered / Supplied Dosage) X Quantity = Amount to Administer

Lets plug in the numbers:

(100 mg / 20 mg) X 1 mL= 5mL NOT 2.5mL

If you read both question carefully, it does not ask anything about time.  So the “over 2 minutes” statement is extra information that she likes to add into problems.  She loves to give you too much information.  

Any help getting this answered before I loss all of my hair would be great.  Tell me that I am not getting and I need to retake the class or the professor is off her rocker.  If you want to see her answer page, please let me know and I will be more then happy to send it to you.  But its going to show you the same thing.


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## MrBrown (Jul 26, 2011)

We threw out lidocaine years ago but for semantics its not important 

If the concentration of your lido is 2g/100ml then you want to administer 5ml 

2g/100ml is broken down into 1g/50ml or 1000mg/50ml

1000mg/50ml is further broken down to 100mg/5ml which is what you want to dish out


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## Canmedic (Jul 26, 2011)

Your answer is correct provided the question is worded EXACTLY as you have written it. 

Her answer looks like she is asking the rate (in ml) per minute instead of the total volume after 2 minutes.


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## ClarkKent (Jul 26, 2011)

Canmedic said:


> Your answer is correct provided the question is worded EXACTLY as you have written it.



The question has been cut, copied, and pasted into this thread to ensure that it is word for word straight from her answer page.  If she wanted milliliters per minute, she would have the mL/min next to the bank.  But that is not the case and that is why it is driven me up the wall. I just don't get it.  Or maybe, just maybe, she is off her rocker.  Don't tell her I said that.


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## 18G (Jul 26, 2011)

Where is the volume(100mL) coming from? It doesn't seem to be apart of the actual question. Just curious.  


Dose x Volume  = Amt to administer (mL)
Concentration

This is the only formula you need to know for parenteral medications. Dosage calculations courses make this stuff seem so much harder then it actually is.  

Here is your problem plugged into the formula:

100mg x 100mL = 5mL
2000mg

As a side note...
Standard lidocaine concentration for bolus in pre-filled is 100mg/5mL. Infusion concentration is pretty standard at 2000mg / 500mL which equals 4mg/mL. Once you learn these little things it makes dosing a little easier too. 

I know its stressful for ya now in the course but once your practicing in the field the majority of your dosing is intuitive. In an urgent situation you dont have time to go through a long formula so the drugs are packaged for easy dosing. And it also helps to eliminate errors.


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## ChorusD (Jul 27, 2011)

I would bet it's a typo on her part.  2.5 just happens to be the gtt/min rate if you had a 60gtt/ml set.


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