Medical Math

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MedicIntern305

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hey all, i am still currently having issues with some of my med math for example questions like this


"you are asked by medical control to administer an iv drug by using a microdrop (60gtts/ml) chamber. you prepare the iv drip by mixing 5 grams in a 250ml iv bag. how many drops per minute should you infuse in order to administer 5mg/min?


could someone set this help and then explain to me how to properly infuse and figure out how many drops per 15 seconds would be needed to get 5mg/min .... i feel so lost please help!
 
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DesertMedic66

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So you have 5gms in a 250mL bag. How many mg of the medication do you have in the bag?

Reduce that down to find out how many mg per mL you have.

From there you need to find out how many mL of the medication you need. To do this use:

Desired dose (5mg) x
________________ X ________________
Dose on hand (mg) mL of dose on hand

Solve for x. Once you get that you will know how many mL of the medication you need to give to get your required dose. Then to find the drip rate:

(mL of medication) X drip rate (60 in this case)
_______________________________________
Time of infusion in mins (1 min in this case)

That will give you the gtts/min. From there divide it by 4 to get how many drops per 15 seconds.
 
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DesertMedic66

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So you have 5gms in 250mL. That is 5,000mg/250mL.

5,000 divided by 250 is 20mg/mL. So your dose on hand is 20mg of medication in every mL of fluid.

5mg x
_____ X _____ = 0.25mL
20mg 1mL

0.25mL x 60 (drip rate)
__________________ = 15gtts/min.
1 min (infusion time)

15gtts/min divided by 4 = 3.75gtts/15 seconds
 
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MedicIntern305

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DesertMedic66 said:
So you have 5gms in a 250mL bag. How many mg of the medication do you have in the bag?

Reduce that down to find out how many mg per mL you have.

From there you need to find out how many mL of the medication you need. To do this use:

Desired dose (5mg) x
________________ X ________________
Dose on hand (mg) mL of dose on hand

Solve for x. Once you get that you will know how many mL of the medication you need to give to get your required dose. Then to find the drip rate:

(mL of medication) X drip rate (60 in this case)
_______________________________________
Time of infusion in mins (1 min in this case)

That will give you the gtts/min. From there divide it by 4 to get how many drops per 15 seconds.
Click to expand...

So in regards to another problem I came across and I'm getting better at the math thing but still a little confused, I understand the formula dose x drip over time.


So if the questions reads "You have a patient that weighs 150 pounds. He has been resuscitated and now has a BP of 60/40. You are going to establish a dopamine drip IV piggyback. You are using a 60gtt set and wish to deliver 5 mcg/kg/min. You mix 200 mg of dopamine into 250 ml of normal saline. What is your flow rate?"


I understand that I have to convert lbs to kg

And use the formula

Order x weight x drip over time. But a little confused on what to plug in for order

Is it 200,000mcg x weight x drip over 1?
 
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DesertMedic66

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Not quite.

Convert lbs to Kg. So you have a 68kg patient.

Next you need to find out your dose on hand and reduce it. So you have 200mg/250ml which is 200,000mcg/250ml. Next you need to reduce it down to find out how many mcg are in each ml. So it reduces down to 800mcg/ml. So 800mcg/ml is your dose on hand.

Now we need to find our desired dose. Desired dose is 5mcg/kg. So our desired dose is 5mcg x 68kg which is 340mcg.

From there we need to divide our desired dose by our dose on hand to figure out how many mLs we need to give the patient.

340mcg (desired dose) x
___________________ X __________
800mcg (dose on hand) 1mL

So x=0.425mL

So in order to give the patient the desired dose of 340mcg/min we need to give the patient 0.425mL of our medication every minute. From here we can multiply it by our drip set (60) to find out how many gtts/min we need.

0.425mL X 60
_________________ =25.5gtts/min
1 min

So to give this patient a dose of 5mcg/kg/min we need to give this patient 25.5gtts/min of our dopamine drip which has a concentration of 800mcg/mL.
 
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DesertMedic66

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IMG_2596.JPG
 
Gurby

Gurby

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MedicIntern305 said:
So in regards to another problem I came across and I'm getting better at the math thing but still a little confused, I understand the formula dose x drip over time.


So if the questions reads "You have a patient that weighs 150 pounds. He has been resuscitated and now has a BP of 60/40. You are going to establish a dopamine drip IV piggyback. You are using a 60gtt set and wish to deliver 5 mcg/kg/min. You mix 200 mg of dopamine into 250 ml of normal saline. What is your flow rate?"


I understand that I have to convert lbs to kg

And use the formula

Order x weight x drip over time. But a little confused on what to plug in for order

Is it 200,000mcg x weight x drip over 1?
Click to expand...

I used to tutor math and I have no idea what is going on in this post. IMO you shouldn't memorize any formulas for doing med math, at least at first. Get comfortable using dimensional analysis which will allow you to figure out any med math problem and actually understand what's going on. Once you understand how they work, then you can memorize formulas to make your life easier. Or don't, whatever.
 
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Handsome Robb

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You could also do order x weight x drip set / concentration....

5 x 68 x 60 / 800

That's the quickest way.

Or just don't use weird concentrations of dope and use the cheater method but that's not a good teaching point as you need to understand the math before you can start using shortcuts.


Sent from my iPhone using Tapatalk
 
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jwk

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Although it's certainly important to understand how to do these types of calculations, it also points out the importance of using pre-mixed/pre-configured infusions whenever possible. Far too many drug administration errors are made because of someone making a math error. Most hospitals have pharmacy pre-mix infusions rather than having the nurses do it at bedside. If you can't have a manufacturer pre-mixed infusion (like dopamine) then have the drug and appropriate sized IV fluid packed together in a ziploc, along with a pre-printed label to help you quickly figure drip rates if you need it. Anything to cut down the chance of errors is helpful.

And although mini-drips and dial-a-flow type devices are nice for infusions, they are horribly inaccurate if you truly need to know an infusion rate. There are plenty of commercially available IV pumps that are small enough for pre-hospital use.
 
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MedicIntern305

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I have gotten how to set up and convert units to be the same, now its just my basic math but the understanding is there! thank youall
 
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