DRUG CALCULATIONS
The Master Formula:
DD x Wt x SS
C x T
DD = Desired Dose
Wt = Weight
SS = Solution Set
C = Concentration
T = Time
What is it?
The Master Formula is a formula that can be used for all infusions. It was developed by my paramedic instructor and is currently a standard in many educational facilities.
How do I use it?
You plug the appropriate numbers into the previously defined variables. This means that if you are not administering a weight based medication & dose that you do not need to use that portion of the formula. I will explain in the fallowing.
DD x Wt x SS
C x T
• Let’s try an example with Dopamine:
• Your dose for Dopamine starts at 5mcg/kg/min.
• This tells us that we will be using the weight (wt) portion of the formula.
• First we plug the dose, and weight into the formula. Lets say our patient is 110 lbs. That equals 50 kg, because 1 kg = 2.2 lbs.
5mcg/kg x 50kg x SS
C x min.
• Notice that the time portion of the dose replaces Time (T) on the bottom of the formula.
• Now we still need our solution set (SS) and concentration (C).
• We will be using a 60gtts/ml drip set.
• Dopamine comes packaged 400mg in a 250ml premixed bag.
5mcg/kg x 50kg x 60gtts/ml
400mg/250ml x min.
• Since our dose is in micrograms we need to convert our concentration from milligrams to micrograms.
• 1mg = 1000mcg
5mcg/kg x 50kg x 60gtts/ml
400,000mcg/250ml x min.
• Next we need to get our concentration simplified to per 1ml (1cc). This means that you divide both sides of the concentration by the volume (milliliters).
• 400,000 divide by 250 = 1,600
• 250 divided by 250 = 1, We do not show the numeral 1 when there is a variable fallowing it(1ml = ml).
C= 400,000mcg/250ml = 1,600mcg/ml
• Now we can plug our simplified concentration into the formula.
5mcg/kg x 50kg x 60gtts/ml
1600mcg/ml x min.
• Now it’s time to start simplifying our formula.
• First you can eliminate all variables that you see twice
• Since mcg is on the top and the bottom we cross them out
5mcg/kg x 50kg x 60gtts/ml
1600mcg/ml x min.
• Do the same with kg &ml.
5mcg/kg x 50kg x 60gtts/ml
1600mcg/ml x min.
• Now you can rewrite the formula without the crossed out variables.
• This will show you what form your dose should end up in. In this example gtts/min.
5 x 50 x 60gtts
1600 x min.
• Now you can just do the math.
• The easiest way is to cross out the zeroes. Cross 1 zero off the top and 1 off the bottom. Then do it again. You will be left with 16 on the bottom.
5 x 50 x 60gtts
1600 x min. =
5 x 5 x 6gtts
16 x min.
• Now divide top and bottom by 2
5 x 5 x 3gtts
8 x min.
• This is as far as you can simplify so multiply the top then divide product by the bottom.
1.)
5 x 5 x 3gtts
8 x min.
a.) 5x5 = 25
b.) 25x3=75
2.)
75gtts
8min.
• 75 divided by 8 = 9.375. Round to the nearest whole number.
• > 0.5 round up. < 0.5 round down.
ANSWER: 9gtts/min.
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Here are some weight conversions to remember:
10kg = 22lbs 60kg = 132lbs 90kg = 198lbs
20kg = 44lbs 65kg = 143lbs 95kg = 209lbs
30kg = 66lbs 70kg = 154lbs 100kg = 220lbs
40kg = 88lbs 75kg = 165lbs 110kg = 242lbs
50kg = 110lbs 80kg = 176lbs 120kg = 264lbs
55kg = 121lbs 85kg = 187lbs 150kg = 330lbs
Concentrations to remember:
NTG (Tridil) = 25mg/250ml = 100mcg/ml
Dopamine = 400mg/250ml = 1,600mcg/ml
Diprivan = 500mg/50ml = 10mg/ml = 10,000mcg/ml
Amiodarone = 150mg/100ml = 1.5mg/ml
Epinephrine (Infusion for anaphylaxis/bradycardia) = 1mg/250ml = 4mcg/ml
Here are some tricks:
Dopamine:
Dose: 5mcg/kg/min
Take the patient’s weight in pounds, drop the last number, and subtract 2. This will give you your drip setting using a 1600mcg/ml conentration, and 60 gtts/ml drip set.
[Weight(lbs), drop last number] – 2 = gtts/min.
ex. 120 lb pt.
1. 120 -- drop the zero
2. 12 -- now subtract 2
3. 10 -- 10 gtts/min.
Amiodarone:
Dose: 150mg/100ml(D5W) over 10min.
Use 10gtts/ml drip set.
The easy way to do this is forget the drug when calculating. Whenever you need to give fluid over a time period just devide the total fluid by the time in min. Then multiply by drip set to get gtts/min.
100(ml) / 10(min) x 10(gtts/ml) = 100 gtts/min
About 3 gtts/2 sec
Mag Sulfate:
Dose: 1g/100ml(D5W) over 10-20min
A Magnesium sulfate dip is similar to an Amiodarone drip because its the same total amount of fluid.
100 gtts/min for 10min. dose
50 gtts/min for 20min. dose
Epinephrine Infusion:
Dose: 2 mcg/min.
Concentration: 4mcg/ml
2mcg x 60gtts/ml
4mcg/ml x min.
30gtts/min. or,
1 drop every 2 seconds
