Medical Math for Dummies :
Medical Math for Dummies The Basics :
The Basics The Metric System
Basic units of measure:
meter – linear
liter – volume
gram – weight (mass)
Each unit is subdivided in units of 10 by moving the decimal point. Abbreviations :
Abbreviations Kilogram is kg
Milligram/Milliliter is mg/ml
Microgram is mcg or ug
Milliequivilant is mEq Slide 4:
KILO – 1,000 times greater than
Centi – 100 times greater than
Deci – 10 times greater than
MILLI – 1,000 times less than
MICRO – 1,000,000 times less than Slide 5:
SO……………
1 kg = 1000 mg
1 mg = 1000 mcg
1 kg = 1,000,000 mcg General Rules :
General Rules DO NOT capitalize: use “ml” or “kg”
No periods in the abbreviations
Single space between quantity and unit of measure (25 mg not 25mg)
Do not use plurals (35 mg not 35 mgs)
Anything less than 1 must have a zero to the left of the decimal (0.25 mg) Other units to remember :
Other units to remember 1 T (tablespoon) = 3 t (teaspoon)
1 t = 5 ml
1 T = 15 ml
cc means the same thing as ml Things to remember!!!! :
Things to remember!!!! Slide 9:
Amount of Drug or Amount on hand
How much drug is in the box or the syringe. For example, Epinephrine comes packaged 1 mg in 10 cc. This will be expressed as 1 mg/10cc. This means that there is 1mg of Epi in 10 cc of fluid (almost always saline). Because of diffusion, it will NEVER separate and each drop will always have the same amount of drug. Slide 10:
Dripsets
gtts means drops per cc/ml
10 gtts set, 15 gtts set, 60 gtts set, means that for every 10, 15, or 60 drops that falls in the drip chamber, 1 cc/ml of fluid has passed and therefore into the patient. Slide 11:
Weight
All weights will be in kg. There are two methods to convert a patients weight from pounds to kilograms.
Example 1:
Take the weight in pounds and divide it by 2.2.
Pt wt is 220 pounds.
100
2.2 220.0 100 kg Slide 12:
Example 2:
Divide the weight in pounds in ½ and then subtract 10% from that number.
220 pounds divided by ½ is 110.
110 minus 10% (11) is 99 which we can round up to 100 kg
220 pounds = 100 kg Rounding off…. :
Rounding off…. When you are trying to figure out drug dosages round off the weight to the closest 5 or 10 digit number. For example 82 kg can be rounded to 80. 93 kg can be rounded to 95. It will make your math much easier and the change in drug amount will be minimal. Four easy steps….. :
Four easy steps….. Calculate the Desired Dose
Calculate the Concentration
Calculating the cc’s or ml’s
Calculating Drip Rates STEP ONE :
STEP ONE CALCULATE THE DESIRED DOSE Slide 16:
Desired Dose or DD
The amount of drug ordered by the physician or standing orders
For example: Dr Smith orders you to push 1mg of Epi. The desired dose is then 1mg.
If the drug to be given depends on the patients weight, then you may have to calculate the desired dose. Calculating the Desired Dose :
Calculating the Desired Dose Dr. Smith orders you to give your 198 pound patient dopamine at a dose of 5 mcg/kg. Since we know that it is a “weight based” drug, we first need to figure out the weight in kg.
90
2.2 1980
So 198 pounds is 90 kg. The DD is 5 mcg/kg. This means that for every one kg of patient body weight, we will give 5 mcg of drug. Therefore 90 x 5 mcg = 450 mcg. The desired dose then is finalized at 450 mcg. STEP TWO :
STEP TWO CALCULATE THE CONCENTRATION Slide 19:
Concentration or Yield
The amount of drug in 1 cc/ml or fluid.
amount of drug
Conc = ----------------------
amount of fluid Calculating the Concentration :
Calculating the Concentration Remember that in all cases, concentration is expressed as mg/cc or mg/ml.
1 mg/10 cc 2 mg/2 cc 50 mg/2 cc 1 gm/250 ml
Don’t forget. We need the concentration to be in its base form. ie., mg per 1 cc or mg/cc
2 mg/2 cc = 1 mg/cc Slide 21:
The units for the desired dose and the concentration must be the same.
For example, if the DD is in mg or g, then the concentration must match that.
Make sure that the concentration is expressed in its most basic form. Ie., mg/cc ? just one cc. So far… :
So far… Step 1 = DD
Step 2 = Concentration
(always weight/cc or ml. Ie., mg/cc or ml Review :
Review Doctor orders you do give 20 mg of Lasix IV. On hand you have Lasix 40 mg in 4 cc. a. What is the desired dose? 20 mg b. What is the Concentration? 10 mg/cc Slide 24:
You have been ordered to give 8 mg of Calcium Chloride. In your box is a vial that contains 20 mg in 5 cc. a. What is the DD? 8 mg b. What is the concentration? 4 mg/cc STEP 3 :
STEP 3 CALCULATING THE cc’s Slide 26:
Calculate the amount of cc’s to be delivered. DD ____ C Slide 27:
The desired dose of Drug A is 500 mg. It comes packaged 50 mg/cc.
You have been ordered to give 1 mg/kg of lidocaine IV. Your patient weighs 176 pounds. The prefilled syringe contains 100 mg in 5 cc. Review… :
Review… Step 1. Calculate the DD
Step 2. Calculate the Concentration
Step 3. Calculate the # of cc to be given.
Remember to always bring your units with you and cancel out any zeros that you can. STEP 4 :
STEP 4 CALCULATING DRIP RATES Slide 30:
Generally speaking, when you…
Have to mix a drug into a bag of fluid
Have a problem that states “hang a drip”
Are dealing with specific drugs such as Dopamine that are always given as a drip
We are talking about hanging a bag of fluid that has had some sort of drug mixed in it and that will be infused over a certain amount of time, or until you have reached a desired effect. Slide 31:
Some general rules regarding hanging a drip.
- We do not add the amount of fluid that is in the vial to the total fluid amount in the bag. For example, if our Dopamine comes in a vial that says 400 mg/10 cc and we are putting it into a 250 cc bag of D5W, we don’t add the 10 cc to the 250 to make 260 cc. The addition of the 10 cc to the 250 will have an insignificant change in the concentration, and to make our math a bit easier, we do not include it.
- Unless otherwise noted, we will always use a 60 gtts/cc drip set Slide 32:
Calculating the drip rate is necessary to deliver the desired dose. cc which we figured in step 3 x Drip set in gtts/cc ------------------- Time always expressed in minutes Slide 33:
The TIME factor will ALWAYS be per minute in your calculations. For example 2 hours is expressed as 120 minutes. Slide 34:
You have been ordered to hang a Lidocaine drip @ 4 mg/min. To get your bag ready, you mix 1 g in a 250 cc bag of D5W. Using a mini drip, calculate how many drops per minute you must deliver.
Step 1. What is the DD? 4 mg/min Step 2. What is the Concentration? Remember, since the desired dose is in mg, then the concentration must be in mg also. 1 g = 1000 mg.
1000 mg/250 cc = 4 mg/cc. 1000 divided by 250 is 4. Therefore the C is 4 mg/cc. Slide 35:
Step 3. Calculate the # of cc to be given. Remember the formula DD
-----
C 4 mg 4 mg/cc 1 cc (remember we can cancel out the mg units) ---------- Therefore, 1 cc of fluid will be given. In that 1 cc, 4 mg of lidocaine will be infused. Slide 36:
Step 4. Calculate the drip rate. Remember the formula…
cc x drip factor
---------------------
time Now we just plug in our numbers from steps 1, 2, and 3. Slide 37:
1 cc (from step 3) x 60 gtts/cc (from our orders)
--------------------------------------------------------
1min (from our rules)
1cc x 60 gtts/cc 60 gtts
------------------- = --------
1 min 1 min Ok, here’s what we know. To get our desired dose of 4 mg/min of lidocaine, we have to have 60 gtts of our bag flow every minute. How do we see that in our drip chamber? Slide 38:
What we know…
60 gtts need to be infused every minute
60 seconds for every minute Therefore, we can write 60 gtts/min as 60 gtts/60 sec or 1 gtt/sec. So in our drip chamber we want to see
one drop fall every second. REVIEW THE STEPS :
REVIEW THE STEPS Step 1. DD
Step 2. C (always weight/cc ie., mg/cc
Step 3. DD/C
Step 4. cc x drip factor
------------------
time
DON’T FORGET YOUR RULES AND UNITS!!!. SQUAD 1 :
SQUAD 1 You have been ordered to hang a dopamine drip to be run at 5mcg/kg/min. Your patient weighs 180 pounds. Your dopamine is premixed with 400 mg in 250 cc. How many gtts/min will you deliver? How will you see that in your drip chamber? SQUAD 2 :
SQUAD 2 Dr. Keeler wants you to hang a procainamide drip at 20 mg/min. On hand is a vial containing 1 gm, a 50 cc bag, and a mini-drip. How many gtts per minute would you deliver with a patient weight of 225 pounds? SQUAD 3 :
SQUAD 3 You have orders to start a Nipride drip to be run @ 0.5 mcg/kg/min. Your Nipride comes packaged 50 mg/250 cc. Your patient weighs 154 pounds . Using a mini drip how many drops per minute would you deliver? SQUAD 4 :
SQUAD 4 You have been ordered to give 0.5 mg of Lidocaine IVP followed by a Lidocaine Drip at 2 mg/min. In your box you have a prefilled lido syringe that reads 100 mg/5cc and a vial containing 1 gram and a 500 cc bag of NS. For your patient that weighs 200 pounds, how many cc would you give IVP and how many drops per minute would you give via the IV drip? SQUAD 5 :
SQUAD 5 You have been ordered to deliver 200 cc/hr of a premixed aminophylline drip for the first hour. After the first hour, the doctor wants you to deliver the remainder at 125 cc/hr. Using a minidrip, what is your gtt/min for the remaining aminophylline and how would you see that in your drip chamber (gtts/sec)