Dimensional Analysis Your Friend not your Enemy!
The First Thing You Always Ask Yourself is: What is the question asking me to find? What is the unknown piece? What is/are the unit/s of measure that need to be calculated? Example: If the question stated, The physician has ordered Wellbutrin 75 mg po for smoking cessation. The medication is supplied in 150 mg tablets. How many tablets will the nurse administer? What item is the question asking you to identify in the question above? Answer: How many tablets. Good Job!
Try it Again How many inches are in 2 feet? What is the unknown piece of information? Answer: inches (in two feet) Good Job!
Were these easy? Unfortunately they are not all that easy The question may require several steps before a final answer can be formulated. Many students get confused because they see a lot of numbers and words in the sentence: e.g. 75mg, 150mg, po, tid, in 24 hours, physician, nurse, tablets, kg etc If you can stay focused on what the ultimate goal or variable is that you want to pull from the sentence then you usually won t go astray
HERE S AN EXAMPLE OF DISTRACTERS A patient weighed 80 kg on admission and is now 75 kg. The doctor s order states that they are to receive 50mg of Drug X mixed in 250mL of Normal Saline every 8 hours. How many mg will the patient receive at 3pm, which is when the next dose is due? What is the question asking? What item/piece of information needs to be extracted from the question? Answer: Good Job if you said How many milligrams at next dose What is the answer? Answer: 50mg
Did we need to know about: A patient weighed 80 kg on admission and is now 75 kg. The doctor s order states that they are to receive 50mg of Drug X mixed in 250ml of Normal Saline every 8 hours for 24 hours. How many mg will the patient receive at 3pm, which is when the next dose is due?
Try It Again With A Little Less Information Dr. Orders: Demerol 50 mg IM STAT Available: Demerol 100mg/2mL What is the piece of information that you must find? Answer: Good Job if you said How many ml
See if you can get this one What is the statement asking you to find? The physician has ordered heparin IV drip at 1200 units per hour. The medication is supplied in 25,000 units/500ml of NS. Calculate the flow rate in ml/hr. Answer: How many ml it must infuse in one hour
Dimensional Analysis Similar to Ratio and Proportion in that you: deal with fractions deal with known and unknowns must set it up a certain way
Now that we know what to retrieve we are ready to set up the problem! Sometimes the answer can be easily retrieved without any written computation. Can you give the answer to this question without writing it down? The physician has ordered Wellbutrin 200 mg p.o. for smoking cessation. The medication is supplied in 100 mg tablets. How many tablets will the nurse administer? Answer: 2 tablets Good Job! Let s work it out using dimensional analysis.
1. The physician has ordered Wellbutrin 200 mg p.o. for smoking cessation. The medication is supplied in 100 mg tablets. How many tablets will the nurse administer? The Item: How many tablets Write it down the same way? Tablets What ever the item is, whether it is tablets, mg, kg, ml, hours etc.. You always write it first and write it as a numerator (e.g. tablets) X
2. Then return to your statement and pull everything from it that deals with your item (the tablets). The physician has ordered Wellbutrin 200 mg p.o. for smoking cessation. The medication is supplied in 100 mg tablets. How many tablets will the nurse administer? 1 tablet = 100mg This is the concentration of the medication you have available to you.
3. The physician has ordered Wellbutrin 200 mg p.o. for smoking cessation. The medication is supplied in 100 mg tablets. How many tablets will the nurse administer?? tablets 1 tablet dose = 100mg
4. Cross elimination- - Although you will need some of the other numbers in the statement in order to produce your answer- the words attached to those numbers have to be eliminated.(i.e. in this case mg.) - The only way to do this is to align them so that you can cross eliminate. You keep the # s but get rid of the title? Tablets = 1tablet x 200mg dose 100mg dose - Multiply the concentration (100mg/tablet) and the ordered dose (200mg/dose) to get the # of tablets required.
5.? Tablets = 1tablet x 200mg dose 100mg dose You got rid of the mg and the only thing left is what you needed- tablets. Now multiply across?tablets = 1 tablet x 200mg= 200 dose 100mg dose 100 Now Divide 200 100 = 2 tablets
There is always another item to know! Conversions!
Conversions Quite often the numbers provided in the statement are in a unit of measure different from the question For e.g. you are asked how many kg the patient weighs when all you have in the statement is the weight in pounds, or a drug is stated in grams and you must give milligrams Example: The physician orders 0.4mg of atropine. The drug label reads 400mcg per 1mL Going back to our 1 st step- what is the piece of information we need to find? Answer: how many ml?
Conversions You must memorize several important conversions or know when and where to look them up!!! They will be a very important step in you completing many problems 1 gm = 1000 mg 1 mg = 1000 mcg 1 gm = 1,000,000 mcg 1 L = 1000 ml 1 oz = 30 ml 1 tsp = 5 ml 1lb = 16 oz 1 kg = 2.2 lbs 1 kg = 1000 gm
1 gm = 1000 mg 1 mg = 1000 mcg 1 gm = 1,000,000 mcg 1 L = 1000 ml 1 oz = 30 ml 1 tsp = 5 ml 1lb = 16 oz 1 kg = 2.2 lbs 1 kg = 1000 gm Looking at the table, which conversion equation do you think we need in order to complete our problem: The Physician orders 0.4 mg of atropine. The drug label reads 400mcg/mL. How many ml will you administer? Answer: You need a conversion that has both mg and mcg. So you would choose 1 mg= 1000mcg? ml = 1mL x 1000mcg x 0.4 mg = 400 = 1mL Dose 400mcg 1mg dose 400 dose
Try this one yourself. The physician has ordered heparin IV drip at 1200 units per hour. The medication is supplied in 25,000 units/500 ml of NS. Calculate the flow rate in ml/hr. ANS: 24 ml/hr