dimensional analysis - central texas...
TRANSCRIPT
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 AgainHow 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/2mLWhat 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.
? tablets 1 tablet
dose = 100mg
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?
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 = 2 tablets
100
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 1st 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