Dosage Calculation Using the Dimensional Analysis Method

Dosage Calculation Using the Dimensional Analysis Method

Dosage Calculation Using Dimensional Analysis Method: Objectives After reviewing this chapter, you should be able to: 1. Define dimensional analysis 2. Implement unit cancellation in dimensional analysis 3. Perform conversions using dimensional analysis 4. Use dimensional analysis to calculate dosages 2

Background: Dimensional Analysis Fancy name simple technique Used to manipulate units in a calculation Involves cancellation of unwanted units NO memorization of formulas Also known as the factor-label method or the unit factor method Only one equation is used, even if a conversion is required Memorization of common equivalents is necessary 3

Performing Conversions Using Dimensional Analysis Conversions can be made within process of calculation rather than in advance Equivalents or conversion factors are expressed as fractions written in one of two ways without changing meaning or value e.g., CF: 1 kg = 1,000 g is the same as 1 kg/ 1,000 g or 1,000 g/1 kg How a fraction is written depends on the unit you want to cancel or eliminate to get the unit desired 4

Making Conversions Using Dimensional Analysis 1. Identify desired unit 2. Identify equivalent needed 3. Write equivalent in fraction format with desired unit in numerator as first part of equation 4. Label all factors in equation, with x being what you desire to have in the end 5

Making Conversions Using Dimensional Analysis (Cont.) 5. Identify unwanted or undesired units and cancel, then reduce to lowest terms 6. Must be able to eliminate all labels except for answer label if not, recheck 7. Perform mathematical process Note: It is key to set up the equivalent correctly! 6

Making Conversions Using Dimensional Analysis (Cont.) Example: 1.5 g = mg 1. The desired unit is mg 2. Equivalent: 1,000 mg = 1 g 3. Write equivalent keep mg in numerator (allows cancellation of unwanted unit, g) 4. Write equivalent as fraction, add multiplication sign 5. Perform calculation 7

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Dosage Calculation Using Dimensional Analysis 1. Identify unit of measure (caps, ml, tabs) and place x on left with appropriate unit 2. On right, place available information as a fraction information matching the x unit will be placed in the numerator 3. Enter additional factors set up so that unit of numerator matches preceding denominator 4. Cancel out like units remaining unit MUST match the unit for x then calculate 9

Dosage Calculation Using Dimensional Analysis (Cont.) Example: Order is Lasix 40 mg p.o. daily Available: 20 mg tablets 1. Place desired unit of measure on left and label as x x = tab 10

Dosage Calculation Using Dimensional Analysis (Cont.) Example: Order is Lasix 40 mg p.o. daily Available: 20 mg tablets 2. Place the information in problem on right in fraction format matching desired unit in numerator 11

Dosage Calculation Using Dimensional Analysis (Cont.) Example: Order is Lasix 40 mg p.o. daily Available: 20 mg tablets 3. Enter additional info (e.g., what is ordered), matching unit in numerator with preceding denominator 12

Amount to administer Available dosage Ordered dosage 13

Dosage Calculation Using Dimensional Analysis (Cont.) 4. Cancel like units on right of equation; remaining unit of measure should be what is desired 14

Dimensional Analysis Using Conversion Factor When a conversion factor is necessary, the conversion factor is placed as the second fraction inside the equation Always match numerator of conversion factor with denominator of previous fraction 15

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Practice Problems Order. 1) Order: Potassium Chloride 40 meq p.o. b.i.d Available: KCl 20 meq tablets How many tablets will you administer per dose? 2) Order: Cleocin 300 mg IV q6h Available: Cleocin 0.6 g per 4 ml How many ml will you administer per dose? 3) Order: Kefzol 0.5 g IV q6h Available: Kefzol 225 mg per ml How many ml will you administer per dose? 17

Case Study 3 Mr. Ross s fever has subsided and he has no other s/s of infection. The doctor has added Flagyl 500 mg IV q6h and has discontinued the PCA morphine. She also wrote an order for Percocet 5/325 2 tab p.o. q6h prn pain. Mr. Ross is now complaining of 7/10 pain. It is also time to hang the Flagyl. At the Pyxis, you have available: Flagyl 0.5 g in 100 ml Percocet 5/325 tablets 18

Case Study 3 (Cont.) Answer: Pull 2 tabs of Percocet and 100 ml of Flagyl from the Pyxis. As long as Mr. Ross remains in stable condition throughout the night, he will be discharged tomorrow. 19