Conversions: Metric and Household Systems Chapter 4 Conversions: Metric and Household Systems

Conversions between Metric and Household Systems Review base units of the metric system

Pre-scribed Medications: Ordered in a unit of weight measurement usually gram or milligram RN responsibility to interpret order RN responsibility to use conversions if necessary Administer correct amount of medication prescribed Give example on board: ORDER: Acetaminophen 650 mg to be given. ON HAND is 325 mg tablets. HOW many?? If sam e amount in liquid: 650 mg ON HAND: 325 mg per 5 mL HOW much?

Identifying Need for Unit Conversion A drug order written in a unit of measurement different from what is supplied Usually written in one metric size (mg) but is supplied in another metric unit (g) Encounter apothecary or household measure Converting a unit of measure does not change amount of ordered dosage of medication! It provides an alternate expression of dosage. Example to be done on board next slide

Identifying Need for Unit Conversion Medication order versus supply on hand Medication order: triazolam 250 mcg orally at bedtime Supply on hand: Halcion 0.25 mg tablets Medication order: Antacid Plus ½ fl oz after meals Supply on hand: Antacid Plus 1 fl oz (30 mL) First example: prescribed quantity must be converted into the unit of measure supplied ANSWER= 0.25 mg tablets; give 1 page 98-99 Second example: nurse selects correct unit of measure from 2 options provided on label before calculating ordered dose

Identifying Need for Unit Conversion First example: prescribed quantity must be converted into the unit of measure supplied Second example: nurse selects correct unit of measure from 2 options provided on label before calculating ordered dose Therefore, calculating unit conversions are the first steps in dosage calculations

RULE FOR PROPORTION/RATIO Recall Equivalents Set up proportion for 2 equivalent ratios Cross multiply to solve for an unknown quantity, X Another method to perform conversions to change from one unit to another within the same measurement system is: How do we do this? We us ratio/proportion to convert from one unit to another, you need to follow these 3 steps

Conversions Proportions and ratios are set up for solving in the same format. THIS IS ESSENTIAL. The like units must be across from each other. The units in the numerators match, and the units in the denominators match. In a proportion, the ratio for a known equivalent equals the ratio for an unknown equivalent Remember: a proportion compares like things to like things

Converting Using Ratio-Proportion Example: How many cups are equivalent to 3 quarts? It is known that 1 qt equals 4 cups Set up proportion by expressing ratios as fractions What we know: the first ratio of the proportion contains the known equivalent; 1:4 the second ratio contains the desired unit of measure and the Unknown Equivalent expressed as X 3:X Express the ratios as fractions… Note: X, in the denominator, is the unknown.

Converting Using Ratio-Proportion Cross-multiply Simplify: Divide both sides by number before X Therefore, 3 qt = 12 cups Do example 2 on the board from page 100: example 2: ask class what is the known equivalent AND then, the desired unit of measurement and the unknown equivalent

In review: Using ratio-proportion method: Recall known equivalent Set up proportion Ratio for known equivalent equals ratio for unknown equivalent Cross-multiply to solve for unknown quantity (X)

Converting within Metric System Most conversions are derived by multiplying or dividing by 1,000 Multiplying by 1,000 Move decimal three places to right Used when converting from larger to smaller unit Note: Sometimes you have to add zeros to hold the places equal to the number of zeros in the equivalent. Example: 2 grams per tablet means the same as 2,000 milligrams per tablet. 1g = 1,000 mg, so 2 x 1,000 = 2.000 = 2,000 mg On board can show ratio proportion method

Converting within Metric System Most conversions are derived by multiplying or dividing by 1,000 Dividing by 1,000 Move decimal three places to left Used when converting from smaller to larger unit Example: Convert 20 mg to g 1g = 1000 mg, so we divide moving decimal 3 places to left; x=0.02g Can show this in ratio proportion method too on board.

Converting within Metric System Example: Convert 0.3 g to equivalent number of mg Recall known equivalent Equivalent is: 1 g = 1,000 mg Set up equivalent ratios Ask class to do this themselves before answering

Converting within Metric System Cross-multiply Answer: Ask class to convert 50 mL to L What is first rule….second rule…third rule…

Converting within Metric System Example: Convert 50 mL to L Recall known equivalent Equivalent is: 1 L = 1,000 mL Set up equivalent ratios

Converting within Metric System Cross-multiply Simplify

Remember: When converting from a larger to a smaller unit of measure, the number of smaller units should be greater than original number of larger units Conversely, When converting from a smaller to a larger unit of measure, the number of larger units should be less than the original number of small units

Conversion Slide Use this diagram when converting dosages within metric system See which way the arrow points then: 1 mcg = 0.001 mg (moved decimal point to the left 3 places) 2 g = 2,000 mg (moved decimal point to the right 3 places) kg g mg mcg Move decimal three places to left for each step Move decimal three places to right for each step On the board: page 101 example 2-6 (practice for converting from larger unit to smaller unit) example 1-5 (practice for converting from smaller unit to larger unit) Have class work on review set 13 pt 106

Approximate Equivalents 1 t = 5 mL 1 T = 3 t = 15 mL = fl oz 1 fl oz = 30 mL = 6 t = 2 T 1 L = 1 qt = 32 fl oz = 2 pt = 4 cups 1 pt = 500 mL = 16 fl oz = 2 cups 1 cup = 250 mL = 8 fl oz 1 kg = 2.2 lb 1 inches (in) = 2.5 cm need to know! Infrequent and may soon be completely obsolete Any conversions based in the houselhold/apothecary system is based on approx. equiv. They are called approx. because it is impractical to use exact equivalents; ie, A more exact equiv of 1 US fl oz is 29.5735296875 mL or approx. 30 mL In the UK, 1 fl oz is 28.4130625 mL

Converting Between Systems of Measurement Use ratio-proportion method Same steps as when converting within same system of measurement

Converting Between Systems of Measurement Convert 2 fl oz to mL -converting from a larger to smaller unit Recall equivalent Equivalent is: 1 fl oz = 30 mL Set up equivalent ratio Cross-multiply Converting between larger to a smaller unit

Converting Between Systems of Measurement Example: Convert 45 mL to t Converting from smaller to larger unit Recall equivalent Equivalent is: 1 t = 5 mL Set up equivalent ratio how about

Converting Between Systems of Measurement Cross-multiply Simplify Then , convert from a smaller to a larger unit of measure On board, examples 2-3 page 108-109 THEN ask: what about kg and lb….what is the known equivalent? Set up problem for determining what 40 kg is in lbs

Converting Between Systems of Measurement Approximate equivalent: 1 kg = 2.2 lb (so our weight in kg is about half of our weight in pounds)

Converting Between Systems of Measurement When converting from a smaller to a larger unit of measure, the resulting number of larger units will be less than the original number of smaller units. When converting from a larger unit of measure to a smaller unit of measure, the resulting number of smaller units should be greater than the original number of larger units. Always look at your answer and ask yourself if it makes sense.

Quick Review Steps for using ratio-proportion method to convert within or between systems of measure: Recall known equivalent Set up proportion ratio Label and match units in numerators and denominators Cross-multiply to find value of x Label units in answer to match unknown x Have class work on review set 14, questions 1-30

Can go over odd problems from review set 14 questions 31, 32, 36, 37