Transcript Chapter 4
Conversions: Between and Within Systems Textbook Assignment: Pickar, G. (2007). Dosage calculations: A ratio-proportion approach. (2nd ed.) Chapter 4 Revised KBurger0808 Copyright © 2008 Thomson Delmar Learning Equivalents 1 grain (gr) = 60 milligrams (mg) 1 teaspoon (t) = 5 milliliters (mL) 1 tablespoon (T) = 3 teaspoons (t) 1 ounce (oz) = 30 milliliters (mL) 1 cup = 8 ounces (oz) 1 Kilogram (Kg) = 2.2 pounds (lbs) 1 liter (L) = 1000 milliliters (mL) 1 gram (g) = 1000 milligrams (mg) 1 milligram (mg) = 1000 micrograms (mcg) The equivalents listed in blue are only considered approximate equivalents Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion • Rule – Recall equivalents – Set up a proportion of two equivalent ratios – Cross-multiply to solve for an unknown quantity, X Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion • Remember – Each ratio in a proportion must have the same relationship and follow the same sequence – A proportion compares like things to like things Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion • Remember – The units of measurement in both numerators and denominators must match – ALWAYS, ALWAYS, ALWAYS label the measurement units in each ratio INCLUDING your unknown quantity X Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion • Example – How many feet are in 36 inches? Copyright © 2008 Thomson Delmar Learning Converting Using Ratio-Proportion 1 foot = 12 inches 1 ft X ft 12 in 36 in 12X 36 12X 36 12 12 X 3 feet • Recall equivalent • Set up a proportion of two equivalent ratios • Cross multiply to solve for “X” • Label units to match the unknown “X” Copyright © 2008 Thomson Delmar Learning Using Ratio Proportion to Convert Within Metric System EXAMPLE: Convert 5 grams to milligrams 1 gm=1,000 mg • Recall equivalent 1 gm 5g 1,000 mg X mg • Set up a proportion of two equivalent ratios X 1,000 5 • Cross multiply to solve for “X” X 5,000 mg • Label units to match unknown “X” Copyright © 2008 Thomson Delmar Learning Converting Within the Metric System Short Cut • Medication conversions within the metric system most often occur between: mg and mcg [ mg are larger than mcg ] g and mg [ g are larger than mg ] L and mL [ L are larger than mL] • These are all 3 decimal place differences [ a difference of 1000 ] • To use this Short Cut you will need to remember -which unit is larger -to always move 3 decimal places Copyright © 2008 Thomson Delmar Learning Conversion Slide • Keep this visual in mind when converting within the metric system Move decimal point three places between each unit kg g mg mcg Copyright © 2008 Thomson Delmar Learning Converting Within Metric System Short Cut continued • Write out the desired equivalent in this format 5 mg = ______ mcg • Then draw an arrow that starts at the larger unit and points toward the smaller unit Larger to Smaller • Move the decimal point in the direction of the arrow by three places. Copyright © 2008 Thomson Delmar Learning Calculating a Drug Dosage that requires Conversion between Systems • Drug order reads Codeine sulfate gr ¾ p.o. q.4h p.r.n., pain • Drug supplied is Codeine sulfate 30 mg per tablet • Calculate one dose Copyright © 2008 Thomson Delmar Learning Converting to Same System • Drug order reads Codeine sulfate gr ¾ p.o. q.4h p.r.n., pain • Drug supplied is Codeine sulfate 30 mg per tablet • What do you notice? – Different system – Needs to be converted Copyright © 2008 Thomson Delmar Learning Approximate Equivalent: gr i = 60 mg • Step 1. Convert – Convert to equivalent units in the same system of measurement. Convert gr to mg. – Approximate equivalent: gr i = 60 mg. Copyright © 2008 Thomson Delmar Learning Convert using Ratio Proportion Method • Start by writing a known ratio: 1 grain = 60 mg [ the known equivalent ] • Then fill in the rest of the proportion • Solve for X 1 gr ¾ gr 60 mg = X mg 1X = 60 x ¾ (0.75) X = 45 mg • Codeine gr ¾ = 45 mg Copyright © 2008 Thomson Delmar Learning Think • Step 2 Stop and think carefully about what a reasonable dosage should be: You have just figured out that the doctor ordered 45 mg. The drug label indicates that each tablet = 30 mg. Will you be giving more or less than 1 tablet? MORE Copyright © 2008 Thomson Delmar Learning Step 3: Calculate using Ratio Proportion Method • Start by writing known ratio from the problem • Complete the proportion with other information you have [doctor’s order ] • Check for matching units. Cross multiply and solve for X • 30mg 45mg 1 tablet = X tablet 30X = 45 X = 45 = 1 15 = 1 ½ tablets 30 30 Copyright © 2008 Thomson Delmar Learning