Chapter 4: Pharmacology I Medical Systems of Weights and Measures

Overview Systems of weight and measurement Application of math principles Drugs are measured using: Weight Volume Equivalents Weight and measurement systems were invented so there could be standard ways of comparing two or more objects for size, strength, function, and condition. Example of a measuring system: Temperature (degrees) Example of dry weight: An Extra-Strength Tylenol tablet contains 500 mg of drug. Example of liquid volume: 5 mL or 1 tsp of Benadryl Equivalents are used to convert from one system to another. 2

Temperature: Fahrenheit and Celsius Fahrenheit to Celsius (°F to °C) (°F – 32) / 1.8 = °C Celsius to Fahrenheit (°C to °F) (°C × 1.8) + 32 = °F Round to the nearest tenth Many hospitals use the metric system for measuring temperature (Celsius) and other health care settings use the Fahrenheit system. What is the freezing point of water in Fahrenheit versus in Celsius? 32º versus 0º What is the boiling point of water in Fahrenheit versus in Celsius? 212º versus 100º Normal human body temperature ranges between 36.1º and 37.8º Celsius. 3

Temperature: Fahrenheit and Celsius (cont’d) Figure 4-1. Comparing Centigrade to Fahrenheit values with a mercury thermometer. 4

Household System Liquid Dry Drops (gtt) Teaspoon (tsp) Tablespoon (Tbsp) Ounce (oz) Cup (C) Larger measures: pint, quart, gallon Dry Pounds (lb), ounce (oz) Note: a liquid ounce is not equal to a dry ounce Household system is the one most used by patients when they are taking liquid drugs at home. The only household (dry) measurement units used in the health care setting are ounces and pounds. Patients are usually weighed in the health care setting using which measurement unit? Kilograms (kg) What are some examples of measuring tools that are calibrated for liquid drugs? Dropper, oral syringe, small medicine cup, hollow-handled spoon 5

Metric System Three basic units: Meter (m) = length Liter (L) = liquid Gram (g) = weight Prefixes determine size (e.g., milli-, centi-) When converting between systems, apply DIMS (does it make sense) test The metric system is based on the number 10 and uses the decimal system. It is the most used system worldwide for drug prescriptions because it is accurate even in small doses. Give an example of each of the three basic units of the metric system. Centimeter, milliliter, and milligram In medicine, most metric units vary by 1000. Example: 1 kilogram is 1000 times larger than 1 gram. How can a simple decimal error impact a drug dose? You could give 10 times too much or 10 times too little! 6

Household and Metric Systems Convert between systems Equivalent table that makes sense To convert weight : 1 kg = 2.2 lb Kilograms to pounds: multiply by 2.2 Pounds to kilograms: divide by 2.2 Nurses must be able to efficiently and comfortably convert between systems. Remember, you are dealing with approximate equivalents. 7

Milliequivalent System Abbreviated “mEq” Used to measure electrolytes Dosage calculations are the same Drugs ordered in mEq are always supplied in mEq Electrolytes are minerals and chemicals in the body that have a positive or a negative charge. Determine the number of tablets or amount of liquid to administer based on what you want versus what you have on hand. Calculate the dose for the following example: potassium chloride (Slow-K) 30 mEq orally; you have Slow-K tablets that contain 20 mEq/tablet. Divide the number you want by the number you have – want 30 mEq; have 20 mEq. 30/20 = 1.5 tablets 8

Unit System Units or international units Insulin, heparin most common Always write out “unit” or “international units” – do not abbreviate Dosage calculations are the same Drugs ordered in units are always supplied in units Other drugs measured in units include injectable penicillin and some vitamins. Insulin is very concentrated and requires special (unit) syringes. Carefully check the concentration of insulin in the bottle with the type of syringe chosen to ensure you have the correct syringe. What could happen if the wrong syringe was used to measure insulin? An incorrect dose of insulin can result in severe hypoglycemia and death. 9

Two-Step Drug Dosage Calculations Used when two measurements are in two different systems First find how the two measurements are related Convert the system you have to the system you want using ratio and proportion Need an equivalent table to help convert one measurement into another. Example: dextromethorphan comes in solutions of 3.5 mg/5 mL (or written as a ratio). You want to give 7 mg, but you need the final dose expressed in teaspoons. Determine how many tsp to give. Box 4-2 tells us that 1 tsp = 5 mL. Step #1: 1 tsp = 5 mL, then 3.5 mg = 5 mL = 1 tsp. Step #2: Want/Have: 7.5 mg/3.5 mg × 1 tsp = 2 tsp. 10

KEY POINTS A liquid ounce is not equal to a dry ounce. Always double check the amount of drug in a measuring device with the amount ordered to prevent giving an overdose. Before using a measuring device (oral syringe, dropper, medicine cup) to administer a liquid drug, check the patient's swallow reflex. The household measurement system is not as precise as the metric system.

KEY POINTS Do not substitute one type of potassium for another. When calculating doses for drugs that are manufactured in milliequivalents or units, determine the amount of tablets or milliliters to administer based on what you want versus what you have on hand, in the same way as for drugs manufactured in milligrams. To prevent insulin dose errors, check the specific markings on the syringe rather than going by the color of the cap, needle hub, or box label. Always write out the word unit or the words international units rather than using abbreviations.