Notes Ch 7 Measurement System and Ch 8 Drug Calculations PDF

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Notes Ch 7 Measurement System and Ch 8 Drug Calculations...

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Pharmacology Ch 7 Measurement



Define all key terms.



Compare the four systems of measurement used for drug dispensing.



State the basic units of measurement in the metric system.



Use conversion methods for each system of measurement correctly and accurately.



Key terms 

Apothecary system



Avoirdupois system



Compound

Measurement Systems 

Before metric system, pharmacists compounded, or mixed, and dispensed drugs



Patients and families measured drugs with whatever was handy



Today medications are ordered using metric system



Four measurement systems 

Avoirdupois



Apothecary



Metric



Household systems

Avoirdupois System 

All units based on the pound and ounces



Most common use is for weighing patients



Need to know conversion from pounds/ounces to kilogram. 1 kilogram = 2.2 pounds (110 pounds ÷ 2.2 = 50 kilogram) Or 1 pounds = 0.45 kilogram (110 pounds multiplied by 0.45 = 49.5 kilogram)

45 pounds = ______kilogram A.

200

B.

20.5

C.

99

D.

100

Critical Thinking Question Harold is seen in the office. He has a history of congestive heart failure and is concerned that he is gaining a lot of weight. You determine that he weighs 185 pounds. When he asks you how much weight he has gained since his last check up the previous month, you check the chart and find that he weighed 79 kilogram. What will you tell him? 185 / 2.2 = 84.09 minus 79 = 5.09 X 2.2 = 11 pounds

Converting pounds to kilograms 

1 pound = 16 ounces



Convert 4 pounds 8 ounces to kilogram 8 ounces ÷ 16 ounces = 0.5 lb 4 pounds 8 ounces = 4.5 lb 4.5 ÷ 2.2 = 2 kg

Convert 7 pounds 4 ounces to kilogram A.

3.4

B.

33

C.

16

D.

1.6

Critical Thinking Sally brings her 3-month-old baby to the clinic because of a fever and possible ear infection. You find that the infant weighs 13 pounds, 4 ounces. The physician writes an order for you to give a dose of antibiotics based on the infant’s weight in kilograms. Before you give the medication, you must convert the child’s weight to kilograms. How will you do this, and what is her weight in kilograms?

Converting kilogram to pounds kilogram multiplied by 2.2

Convert 37 kilogram to pounds A.

8.1 pounds

B.

81.4 pounds

C.

17 pounds

D.

168 pounds

Would you rather be weighed in kilograms or pounds? Why? Kilograms, because it would say I weigh less.

Apothecary System 

One of oldest measurement systems 

Minims, drams, grains



Uses fractions



Roman numerals



Less accurate and rarely used today, not precise



Common drugs still using this system 

Tylenol gr X



Morphine gr ¼

Roman Numerals

Household System 

Teaspoons, tablespoons, cups, glasses, etc.



Unsafe as these units not standardized



Encourage patients to use standardized devices



1 ounce = 2 Tablespoons = 6 teaspoons = 360-480 gtt/grains/minims = 30mL 

If you can memorize this, you can work through most problems

Equivalents Between Apothecary and Household Systems 

1 dram = 60 minims 



1 Drama = 60 minutes

1 ounces = 

360–480 drops (gtt)



360–480 grains



360–480 minims

Question 

Does it bother you that 1 ounce is equivalent to something between 360 and 480 gtt, minims, or grains? What does this say about the accuracy of these systems?

Metric System 

Based on decimal system places of multiples of 10



Used by many countries and most researchers



Gold standard for calculating drugs due to accuracy



Uses Arabic numbers (1, 2, 3, 4, 5, 6, etc.)

Base units 

Weight/mass = gram



Length = meter



Volume/liquid = liter

Converting Units in the Metric Systems

Prefix

Level of Measurement

Deci-

Tenths

Centi-

Hundredths

Milli-

Thousandths

Micro-

Millionths

Kilo-

Thousands



Ordered units often do not match hand



Example: Physician orders 0.5 grams and you have a vial with each mL containing 250 milligrams



0.5 g multiplied by 1000 = 500 mg

Simple Unit Conversions

units on

Grams to kilograms

Divide by 1,000

Grams to milligrams

Multiply by 1,000

Kilograms to grams

Multiply by 1,000

Milligrams to grams

Divide by 1,000

Liters to milliliters

Multiply by 1,000

Milliliters to liters

Divide by 1,000

How many milligrams are in 1 gram? A.

10

B.

100

C.

1,000

D.

10,000

Do you have trouble seeing the decimal point? Do you think pharmacists ever do? How can you be sure that a patient is given 0.5 gram instead of 5 grams?

Common Medication Conversions 

1 gram = 1,000 mg



1 gram = 15 grains



1 grain = 60 mg



1 mg = 1/1,000 gram



1 grain = 1/15 gram



1 mg = 1/60 grain

Pharmacology Chapter 8 Dosage Calculations 

Define key terms.



Learn and understand the four methods for calculating drug dosages.



Explain why certain calculations are considered special and which populations are affected.



Explain how to reconstitute powdered medication and calculate the desired dosage.



Discuss the factors consider when calculating the dosages of parenteral medications and the two ways intravenous medications are administered.



Explain the calculation process for determining fluid intake. 

Available dose



Body surface area (B S A)



Conversion factor



Desired dose



Dimensional analysis



Diluent



Formula



Infiltrate



Ordered dose



Reconstitute

Methods for Calculating Drug Dosages 

Ratio and proportion



Formulation



Dimensional analysis



Fractions

Steps for Any Method 

Read drug label accurately



Convert numbers into same unit of measurement



Write problem on paper



Check and check again

Ratio and Proportions 

Uses ratios—comparisons between two objects (numbers, in this case) 



4:3

Proportion is a statement saying two ratios are equal 

4:3::8:6

Physician has ordered 650 mg of a pain medication by mouth. You have on hand a bottle with 325 mg tablets. 

Step 1: Write the ratio that you know (what is available) 



Step 2: Write the ratio that you need to solve for (ordered dose) 



325 mg / 1 tab

650 mg / ? tab

Write the proportion 325 mg/1 tab= 650 mg/ ? tab



Cross multiply 



325? = 650

Divide both by 325 

?=2



Physician has ordered 650 mg of a pain medication by mouth. You have on hand a bottle with 325 mg tablets. 

Step 1: (ratio from label) 325 mg:1 tablet



Step 2” Ratio you need to solve for 650 mg : ? tablets



Step 1: (ratio from label) 



325 mg : 1 tab

Step 3: Write proportion 325 mg:1 tab::650 mg:? tab



Step 4: Multiply means and extremes 



325 ? = 650

Step 5: Isolate ? ?=2

Ratios and Proportions Using ratio and proportions, calculate the dosage amount that must be administered. 1. 400 mg = 200 mg 1 mL

? mL __________

2. 250 mg = 750 mg 1 mL

? mL __________

3. 200 mg = 100 mg 2 mL

? mL __________

4. 50 units = 150 units 1 mL

? mL __________

Formulation Method 

D = Desired dose (ordered dose)



H = On-hand or available amount



Q = Quantity of liquid, tablets, capsules, etc.



D and H must be in same units



Physician orders 



450 mg of Ampicillin IV

Available medication 

450mg 200 mg

200 mg in 1 mL



D = 450 mg

2.25 X 1 mL



H = 200 mg

2.25 mL



Q = 1 mL

The physician orders 500 mg. You have a vial with 250 mg medication in 1 mL. How many milliliters will you give? A.

1

B.

2

C.

3

D.

4

Formulation method 

If units of measurement do not match



Example 1: Ordered dose is 0.4 g



Available is a vial with 400 mg/2 mL



D = 0.4 g



H = 400 mg



Q = 2 mL 

D ≠ H: must convert 0.4 to mg 

0.4 X 1000

400 mg 400 mg 2 mL 1 multiplied by 2 mL 2 mL



Example 2: The medication order is for gr V (5 grains), and the label says 300 mg per tablet. 

D = 5 grains



H = 300 mg



Q = 1 tablet



D≠H 

Step 1: Convert grains to mg 1 grain = 60 mg 5 grains = 5 multiplied by 60 mg = 300 mg



Step 2: Set up formula 300/300 mg X 1 tablet



Step 3: Solve formula 1 multiplied by 1 tablet 1 tablet

Special Circumstances: Pediatric Calculations



Weight frequently used to calculate



Example: The physician orders 20 mg/kg/day. The patient weighs 50 pound. 

Step 1: Convert 50 pound to kilograms



Weight frequently used to calculate



Example: The physician orders 20 mg/kg/day. The patient weighs 50 pound. 

Step 1: Convert 50 pound to kilograms



Other times medications are ordered as mg/kg/dose.



Example: Order is for 10 mg/kg/dose q.i.d. Patient weighs 100 pound. 

Step 1:



Step 2: Multiply 45 kg multiplied by 10 mg/dose = 450 mg/dose



Step 3: daily dose Multiply 450 mg multiplied by 4 doses = 1800 mg/day

How much does a 75 pound child weigh in kilograms? A.

31.5

B.

33.75

C.

35.5

D.

37.75

Critical Thinking Some dosages may be numbers that are difficult to decide how to administer. For example, when the dose is 337 mg/dose and the medication comes in 200 mg tablets, what would you do? How many tablets will you give? Whom do you ask for advice?

Special Circumstances: Geriatric Calculations 

Elderly patients are at high risk for toxicity due to aging body systems



Most common adjustments are reduced dosages



There is no magic formula



Physicians adjust on individual basis after evaluating organ function and body weight

Reconstituting Powders 

Powdered medications need to be converted to a liquid form



Diluent: Fluid used to reconstitute



Directions for reconstitution are on medication label 

Example: Label states that if 2.1 mL of sterile water is added, resulting solution will contain 250 mg/mL Example: Drug label indicated you should add 62 mL of diluent to powder. Resulting solution will be 200 mg/teaspoon (5 mL). You are to administer 300 mg/dose.





Your formula for this calculation would be:

Add 4 mL of sterile water. Resulting solution will contain 250 mg of medication/5 mL. 

What amount of sterile water would you add to the vial?



What conversion factor would you use to calculate the dosage?

Parenteral (IV Drip) Calculations 

Dimensional analysis used



If electronic regulator pump is used



Order written as milliliters over a certain period of time



Example: 2000 mL to be administered over 5 hours

What is the electronic milligrams per hour for the following? 1. 1,000 mL over 3 hours mL/hr 2. 500 mL over 2 hours mL/hr 3. 1,000 mL over 4 hours mL/hr 4. 250 mL over 1 hour mL/hr 5. 150 mL over 3 hours mL/hr

Critical Thinking Question In a situation in which the flow rate has not been consistent, you see that the IV insertion site is swollen. Would you increase the flow rate to make up the difference? What would you do?



Manual IV Sets: 60, 10, or 15 gtt/mL stated on packaging (how many drops are in 1 mL)



Formula



Example: 250 mL to be infused over 2 hours. Tubing is 15 gtt/mL.

Fluid Calculations 

Fluid intake and output are important parts of health care.



Educating patients depends on you knowing proper conversions. 

1 ounce = 30 mL is the most important conversion



Example: Patient had four 12-oz sodas today 12 ounces multiplied by 30 mL = 360 ml per soda 4 multiplied by 360 mL = 1440 mL total If on 1000 mL daily fluid restriction, the patient is over limit...