Transcript The metric system. - McGraw Hill Higher Education

CHAPTER
52
Dosage Calculation
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52-2
Learning Outcomes
52.1 Explain the role of the medical
assistant to ensure safe dosage
calculations.
52.2 Identify systems of measurements and
their common uses.
52.3 Convert among systems of
measurements.
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52-3
Learning Outcomes
52.4 Execute dosage calculations
accurately.
52.5 Calculate dosages based upon body
weight and body surface area.
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52-4
Introduction
• Administer medications
– Close attention to detail
– Dosage calculation must be accurate
• Basic math
• Systems of measurement
• Conversions within or between measurement
systems
• Calculations for special patient populations
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52-5
Ensuring Safe Dosage Calculations
• Minor math errors major error in dosage
• Each time you calculate a dosage
– Analyze the problem
– Estimate a range
– Is the answer reasonable?
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52-6
Measurement Systems
• Three systems
– Metric ~ most common
– Apothecary
– Household
• Focus on the basic units of
– Volume
– Weight
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52-7
Metric System
• Based on multiples of 10
• Basic units
– Volume = liter (L)
– Weight = gram (g)
• Prefixes indicate multiples of each
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52-8
Metric System
Prefix
Kilo-
Base
unit
Centi-
Milli-
Micro-
Value
× 1000
--
÷ 100
÷ 1000
÷ 1,000,000
Weight
kilogram
gram
centigram
milligram
microgram
(kg)
(g)
(cg)
(mg)
(mcg)
1000 g
1g
0.01 g
0.001 g
0.000001 g
kiloliter
liter
centiliter
milliliter
microliter
(kL)
(L)
(cL)
(mL)
(mcL)
1000 L
1L
0.01 L
0.001 L
0.000001 L
Volume
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52-9
Apothecary and Household Systems
• Apothecary
– Weight
– Volume
• Fluid ounces
• Drams
• Fluid drams
• Ounces
• Pint quart
• Pounds
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52-10
Apothecary and Household Systems
• Household – only volume
– Drops
– Ounces
– Teaspoons and
tablespoons
– Cups
– Pints, quarts, and
gallons
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52-11
Apothecary and Household Systems
Apothecary Units
Equivalent
Measures of Volume
8 fluid drams (fl dr) 1 fluid ounce (fl oz)
16 fl oz 1 pint (pt)
2 pt 1 quart (qt)
4 qt 1 gallon (gal)
Measures of Weight
60 gr 1 dram (dr)
8 dr 1 ounce (oz)
16 oz 1 pound (lb)
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52-12
Apothecary and Household Systems
Household
Units
Equivalent
Measures of Volume
60 drops* (gtt) = 1 teaspoon (tsp)
3 tsp = 1 tablespoon (tbsp)
6 tsp =
8 oz =
2c=
4c=
1 ounce (oz) or 2
tbsp
1 cup (c)
1 pint (pt)
1 quart (qt) or 2 pt
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52-13
Apply Your Knowledge
Match the following: ANSWER: A. Metric
B units of measurements are the
same as in the household system B. Apothecary
C. Household
C drops
A multiples of ten
C only uses units of volume
B fluid dram
A microliter
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52-14
Conversions Within and Between
Measurement Systems
• Rules for converting within the Metric
System
1. Move the decimal point to the right when you
convert from a larger to a smaller unit.
This is dividing.
2. Move the decimal point to the left when you
convert from a smaller to a larger unit.
This is multiplying.
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52-15
Converting Within the Metric System
• Determine the direction and number of
places to move the decimal point
– A milliliter is three
decimal places
to the right of liter
– A kilogram is three
decimal places
to the left of a gram
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52-16
Converting Within the Metric System (cont.)
LEFT
3
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52-17
Converting Between Systems
• Be familiar with equivalent measures
• Conversion charts – check carefully before
administering a drug
1 teaspoon = 5 mL
1 tablespoon = 15 mL
1 pint = 480 mL
1 kg = 2.2 lbs
1 fl oz = 30 mL
1 fl oz = 2 tbsp
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52-18
Converting Between Systems (cont.)
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52-19
Converting Between Systems (cont.)
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52-20
Apply Your Knowledge
1. Which measuring system is most commonly
used?
ANSWER: The metric system.
2. Convert 25 grams to milligrams.
ANSWER:
1. Add a decimal point to the measurement: 25. g
2. Add 3 zeros so you can move the decimal point
three places to the right: 25.000 g
3. Move the decimal point to the right three places:
25,000
4. Change the unit: 25,000 mg
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52-21
Dosage Calculations
• Must be accurate
– Double check
– Have coworker check
• Use
– Proportion method
– Formula method
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52-22
Proportion Method for Dosage
Calculations
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52-23
Proportion Method for Dosage
Calculations (cont.)
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52-24
Proportion Method for Dosage
Calculations (cont.)
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52-25
Formula Method for Dosage Calculations
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52-26
Formula Method
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52-27
Dosage Calculations (cont.)
• Prevent errors
– During calculations
– When selecting medication
– When reading the label
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52-28
Apply Your Knowledge
1. The physician has ordered ampicillin 500 mg,
on hand 250 mg capsules. How much would
you give?
2 capsules
2. You have 50 mg metoprolol as a scored tablet
on hand and the doctor tells you to give 25 mg.
How much would you give?
½ tablet
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52-29
Body Weight and Body Surface
Area Calculations
• More common for pediatric or geriatric
dosages
– Greater risk of harm
– Calculations must be precise
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52-30
Dosages Based on Weight
• Order
– Dosage of medication / weight of patient
– Over a unit of time
• Weight must be calculated in kilograms
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52-31
Dosages Based on Weight (cont.)
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52-32
Dosages Based on Weight (cont.)
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52-33
Dosages Based on Weight (cont.)
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52-34
Dosages Based on Weight (cont.)
4. Calculate the amount to administer. Refer to
label for medication on hand
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52-35
Dosages Based on Weight (cont.)
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52-36
Dosages Based on Weight (cont.)
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52-37
Dosages Based upon Body Surface Area
• Precise dosages
– Pediatric patients
– Burn patients
– Chemotherapy patients
• Nomogram
– Plot height and weight
– Third scale will give
the BSA
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52-38
Apply Your Knowledge
A 5-year-old child weighs 44 lbs. The
physician orders him to receive Zinacef 50
mg/kg/day IM q6h.
What is the child’s weight in kg? 20 kg
How many milligrams of medication should
the child receive in one dose?
250 mg
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52-39
In Summary
52.1 A medical assistant must be proficient in
math and determine if the answer to every
calculation he performs is reasonable.
52.2 The metric system is based on 10 and is the
most common system of measurement for
dosage calculations.
Metric units commonly used for dosage
calculations include g, mg, mcg, and mL.
The apothecary and household systems
have some equal measures, but they are
used rarely.
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52-40
In Summary (cont.)
52.3 To convert among systems of
measurements, you can refer to a
conversion chart or perform a proportion
method calculation.
Keep in mind that measurements between
the metric and apothecary and household
systems are only approximations
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52-41
In Summary (cont.)
52.4 Use the formula method or proportion
method to perform dosage calculations.
Formula Method
D/H × Q
Proportion Method D/X = H/Q
52.5 Dosages based on body weight and BSA are
used when precise amounts of medication
must be administered.
Body weight calculations are usually ordered
in mg/kg/day. BSA calculations use special
formulas or a nomogram
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52-42
End of Chapter 52
Math is like
love -- a
simple idea
but it can get
complicated.
~ Unknown
© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.