Transcript moini_ch07_lecture

Focus on
PHARMACOLOGY
ESSENTIALS FOR HEALTH PROFESSIONALS
CHAPTER
7
Adult and
Pediatric Dosage
Calculations
Accuracy in Drug Calculations
• Accurately calculating drug dosages is
an essential skill in health care.
• Serious harm to a patient may occur
from a mathematical error during
dosage calculation.
• Health-care professionals who
administer drugs must follow medical
orders precisely and efficiently.
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Background
• Dose: amount of drug taken by patient
for intended therapeutic effect
• Dosage regimen: schedule of taking
drug dose
• Most drug doses are measured in units
of weight (e.g., 500 mg).
• Many drug doses are general, not
patient specific.
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Dosage Calculation Terms
• Desired dose: amount to be
administered at one time
• Amount to administer: volume
containing desired dose
• Dosage ordered: total amount of
ordered drug and its frequency of
administration
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Dosage Calculation Terms
• Dosage unit: volume of medication
containing quantity of drug as listed on
drug label
• Dose on hand: amount of drug in
dosage unit
• Dosage strength: dose on hand per
dosage unit
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Calculating Dosages
• First step: make sure the strength of
drug ordered and the strength of drug
available are in same unit of measure
• If necessary, convert to a single unit.
• Then, use formula: D/H × Q = X, which
can also be expressed as D × Q/H = X
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Using the Formula
• D/H × Q = X
– Divide D (desired dosage) by H (dose on
hand or dosage of drug available).
– Then multiply by Q (number of tablets,
milliliters, etc., that contains available
dosage).
– Equals X (amount to be administered)
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Ratio Formula
of Dosage Calculation
Dose on hand (H)
Desired dose (D)

Quantity on hand (Q) Quantity desired (X)
• Example: Amoxil 500 mg is ordered. It
is supplied as 250 mg/5 mL liquid.
– 250 mg ÷ 5 mL = 500 mg ÷ X
– In this example, X = 10 mL (the dose
ordered)
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Oral Dosages
• Oral medications are either liquid or
solid.
• Tablets are most common solid oral
medications.
• Oral medications may be chewable,
administered sublingually or buccally,
swallowed, or dissolved in water. Check
the label before administering.
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Solid Medications
• Scored (notched) tablets can be broken
only along the line of scoring.
• Unscored tablets must not be broken.
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Calculating Solid Doses
• Dose on hand (H) divided by dosage
unit (Q) equals desired dose (D)
divided by amount to administer (X).
• H/Q = DX or D × Q/H = X, which can
also be expressed as D/H × Q = X.
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Zocor Calculations
• Ordered: Zocor 40 mg bid
• On hand: Zocor 20 mg tablets
– D × Q/H = X
– 40 × 1 tablet/20 mg = X
– X = 2 tablets
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(For educational purposes only)
Figure 7-1
Drug label for doxazosin mesylate.
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Cardura Calculations
Q
D  X
H
1 tab
40 mg 
X
20 mg
2 tablets = X
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(For educational purposes only)
Figure 7-2
Drug label for amlodipine besylate.
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Norvasc Calculations
Q
D  X
H
1 tab
5 mg 
X
2.5 mg
5
 2 tablets
2.5
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Liquid Medications
• Liquid medications can be measured in
small units of volume.
• Examples of liquid drugs are:
– Syrups
– Spirits
– Elixirs
– Suspensions
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Measuring Liquid Doses
• Liquids can be measured by:
– Measuring cup: calibrated in fluidounces,
fluidrams, milliliters, teaspoons, or
tablespoons
– Medicine dropper or oral syringe: calibrated
in milliliters, minims, or drops
– Calibrated spoon: calibrated in teaspoons
and milliliters
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Calculating Liquid Doses
• The physician orders 400 mg of the
antibiotic cefdinir (Omnicef). How many
milliliters will you administer?
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(For educational purposes only)
Figure 7-3
Drug label for cefdinir..
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Calculating Liquid Doses
• Label on bottle indicates
that 5 mL contains 125 mg
of cefdinir.
5 ml
125 mg
5 mL
2,000 mL
400 

 16 mL
125 mg
125 mg
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(For educational purposes only)
Figure 7-4
Drug label for erythromycin ethylsuccinate.
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Administering Medicine
to Pediatric Patients
• Dosage cups: for children who know
how to drink from a cup
• Syringes: for infants who cannot drink
from a cup; can be squirted into back
of mouth
• Droppers: safe for infants; measure at
eye level
• Cylindrical dosing spoons: easily used
by small children
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Smart Bottles
for Elderly Patients
• Bottles that release correct amount of
medication dosages
• Self-locking feature to prevent
accidental overdose
• System alerts pharmacist when
medication supply is low
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Parenteral Medications
• Injectable mixtures that contain drug
dissolved in appropriate liquid
• Prescribed in grams, milligrams,
micrograms, grains, or units
• Can be packaged as premixed
solutions, or in powdered form that
requires mixing with a solution
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Intradermal Injection
• Usually used for skin testing to
diagnose an allergy or for tuberculin
test
• Tuberculin syringe with a 25-gauge
needle
• Site of injection: inner portion of
forearm or upper back
• A reddened or raised area within 48 to
72 hours indicates a positive reaction
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Subcutaneous Injection
• Drugs that must be absorbed slowly are
usually injected subcutaneously
• Tuberculin syringe or a 3-mL syringe is
used
• Heparin is a drug commonly
administered subcutaneously
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Insulin Injection
• Injection sites usually rotated
• May be administered subcutaneously,
IV, or with insulin pump
• Four types of insulin:
– Quick-onset, short-duration
– Intermediate-acting
– Long-acting
– Ultra-long-acting
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Figure 7-5
Mixing regular and NPH (Neutral Protamine Hagedorn) insulins in one
syringe.
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Figure 7-6
Insulin syringes: 100 unit (A), 50 unit (B), and 30 unit (C).
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Figure 7-7
Tuberculin syringe.
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Figure 7-8
100-unit insulin syringe.
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Figure 7-9
50-Unit insulin syringe.
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Figure 7-10
30-unit insulin syringe.
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Figure 7-11
A partially filled 50-unit insulin syringe.
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Figure 7-12
A partially filled 100-unit insulin syringe.
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Preparing Insulin Injections
• Use 30-unit insulin syringe for insulin
doses less than 30 units.
• Use 50-unit insulin syringe for insulin
doses less than 50 units if standard
100-unit syringe is not available.
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Preparing Insulin Injections
• To remove insulin from vial, first inject
same quantity of air as ordered insulin
volume
• Then withdraw appropriate insulin
• Two types of insulins can be combined
in one syringe; draw up shorter-acting
insulin first.
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Safe Insulin Practices
• Avoid keeping insulin on top of
medication carts or counters because it
can be confused with heparin.
• If insulin concentration is not 100
units/mL, apply bold warning labels.
• Order insulin cartridges for outpatients
to help ensure correct dispensing.
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Heparin Calculation
• Heparin is an anticoagulant that is
measured in USP (United States
Pharmacopeia) units.
• It can be administered IV or
subcutaneously.
• The term units should always be
written out, and the abbreviation U
avoided.
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Heparin Calculation
• With a premixed parenteral solution of
heparin, you have to convert the
physician’s order to the volume of
solution that contains the amount of
heparin ordered.
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Heparin Calculation
• Ordered: heparin 5,000 units
subcutaneously q8h
• On hand: heparin 10,000 units/mL
• Convert units to milliliters: If 10,000
units equal 1 mL, then 5,000 units
equal 0.5 mL
• 0.5 mL should be administered
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Intravenous Drugs
• IV fluid therapy: to administer fluids
containing water, dextrose, vitamins,
electrolytes, and drugs
• Medications for IV administration are
usually available in small-volume vials.
• Amount of drug can be listed in terms
of concentration (5mg/5mL) or as
amount of drug in vial (80 mg in a 2mL vial).
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Dosage Calculation
in Pediatrics
• Dosages for infants and children are
usually less than adult dosages for
same medication.
• Dosage calculations for pediatric
patients must be precise.
• The most accurate methods of
determining appropriate pediatric doses
are by weight and body area.
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Drug Dosage Calculation
by Body Surface Area
• Body surface area (BSA) is determined
by using a nomogram (a numerical
relationship chart) and the child’s
height and weight.
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Figure 7-13
Pediatric nomogram used for determining body surface area.
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Calculating Drug Dose by BSA
• First determine BSA using nomogram.
Then use formula calculation:
surface area of child (m2 )
Child's dose 
 normal adult dose
2
1.7 m
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Calculating Ampicillin Dose
• Child’s BSA is 0.4 m2; adult dose of
ampicillin is 250 mg.
2
0.4 m
 250 mg  0.23  250 mg
2
1.7 m
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Pediatric Doses
in Package Inserts
• Drug data for pediatric doses are
usually supplied in the drug’s package
insert.
• Pediatric dosages are often based on
mg/kg, mcg/kg, or units/kg.
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Finding Safe Pediatric Dosage
• Ordered: amoxicillin 60 mg PO tid
• On hand: 125 mg/5 mL
• Child weighs 12.5 lb. First determine
child’s weight in kilograms:
– 12.5  2.2 = 5.7 kg
• Safe pediatric dosage is 20–40
mg/kg/day in 3 equal doses.
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Finding Safe Pediatric Dose
• Find a range of lowest to highest safe
dosages:
– 20 mg × 5.7 kg =114 mg/day;
40 mg × 5.7 kg =228 mg/day
• 60 mg PO tid = 180 mg/day, which falls
in the safe range of 114 mg to 228 mg
per day.
• Ordered dose is safe for this child.
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Clark’s Rule
• Clark’s rule: based on weight of child,
which is much more accurate than
either Young’s or Fried’s rules
Child's weight (lb)
 adult dose
Average adult's weight (150 lb)
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Young’s Rule
• Young’s rule: for children older than
1 year
Child's age (in years)
 adult dose
Child's age (years)  12
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Fried’s Rule
• Fried’s rule: for infants younger than
1 year:
Child's age (in months)
 adult dose
Average adult weight (150 lb)
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