ip121lec - topic 3 - calculation of doses.pdf

8/20/2019 IP121lec - Topic 3 - Calculation of Doses.pdf

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alculation of Doses

IP 121 Lecture

2nd semester, AY 2012-2013

Department of Industrial Pharmacy

College of Pharmacy

UP Manila


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A Dose of Definitions… Dose

• Quantitative amount administered or taken by

a patient for an intended medical effect

- Single dose- Daily dose

- Total dose

- Divided doses

Dosage Regimen (vs. dosing frequency/interval)

• Schedule of dosing with duration of treatment

• Ex. 3x a day for 7 days

• Ex. 5 mL q12h s.o.s.


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A Dose of Definitions… Usual Adult Dose

• the amount that ordinarily produces the

medicinal effect intended in adults

Usual Pediatric Dose

• amount that ordinarily produces the medicinal

effect intended in infants or children

Usual Dosage Range

• indicates the quantitative range or amounts of

the drug that may be prescribed within the

guidelines of usual medical practice


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A Dose of Definitions… Prophylactic Dose

• Given to protect the patient from

acquiring/contracting a specific disease

Therapeutic Dose

• Given to counter an existing condition, sign,

symptom, or disease once contracted

Maintenance Dose

• Based from a usual dose, given according to

dosage regimen to sustain drug levels or effects

throughout the course of therapy


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DosingCalculating the amount of a medicinal substance

or preparation to be administered to a patient to

obtain the desired therapeutic effect


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Dosage Problem

Calculations:Miscellaneous


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Number of Doses in a Specified

Amount of Medicine

Example:

If the dose of a drug is 150 mg, how many dosesare contained in 6g?

6g 6,000 mgNo. of doses = ------------- = ------------------- = 40 doses

150 mg 150 mg

doseoneof Size

amountTotaldosesof Number


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Size of each dose and total

amount of medicine

Example:

How many drops would be prescribed in each doseof a syrup if 15 mL contained 30 doses? The

dropper to be used calibrates 16 drops/mL.

1. Size of dose = total amt. / # of doses

= 15 mL / 30 doses

= 0.5 mL/dose

2. # of drops = 0.5 x 16 = 8 drops/dosedose

mL

mL

drops

doseoneof Size

amountTotal

dosesof Number


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Quantity of ingredient in each specified

dose, given quantity in total amount

Example:

If 0.05 g of a substance is used in preparing 200tablets, how many micrograms does one tablet

contain?1. 0.05 g x 1000 mg/g x 1000 μg/mg = 50,000 μg

2. μg/tablet = 50,000 μg / 200 tablets = 250 μg

dosesofNumber

amounttotalinQuantitydoseeachinQuantity


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Quantity of ingredient in each specified

dose, given quantity in total amount

When the number of doses is not given, a simpledimensional analysis is used:

Example:

1.8 g of fluconazole was used in compounding a 60mLsuspension. If one dose is one teaspoonful, how

many mg are contained in one dose?

x = 0.15 g = 150 mg

doseofSize

doseeachinQty.

amt.Total

totalthatindrugofQty.

mL5

x

mL60

g1.8


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Easing Patients Into or

Out of Doses

Dose TitrationDesensitization

Tapering


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Dose Titration• The process of gradually adjusting the dose

of a medication (based on recommended

dose range and frequency) until optimal

results are observed

- Helps the body adapt to certain medications

- Often reduces side effects

- Adjustment can be increasing or decreasing


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Desensitization• Process starting a patient with very small

doses and then rapidly increasing the dose

for the patient to develop tolerance to the

immune response caused by the drug

• When patient is already immune to the drug for

which there is no optimal alternative


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Tapering• Process of easing a patient

into or out of a certain

medication dose

• Not starting on a full dose ofa drug right away or not

discontinuing a medication

dose abruptly


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Sample problem

A patient brings in a prescription for a prednisone taperas follows:

prednisone 30 mg po tonight

20 mg po BID tomorrow and day 3

10 mg po BID days 4 and 510 mg po qd days 6 and 7

5 mg po qd days 8 and 9

then off

The available prednisone tablets in the pharmacy are in1-, 5-, 10, 20, and 50-mg. As the pharmacist, whatstrength and quantity should you dispense? Whatinstructions will you give the patient?


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Sample problem (cont’d.) The patient will find it easier if he only has to handle 1 type of tabletfor all the doses. The tablet strength that may be used for all dosesis the 5-mg. A total of 36 tablets of the 5-mg strength will bedispensed. You should give the following instructions to the patient:

Day 1: 6 tablets tonight

Day 2: 4 tablets twice daily




Day 6: 2 tablets once daily

Day 7: 2 tablets once dailyDay 8: 1 tablet daily

Day 9: 1 tablet daily

Day 10: off (stop taking tablets)


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Drug Dosage

Adjustments

Based on AgeBased on Body WeightBased on Surface Area


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The Pediatric Population

Neonates: 0-30 days

Toddlers: 1-3 years

Infants: Birth to

12 months

Preschoolers:3-5 years

School age aka“late childhood”:

6-12 years

Adolescence: 13-17 years


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Special Dosing Considerations

Doses in children need to be

adjusted because…

• Underdeveloped body systemso Kidney function

o Liver enzymes

o Others

• Smaller total body mass

• Similarly, dosing in geriatric patients can be

lower than typical adult dosing due to

degeneration of drug handling systems


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Young’s Rule

For children from 1-12 years

doseAdult12Age

Age doseChild


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Cowling’s Rule

doseAdult24

1(years)AgedoseChild


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Fried’s Rule for Infants

For infants and children up to 2 years

doseAdult150(months)Age doseInfant

Disadvantage of the 3 rules: children treated as

“miniature adults”


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Drug Dosage based on Body

Weight

• Considered more dependable than relying

strictly on age

• Usual “adult” dose is in general most suitable

for a 70-kg 154-lb) man• Dosage based on body weight frequently

expressed on a “mg/kg” basis (mg of drug per

kg of body weight )

• Consideration of physiological/pathological

state is still more superior than this


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Weight

S a m p l e p r o b l e m :

The initial dose of ondansetron is 0.15 mg/kg

given 30 minutes before the start ofemetogenic therapy. How many milligramsshould be administered to a 129-lb individual?

129 lb. x x = 8.8 mglb2.2

kg1

kg

mg0.15


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Clark’s Rule

Based on weight

*150 conveniently based on 154 lb, which is the averageweight of an adult

doseAdult150

(lb)WeightdoseChild


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Surface Area (BSA)

• Measure of drug dosage based on a

measured or calculated surface of a human

body

• BSA unit is in m2 (doses can be expressed

per m 2 )

• Better indicator of metabolic mass than body

weight alone

• Often used in calculation of doses of: Anticancer agents in chemotherapy

Pediatric patients (except premature and neonates)


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Surface Area (BSA)

Commonly used methods to calculate BSA:

1. DuBois formula – most widely used

2. Mosteller formula

Where… W = weight (expressed in kg)

H = height (expressed in cm)

BSA units: m 2

007184.0725.0425.0 H W BSA

3600

H W BSA


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Surface Area (BSA)

Mosteller formula

• If weight is expressed in pounds (lbs) and

height in inches (in):

• BSA is still in m2

3131

H W BSA


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Surface Area (BSA):

Using a nomogramBSA Nomogram – a standard table which allows

simple graphical determination of BSA using

two columns of weight and height and a

corresponding column for BSA

How to use: Draw a straight line connecting the

height and weight of the patient, found in their

specific columns. The line will intersect theBSA column in between them, and the value it

intersects is the corresponding BSA.


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West nomogram

Allows calculation of

BSA for children ofnormal height and

weight using weight

alone


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Dosage Calculation in Children

based on BSA

Using the adult dose and a child’s BSA:

• 1.73 m2 – average adult BSA

If dose per sqm. Is given:

Child dose = child BSA x dose per sqm.

doseadult

m

(mBSAsChild'doseChild

2

2

73.1

)


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BSA Dosage Calculation:

Sample problemA physician prescribed Benadryl for an 8-year old childwho weighs 74 pounds and is 4 feet, 2 inches tall. Thenormal adult dose is 25 mg t.i.d. How much Benadrylmust be administered to the child? Use both DuBois and

Mosteller formulas.

1. Change feet to inches and corresponding cm.

(4 ft x 12 in/ft) + 2 in = 50 inches

50 in x 2.54 cm/in = 127 cm2. Use the Mosteller formula to compute for BSA.

2087.13131

5074mBSA


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Mosteller formulas.

3. Convert pounds to kg and then use the DuBois eqn.or integrate the conversion already in the DuBois eqn.

007184.0)127()2.2

174( 725.0425.0 cmlb

kglbBSA

2073.1 mBSA


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Mosteller formulas.

4. Ratio with 1.73 m2 and multiply with the usual adultdose.

Using DuBois result: Using Mosteller result:

Child dose = 15.5 mg Child dose = 15.7 mg

The practical dose to give is 16 mg t.i.d.

mgm

mdoseChild

2

2

2573.1

0731

.mg

m

mdoseChild

2

2

2573.1

0871

.


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Intravenous

Admixtures and

Rate of Flow of

Intravenous Fluids


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Intravenous Fluids

• Sterile preparations intended for parenteral

administration into a vein

Functions

• Fluid volume replenishment

Such as in severe vomiting and diarrhea

• Electrolyte imbalances

• Administration of nutrients• As a means of administering medication

Rapid delivery of a large dose of medication when

needed

Delivery of small amounts of medication over

controlled and pre-determined periods of time


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Intravenous (IV) Injections

Large volume parenterals (LVP)

• Contain 100-1000 mL of liquid, administered by infusion

• When used alone:

• Fluid/electrolyte/blood volume restoration

• Nutrition

• Vehicle for administering other drugs

• Packaged into collapsible plastic bags or glass bottles

Small volume parenterals (SVP)• Contain up to a maximum of 100 mL of liquid

• Contains drugs or additives for dilution into LVPs or for

direct administration

• Packaged also into ampules, vials, and pre-filled

syringes


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IV Infusion

• Continuous infusions are LVPs with or without

drugs and are run into a vein uninterrupted for

a specified time duration

• Intermittent infusions are administered during

scheduled periods

• In glass or plastic bottles or collapsible plasticbags in volumes of 50, 100, 250, 500 and

1000mL


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IV Push (IVP)

• A relatively large dose of medication

administered into a vein in a short period or in

“one immediate shot” (usually less than 1

minute)

• rapid injection in an emergency or critical caresituation

• Also called IV stat or IV bolus

• Intended to quickly control heart rate, bloodpressure, cardiac output, respiration, or otherlife threatening conditions

• Depends on precise calculations of dose andrate of administration


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Intravenous Admixtures• Preparation involves the addition of one or

more drugs (with or without additives) to LVPse.g. NaCl Injection, Dextrose Injection,Lactated Ringer’s Injection, etc.

• Additives include electrolytes, antibiotics,vitamins, trace minerals- As SVPs packaged in ampoules, vials, “piggybacks”

or sterile solids requiring constitution


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Rate of Flow of IV Fluids• LVPs are intended for infusion of medications

and/or nutrients in the institutional or home-care setting

• On medication orders, the physician specifiesthe rate of flow of IV fluids in mL per minute,drops per minute, amount of drug per hour(mg/hr), or, more frequently as theapproximate duration of administration of thetotal volume of the infusion

• Pharmacists may be called upon to perform orcheck rate of flow calculations• Calculations usually involve the relation of the

dosing regimen to the flow rate of the parenteral


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IV Fluids & Rate of Flow

Sample Problem #1A medication order for a patient weighing 154 lb callsfor 0.25 mg of amphotericin B per kg of body weight tobe added to 500 mL of 5% dextrose injection. If theamphotericin B is to be obtained from a constituted

injection that contains 50 mg/10mL, how many mLshould be added to the dextrose injection?

kglb2.2

kg1lb 70154

mgkg

0.25mgkg 5.1770

mg50

mL10

mg17.5

mLx x = 3.5mL


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Sample Problem #1A medication order for a patient weighing 154 lb callsfor 0.25 mg of amphotericin B per kg of body weight tobe added to 500 mL of 5% dextrose injection. If theamphotericin B is to be obtained from a constituted

injection that contains 50 mg/10mL, how many mLshould be added to the dextrose injection?

mL3.5

mg50

mL10

kg1

mg0.25

lb2.2

kg1lb 154


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Sample Problem #2A medication order calls for 1000 mL of D5W to beadministered over an 8-hour period. Using an IVadministration set that delivers 10 drops/mL, how manydrops per minute should be delivered to the patient?

drops/min20.833min60

hr 1

mL

drops10

hr 8

mL1000 21


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Sample Problem #3If 5 mg of a drug are added to a 500 mL LVP fluid, whatshould be the rate of flow, in mL/hr, to deliver 250 mcgof drug per hour? If the infusion set delivers 18 gtt/mL,what should be the rate of flow in drops/min? How

many hours should the total infusion last?

mg0.25

mLx

mg5

mL500250 mcg = 0.25 mg

x = 25 mL;

therefore, 25 mL/hr

drops/min7.5min60hr 1

mLdrops18

hr mL 25

mL500

hr x

mL25

hr 1

mL/hr 25

mL500= 20 hours


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Sample Problem #4Compare:

a) the number of drops and

b) the length of time, in minutes , required to deliver150-mL of intravenous solution, when using a

microdrip set at 60 drops/mL, and a standardadministration set, at 15 drops/mL, if in each caseone drop is to be administered per second.

drops9000

mL

drops60mL150

Microdrip set:

= 2.5 hoursmin60

hr 1

s60

min1

drop1

s1drops9000

Standard set:

drops2250mL

drops15mL150

min60

hr 1

s60

min1

drop1

s1drops2250

ip121lec - topic 3 - calculation of doses.pdf

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