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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5854_Библиотеки_им_академика_М_И_Перельмана
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The product shown in Figure 8.1 was used to prepare an IV bag
containing 600 mg/50 mL of injectable solution. How many milliliters
of this solution should be given for each divided dose?
CASE IN POINT 8.2
A pediatric patient is being administered enalaprilat every 12 hours by
intravenous injection to manage hypertension and possible heart
failure.4 Based on a dose of 5 mcg/kg, the patient is receiving 55 mcg
of enalaprilat per dose. The physician wishes to convert the patient to
oral enalapril at a dosage of 100 mcg/kg as a single daily dose. The
standard procedure is to crush a 2.5-mg tablet of enalapril, mix with
sterile water to make 12.5 mL, and administer the appropriate dose
using a calibrated oral dispenser. Calculate the dose, in milliliters, to be
administered to this patient.
FIGURE 8.1 Product label showing the drug concentration in
mg/mL for an injectable product. (Source:
https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?
setid=d157983f-4794-400d-a3cf-c515d0c24b62. Courtesy of
Pfizer, Inc.)
Geriatric Patients
Although the term elderly is subject to varying definitions with regard to
chronologic age, it is clear that the functional capacities of most organ
systems decline throughout adulthood, and important changes in drug
response occur with advancing age. Geriatric medicine or geriatrics is
the field that encompasses the management of illness in the elderly.
In addition to medical conditions affecting all age groups, some
conditions are particularly common in the elderly, including degenerative
osteoarthritis, congestive heart failure, venous and arterial insufficiency,
stroke, urinary incontinence, prostatic carcinoma, parkinsonism, and
Alzheimer’s disease. Many elderly patients have coexisting pathologies
that require multiple-drug therapies.
Most age-related physiologic functions peak before age 30, with
subsequent gradual linear decline.6 Reductions in physiologic capacity
and function are cumulative, becoming more profound with age. Kidney
function is a major consideration in drug dosing in the elderly because
reduced function results in reduced drug elimination.
Because reduced kidney function increases the possibility of toxic
drug levels in the body and adverse drug effects, initial drug dosing in the
elderly patient often reflects a downward variance from the usual adult
dose. There is also a frequent need for dosage adjustment or medication
change due to adverse effects or otherwise unsatisfactory therapeutic
outcomes.
There are a number of other common features of medication use in
the elderly, including the long-term use of maintenance drugs; the need
for multidrug therapy, with the attendant increased possibility of drug
interactions and adverse drug effects; and difficulties in patient
adherence. The latter is often due to impaired cognition, confusion over
the various dosing schedules of multiple medications, and economic
reasons in not being able to afford the prescribed medication.
Special considerations in dose determinations for
elderly patients
Dose determinations for elderly patients frequently require consideration
of some or all of the following:
Therapy is often initiated with a lower-than-usual adult dose.
Dose adjustment may be required based on the therapeutic response.
The patient’s physical condition may determine the drug dose and
the route of administration used.
The dose may be determined, in part, on the patient’s weight, body
surface area, health and disease status, and pharmacokinetic factors.
Concomitant drug therapy may affect drug/dose effectiveness.
A drug’s dose may produce undesired adverse effects and may affect
patient adherence.
Complex dosage regimens of multiple drug therapy may affect
patient adherence.
The adult dose of a drug is 500 mg every 8 hours. For an elderly patient
with impaired renal function, the dose is reduced to 250 mg every 6
hours. Calculate the reduction in the daily dose, in milligrams.
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Dosage Forms Applicable to Pediatric and
Geriatric Patients
In the general population, solid dosage forms, such as tablets and
capsules, are preferred for the oral administration of drugs because of
their convenience, precise dose, ease of administration, ready
identification, transportation, and lower cost per dose relative to other
dosage forms. However, solid dosage forms are often difficult or
impossible for the pediatric, geriatric, or infirm patient to swallow. In
these instances, liquid forms are preferred, such as oral solutions, syrups,
suspensions, and drops. With liquid forms, the dose can be adjusted by
changing the volume administered. When necessary, liquid forms of
medication may be administered by oral feeding tube. Pharmacists are
sometimes asked to compound an oral liquid from a counterpart solid
dosage form when a liquid product is not available. Chewable tablets and
solid gel forms (medicated “gummy bears”) that disintegrate or dissolve
in the mouth are often used for pediatric and geriatric patients. In
addition, and as noted in Chapter 7, tablet splitting and tablet crushing
are options for individuals unable to swallow whole tablets.
For systemic effects, injections may be used rather than the oral route
of administration when needed for pediatric and elderly patients, with the
dose or strength of the preparation adjusted to meet the requirements of
the individual patient.
Drug Dosage Based on Age
For reasons stated earlier, the young and the elderly require special
dosing considerations based on factors characteristic of these groups.
Before the physiologic differences between adult and pediatric
patients were clarified, the latter were treated with drugs as if they were
merely miniature adults. Various rules of dosage in which the pediatric
dose was a fraction of the adult dose, based on relative age, were created
for youngsters (e.g., Young’s rule). Today these rules are not in general
use because age alone is no longer considered a singularly valid
criterion in the determination of accurate dosage for a child, especially
when calculated from the usual adult dose, which itself provides wide
clinical variations in response. Some of these rules are presented in the
footnote for perspective and historical purposes.
a
a
Young’s rule, based on age:
NOTE: The value of 150 in Fried’s rule was an estimate of the age (12.5 years or 150
months) of an individual who would normally receive an adult dose, and the number 150 in
Clark’s rule was an estimate of the weight of an individual who likewise would receive an
adult dose.
Currently, when age is considered in determining dosage of a potent
therapeutic agent, it is used generally in conjunction with another factor,
such as weight. This is exemplified in Table 8.1, in which the dose of the
drug digoxin is determined by a combination of the patient’s age and
weight.
TABLE 8.1 ILLUSTRATIVE PEDIATRIC DOSAGES OF
DIGOXIN BASED ON AGE AND WEIGHT
ª
a
These are estimated oral maintenance doses for patients with normal renal function. Specific
pediatric doses for various clinical conditions and by various routes of administration may be
found at Facts & Comparisons eAnswers [book online]. Baltimore, MD: Wolters Kluwer
Clinical Drug Information Inc.; 2020.
Example calculations of dose based on age
1. An over-the-counter cough remedy contains 120 mg of
dextromethorphan in a 60-mL bottle of product. The label states the
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dose as 1½ teaspoonfuls for a child 6 years of age. How many
milligrams of dextromethorphan are contained in the child’s dose?
2. The dose of a drug for a child is acceptable as either 10 mg/kg or 300
mg. Calculate the difference in these alternative doses for a 9-yearold child weighing 70 lb.
Dose at 10 mg/kg: 70 lb ÷ 2.2 lb/kg = 31.82 kg; 31.82 kg × 10
mg/kg = 318.18 mg
Difference in dose = 318.18 mg − 300 mg = 18.18 mg
3. From the data in Table 8.1, calculate the dosage range for digoxin for
a 20-month-old infant weighing 6.8 kg.
Dosage range between 38.08 and 63.92 mcg digoxin administered
twice daily
Drug Dosage Based on Body Weight
Drug doses based on weight are expressed as a specific quantity of drug
per unit of patient weight, such as milligrams of drug per kilogram of
body weight (abbreviated [mg/kg]). Dosing in this manner makes the
quantity of drug administered specific to the weight of the patient being
treated.
Example calculations of dose based on body
weight
A useful equation for the calculation of dose based on body weight is:
This equation is based on a drug dose in mg/kg and the patient’s weight
in kilograms. When different units are given or desired, other units may
be substituted in the equation as long as the terms used are consistently
applied.
1. The usual initial dose of chlorambucil is 150 mcg/kg of body weight.
How many milligrams should be administered to a person weighing
154 lb?
Solving by the equation:
150 mcg = 0.15 mg and 1 kg = 2.2 lb
Or, solving by ratio and proportion:
150 mcg = 0.15 mg and 1 kg = 2.2 lb
Or, solving by dimensional analysis:
2. The usual dose of trimethoprim for infants over 6 months of age and
children is 5 mg/kg administered every 12 hours. What would be the
daily dose for a child weighing 44 lb?
3. The dose of extended-release minocycline to treat acne vulgaris is
given as 1 mg/kg/day × 12 weeks. Tablet strengths available include
45 mg, 55 mg, 65 mg, 80 mg, 90 mg, 105 mg, and 115 mg of
minocycline. What strength tablet and how many tablets should be
prescribed for the entire course of treatment for a 100-lb patient?
4. A dose of enoxaparin sodium injection (LOVENOX) is “1 mg/kg q12h
SC.” If a graduated prefilled syringe containing 80 mg/0.8 mL is
used, how many milliliters should be administered per dose to a 154lb patient?
CASE IN POINT 8.3
A hospital pharmacist is called to a pediatric nursing station to
calculate the quantity of an injection to administer to a pediatric
patient. The daily dose of the injection for the child’s weight is stated
as 15 mg/kg/day, divided into three equal portions. The child weighs
10 kg. The injection contains 5 mg/mL of the prescribed drug. How
many milliliters of injection should be administered?
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Dosing tables based on body weight
For some drugs dosed according to body weight or body surface area,
dosing tables appear in product literature to assist the physician and
pharmacist. An example is presented in Table 8.2.
TABLE 8.2 DOSING BY BODY WEIGHT FOR A
HYPOTHETICAL DRUG
1. Using Table 8.2 and a daily dose of 0.5 mg/kg, how many 20-mg
capsules of the drug product should be dispensed to a patient
weighing 176 lb if the dosage regimen calls for 15 weeks of therapy?
According to table 8.2, the patient should receive 40 mg/day, or two
20-mg capsules/day
2 capsules/day × 7 days/week × 15 weeks = 210 capsules
2. A pharmacist compounds a suspension from oseltamivir phosphate
capsules to contain 15 mg of drug per milliliter. Using Table 8.3,
calculate the single dose in milliliters for a pediatric patient
weighing 40 lb.
From Table 8.3, the dose for the pediatric patient is 45 mg twice
daily.
TABLE 8.3 DOSING OF OSELTAMIVIR PHOSPHATE IN
THE TREATMENT OF INFLUENZA IN PEDIATRIC
PATIENTS
a
a
Adapted from product literature for oseltamivir phosphate (TAMIFLU); Genentech, 2014.
Available at:
https://www.accessdata.fda.gov/drugsatfda_docs/label/2019/021087s071,021246s054lbl.pdf
Drug Dosage Based on Body Surface Area
Body surface area (BSA) of a patient is determined based on height and
weight as discussed in following sections. The BSA method of
calculating drug doses is widely used for two types of patient groups:
cancer patients receiving chemotherapy and pediatric patients.
Example calculations of dose based on body
surface area
A useful equation for the calculation of dose based on BSA is:
If the adult dose of a drug is 100 mg, calculate the approximate dose
for a child with a BSA of 0.83 m2.
Dosing tables based on body surface area
For certain drugs, dosing tables may be provided to determine the
approximate dose based on a patient’s body surface area. Table 8.4
presents an example for a hypothetical drug.
TABLE 8.4 PEDIATRIC DOSING GUIDELINE FOR A
HYPOTHETICAL DRUG BASED ON BSA
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Using Table 8.4, find the dose of the hypothetical drug at a dose level of
300 mg/m2 for a child determined to have a BSA of 1.25 m2. Calculate to
verify.
From Table 8.4, the dose is 375 mg
From calculations, the dose is 300 mg/m2 × 1.25 m2 = 375 mg
Nomograms for determining body surface area
Most BSA calculations use a standard nomogram, which includes both
weight and height. Nomograms for children and adults are shown in
Figures 8.2 and 8.3. The BSA of an individual is determined by drawing
a straight line connecting the person’s height and weight. The point at
which the line intersects the center column indicates the person’s BSA in
square meters. In the example shown in Figure 8.2, a child weighing 15
kg and measuring 100 cm in height has a BSA of 0.64 m2.
FIGURE 8.2 Body surface area of children. (Reprinted with
permission from Diem K, Lentner C, Geigy JR. Scientific
Tables. 7th Ed. Basel, Switzerland: Ciba-Geigy; 1970:538.
Copyright © Novartis AG.)
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