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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5576_Библиотеки_им_академика_М_И_Перельмана
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FIGURE 8.3 Body surface area of adults. (Reprinted with
permission from Diem K, Lentner C, Geigy JR. Scientific
Tables. 7th Ed. Basel, Switzerland: Ciba-Geigy; 1970:538.
Copyright © Novartis AG.)
1. If the adult dose of a drug is 75 mg, what would be the dose for a
child weighing 40 lb and measuring 32 inches in height using the
BSA nomogram?
From the nomogram, the BSA = 0.60 m
2
2. The usual pediatric dose of a drug is stated as 25 mg/m2. Using the
nomogram, calculate the dose for a child weighing 18 kg and
measuring 82 cm in height.
From the nomogram, the BSA = 0.60 m
2
The nomogram in Figure 8.3 designed specifically for determining
the BSA of adults may be used in the same manner as the one
previously described. The adult dose adjusted for BSA is then
calculated as follows:
CALCULATIONS CAPSULE
Dose Based on Body Surface Area
A useful equation for the calculation of dose based on body surface
area is:
If there is need to determine a patient’s BSA, a nomogram or the
following equation may be used:
1. If the usual adult dose of a drug is 120 mg, what would be the dose
based on BSA for a person measuring 6 feet tall and weighing 200
lb?
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2. If the dose of a drug is 5 mg/m2, what would be the dose for a patient
with a BSA of 1.9 m2?
BSA equation
In addition to the use of the nomogram, BSA may be determined through
use of the following Mosteller formula7:
Calculate the BSA for a patient measuring 165 cm in height and weighing
65 kg.
NOTE: For the sake of comparison, check Figure 8.3 to derive the BSA
for the same patient using the nomogram.
Dosage Based on the Medical Condition to Be
Treated
In addition to the factors previously discussed that might be used to
determine a drug’s dose, the medical condition to be treated and the
severity of that condition must also be considered.
Table 8.5 presents an example of a dosage schedule for a drug based
both on a patient’s age and the medical condition to be treated.
TABLE 8.5 PARENTERAL DOSAGE SCHEDULE FOR A
HYPOTHETICAL ANTI-INFECTIVE DRUG BASED ON
PATIENT AGE AND CONDITION BEING TREATED
By using Table 8.5, calculate the IV drug dose for a 3-lb 3-oz neonate.
By using Table 8.5, calculate the daily IV dose of the drug in the
treatment of a lung infection for a patient weighing 160 lb.
Dosage Adjustment Based on Coadministered
Drugs
The usual dose of a drug may require adjustment based on the
coadministration of another drug when there is a known or suspected risk
for a drug interaction. Drug interactions may result in diminished drug
efficacy and/or in increased toxicity due to a number of factors including
those affecting a drug’s pharmacokinetics (i.e., absorption, distribution,
metabolism, and elimination).
The usual adult dose of colchicine in the prevention of gout flares is
0.6 mg once or twice a day. However, when coadministered with protease
inhibitors (e.g., ritonavir), the dose is reduced to 0.3 mg once daily or
once every other day. For “once every other day” treatment, how many
whole or split 0.6-mg tablets are required for a 30-day supply?
Dosage Based on Reduced Kidney and/or Liver
Function
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The status of a patient’s hepatic (liver) and renal (kidney) function plays a
major role in determining drug dosage due to their roles in drug
metabolism and elimination. Specific calculations of dosage based on
reduced kidney function are presented in Chapter 10.
Other Patient Factors Affecting Drug Dosage
and Utilization
In addition to factors of renal and/or hepatic impairment and age
(pediatric, geriatric), other patient factors play a role in drug selection and
dosage including gender, genetics (e.g., pharmacogenetics), metabolic
disorders, pregnancy, breastfeeding, current health status, medical and
medication history, and others.
Special Dosing Considerations in Cancer
Chemotherapy
The term chemotherapy applies to the treatment of disease with chemical
drugs or chemotherapeutic agents. Chemotherapy is primarily associated
with the treatment of cancer patients and is considered the mainstay of
such treatment in that it is effective against widespread or metastatic
cancer, whereas treatments such as surgery and radiation therapy are
limited to specific body sites. Chemotherapeutic agents most often are
administered orally, by intravenous injection, or by continuous
intravenous infusion.
CASE IN POINT 8.4
A hospital pharmacist is consulted on the appropriate dose of
lopinavir/ritonavir (KALETRA) oral solution in the treatment of an
HIV-1 infection in a 12-month-old pediatric patient. The oral solution
contains, in each milliliter, 80 mg of lopinavir and 20 mg of ritonavir,
expressed as “KALETRA 80/20.” According to the pharmacy’s
protocol, the pediatric dose for patients >6 months of age, not
receiving other concomitant therapy, may be calculated based on either
BSA or body weight as follows:
230/57.5 mg/m2, administered twice daily
12/3 mg/kg for patients <15 kg, administered twice daily
10/2.5 mg/kg for patients >15 kg administered twice daily
The patient measures 28 inches in length and weighs 22 lb.
a. Calculate the single dose, in mg, using the BSA equation.
b. Translate the calculated single dose from (a) into corresponding
milliliters of the oral solution.
c. Calculate the daily dose, in mg, based on the patient’s weight.
d. Translate the daily dose from (c) into corresponding milliliters of
oral solution.
Although a single anticancer drug may be used in a patient’s treatment
plan, combination chemotherapy is perhaps more usual. By using
combinations of drugs having different mechanisms of action against the
target cancer cells, the effectiveness of treatment may be enhanced, lower
doses used, and side effects reduced. Combination chemotherapy plans
often include two-, three-, and four-agent regimens.8–
11
Cancer chemotherapy is unique in the following ways:
It may involve single or multiple drugs of well-established drug
therapy regimens or protocols, or it may involve the use of
investigational drugs as a part of a clinical trial.
Combinations of drugs may be given by the same or different routes
of administration, most often oral and/or intravenous.
The drugs may be administered concomitantly or alternately on the
same or different days during a prescribed treatment cycle (e.g., 28
days). The days of treatment generally follow a prescribed format of
written instructions, with D for “day,” followed by the day(s) of
treatment during a cycle, with a dash (−) meaning “to” and a comma
(,) meaning “and.” Thus, D 1–4 means “days 1 to 4,” and D1,4
means “days 1 and 4.”
8
The drugs used in combination chemotherapy often fit into a
standard drug/dosage regimen identified by abbreviations or
acronyms. For example, a treatment for bladder cancer referred to as
MVAC consists of methotrexate + vinblastine + doxorubicin +
cisplatin; a treatment for colorectal cancer called FU/LV consists of
fluorouracil + leucovorin; a treatment for lung cancer called PC
consists of paclitaxel + carboplatin; and one for acute lymphocytic
leukemia called CVAD consists of cyclophosphamide + vincristine +
doxorubicin (Adriamycin®) + dexamethasone.
In addition to the use of abbreviations for the drug therapy regimens,
the drugs themselves are commonly abbreviated in medication
orders, such as MTX for “methotrexate,” DOX for “doxorubicin,”
VLB for “vinblastine,” and CDDP for “cisplatin.” Tables of standard
chemotherapy treatments, dosing regimens, and abbreviations of the
drugs and treatment regimens may be found in the indicated
references.8–
11
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For systemic action, chemotherapeutic agents are usually dosed
based either on body weight or on body surface area. Often, the drug
doses stated in standard regimens must be reduced, based on a
particular patient’s diminished kidney or liver function and, thus, his
or her ability to metabolize and eliminate the drug(s) from the body.
For certain patients, high-dose chemotherapy is undertaken in an
effort to kill tumor cells.
To help prevent errors in chemotherapy, pharmacists must correctly
interpret medication orders for the chemotherapeutic agents prescribed,
follow the individualized dosing regimens, calculate the doses of each
medication prescribed, and dispense the appropriate dosage forms and
quantities/strengths required.
12
Example calculations of chemotherapy dosage
regimens
1. Regimen: VC
10
Cycle: 28 days; repeat for 2–8 cycles
Vinorelbine, 25 mg/m2, IV, D 1,8,15,22
Cisplatin, 100 mg/m2, IV, D 1
For each of vinorelbine and cisplatin, calculate the total intravenous
dose per cycle for a patient measuring 5 feet 11 inches in height and
weighing 175 lb.
From the nomogram for determining BSA, (a) find the patient’s
BSA and (b) calculate the quantity of each drug in the regimen.
a. BSA = 2.00 m
2
b. Vinorelbine: 25 mg/m2 × 2.00 m2 × 4 days/cycle = 200 mg
Cisplatin: 100 mg/m2 × 2.00 m2 × 1 day/cycle = 200 mg
2. Regimen: CMF
10
Cycle: 28 days
Cyclophosphamide, 100 mg/m2/day PO, D 1–14
Methotrexate, 40 mg/m2, IV, D 2,8
Fluorouracil, 600 mg/m2, IV, D 1,8
Calculate the total cycle dose for cyclophosphamide, methotrexate,
and fluorouracil for a patient having a BSA of 1.5 m2.
3. Using Table 8.6 as a reference, calculate the quantities of doxorubicin
and cyclophosphamide administered per treatment cycle to a woman
measuring 5 feet 4 inches in height and weighing 142 lb during the
“AC” protocol for breast cancer.
4. A variation of the “AC” protocol, referred to as “AC → T,” follows 4
cycles of the AC protocol with paclitaxel (TAXOL), 80 mg/m2 by
intravenous infusion every 7 days for 12 cycles.11 Calculate the total
quantity of paclitaxel, in milligrams, that the patient in the previous
problem would receive during this treatment plan.
5. If an injection is available containing paclitaxel, 6 mg/mL, calculate
the volume required per cycle to treat the patient in the previous
problem.
TABLE 8.6 EXAMPLES OF DOSAGE REGIMENS IN
CANCER CHEMOTHERAPY
a
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1.
a
Table from Lexicomp [book online]. Baltimore, MD: Wolters Kluwer Clinical Drug Information
Inc.; 2020. Ref.
11
b
Types of cancer are stated broadly and not differentiated by subclassifications.
c
Some abbreviations are based on brand name(s) of the drug(s) utilized in the regimen. For
example, “X” in “EOX” represents XELODA, the brand name for capecitabine.
d
The number of treatment cycles varies according to the specific protocols employed.
CASE IN POINT 8.5
13
In treating a 54-year-old female patient, an oncologist selects the
drug temozolomide, an antitumor agent used in the treatment of
refractory astrocytoma (brain tumor). The drug is used as part of a 28day regimen, during which the first 5 days of treatment include
temozolomide at a once-daily dose of 150 mg/m2/day. The patient’s
medical chart indicates that she measures 5 feet in height and weighs
117 lb. The physician asks the pharmacist to determine the proper
combination of available capsules to use in dosing the patient. The
drug is available in capsules containing 5, 20, 100, and 250 mg of
temozolomide. What combination of capsules would provide the daily
dose of this drug?
PRACTICE PROBLEMS
Calculations Based on Body Weight
The dose of a drug is 500 mcg/kg of body weight. How many
milligrams should be given to a child weighing 55 lb?
2.
3.
4.
5.
7.
8.
9.
The dose of gentamicin for premature and full-term neonates is 2.5
mg/kg administered every 8 hours. What would be the daily dose
for a newborn weighing 5.6 lb?
The dose of gentamicin for patients with impaired renal function is
adjusted to ensure therapeutically optimal dosage. If the normal
daily dose of the drug for adults is 3 mg/kg/day, administered in
three divided doses, what would be the single (8-hour) dose for a
patient weighing 165 lb and scheduled to receive only 40% of the
usual dose, based on renal impairment?
A patient weighing 120 lb was administered 2.1 g of a drug
supposed to be dosed at 30 mg/kg. Was the dose administered
correct, or was it an overdose, or was it an underdose?
In a clinical trial of ciprofloxacin (CIPRO), pediatric patients were
initiated on 6 to 10 mg/kg intravenously every 8 hours and
converted to oral therapy, 10 to 20 mg/kg, every 12 hours.
Calculate the ranges of the total daily amounts of ciprofloxacin that
would have been administered intravenously and orally to a 40-lb
child.
If the dose of erythromycin ethylsuccinate is given as 40
mg/kg/day:
a. What would be the proper dose of the medication in the Signa, if
the prescription is for a 44-lb child?
b. How many days will the prescribed medication last?
If the pediatric dosage of chlorothiazide (DIURIL) is 10 to 20
mg/kg of body weight per day in a single dose or two divided
doses, not to exceed 375 mg/day, calculate the daily dosage range
of an oral suspension containing 250 mg chlorothiazide per 5 mL
that should be administered to a 48-lb child.
Cyclosporine is an immunosuppressive agent administered 4 to 12
hours before organ transplantation at a single dose of 5 mg/kg.
How many milliliters of a solution with a concentration of 50 mg
of cyclosporine per milliliter would be administered to a 140-lb
kidney transplant patient?
The adult dose of a liquid medication is 0.1 mL/kg of body weight.
How many teaspoonfuls should be administered to a person
weighing 220 lb?
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