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SECTION 5

Dosage Calculation from Body Weight and Body Surface Area CHAPTER 13

Adult and Pediatric Dosages Based on Body Weight CHAPTER 14

Adult and Pediatric Dosages Based on Body Surface Area

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CHAPTER 13 ADULT AND PEDIATRIC DOSAGES BASED ON BODY WEIGHT

117

CHAPTER 13 Adult and Pediatric Dosages Based on Body Weight Objectives The learner will: 1.

convert body weight from lb to kg.

2.

convert body weight from kg to lb.

3.

calculate dosages using mcg/mg per kg, or per lb.

4.

determine if dosages ordered are within the normal range.

Suggested Review Questions 1.

Why is body weight a factor considered by the prescriber when ordering medication dosages?

2.

What are the nurse’s responsibilities when medications are prescribed by weight?

3.

How is weight in pounds (lb) converted to weight in kilograms (kg)?

4.

How can dimensional analysis be used to calculate medication dosages of mg/kg when the patient’s weight is given in lbs?

Answers to Review Questions 1.

Body weight is one of the factors that influences an individual’s response to a dose of medication. Other factors include the amount of the dose and route of drug administration, as well as the individual’s age, general health status, hepatic and renal function, drug history, presence of allergies, etc. Weight is especially important when considering drug dosage for infants and neonates since their ability to metabolize drugs is decreased due to organ immaturity. Drug doses for children in general are frequently prescribed by body weight because of the wide variation in weight and development in children of similar age.

2.

As with all medication orders, when a medication dose is prescribed based on body weight, it is the nurse’s responsibility to prepare the prescribed dose correctly as well as verify that the order is safe. For medications prescribed by weight, the safe dose is determined by comparing the ordered dose and frequency of administration with the

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SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA

recommended total daily dose as noted on the drug label, package insert, or a reliable reference, e.g., PDR. The recommended total daily dose is usually noted as the number of micrograms (mcg) or milligrams (mg) per kilogram (kg) of body weight. Occasionally, doses for adults may be noted as micrograms (mcg) or milligrams (mg) per pound (lb) of body weight. 3.

There are 2.2 lb in one kg, so to convert weight in lbs to weight in kg, divide the number of lbs by 2.2. Stress that fractions of lbs are always noted as decimals and, in children, the number of kg needs to be exact; that is, ounces cannot be rounded to the nearest quarter lb. The medication calculation problem is set up in the same manner as described in Chapter 12, with the conversion for lbs to kg noted in the equation written to the right of the equal (=) sign. Here is an example: A 48 lb child has an order for cefaclor 45 mg/kg PO every 12 h. Using the information on the label in Figure 13-1, how many milliliters are needed per dose? The problem would be set up as follows:

mL =

5 mL 45 mg 1 kg 45 lb 10,800 × × × = = 39.27 rounded to nearest tenth = 39.3 mL/dose 125 mg 1 kg 2.2 lbs 1 275

Used with permission from Ranbaxy Pharmaceuticals Inc.

4.

Figure 13-1

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CHAPTER 13 ADULT AND PEDIATRIC DOSAGES BASED ON BODY WEIGHT

119

Name ____________________________________________ Date ____________________

Chapter 13: Adult and Pediatric Dosages Based on Body Weight Additional Practice Problems Convert the following body weight to kg or lb as directed below. Round weights to nearest tenth. 1.

25 lb

=

___________ kg

2.

62.5 lb

=

___________ kg

3.

115 lb

=

___________ kg

4.

15.25 lb

=

___________ kg

5.

156 lb

=

___________ kg

6.

252 lb

=

___________ kg

7.

58.2 kg

=

___________ lb

8.

13.5 kg

=

___________ lb

9.

5.4 kg

=

___________ lb

10.

23.6 kg

=

___________ lb

Determine the amount of medication needed based upon physician orders and patient’s weight. When available, read the label to determine how the medication is supplied. Work problems using dimensional analysis when appropriate. Calculate doses to nearest tenth. Calculate safe dose when requested. 11.

A patient with rheumatoid arthritis is ordered azathioprine (Imuran®) twice a day. The label reads: recommended dose 1 mg/kg/day in 2 divided doses. The patient weighs 145 lbs. How many mg should the patient get with each dose?

________________ mg

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SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA

12.

You are to administer furosemide (Lasix®) 2 mg/kg to a child weighing 45 lbs. The furosemide available is 8 mg/mL.

13.

You are to give digoxin 15 mcg/kg to a child weighing 15 kg. The digoxin available is 0.1 mg/mL.

14.

________________ mL

________________ mL

Clindamycin 100 mg/kg/day is ordered for an infant weighing 6 lb 4 oz. Refer to the package information shown in Figure 13-2. You are to give this in divided doses 12 hours apart. ________________ mL

Used with permission from Bedford Laboratories, A Division of Ben Venue Laboratories Inc. A Boehringer -Ingelheim Company.

How many mL will be given for each dose?

Figure 13-2

15.

The physician orders 40 mg of gentamycin (Garamycin®) every 8 hours for a patient who weighs 20 kg. The recommended safe range dose for gentamycin is 6 to 7.5 mg/kg/day in 3 to 4

16.

divided doses. What is the safe dose range for this patient?

________________

Is the ordered dose within the safe range?

________________

A patient is to receive 20 mg/kg of nafcillin sodium (Nafcil®) every 12 hours. The patient weighs 5 kg and the nafcillin label reads 500 mg/mL. How many mL will you give?

________________ mL

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CHAPTER 13 ADULT AND PEDIATRIC DOSAGES BASED ON BODY WEIGHT

17.

121

A child who weighs 35 lb has been ordered amoxicillin 250 mg po every 8 h. The label on the amoxicillin bottle reads: amoxicillin 250 mg/5 mL. How many mL are needed for each dose?

________________ mL

The label also reads: Usual Child Dosage: 20 to 40 mg/kg/day in divided doses every 8 hrs. Is this dose safe for this patient? 18.

________________

The physician orders ampicillin 5 mg/kg/day in 4 equally divided doses for a child who weighs 70 lbs. How many mg are needed for each dose?

19.

________________ mg

Procainamide 0.5 mg/kg/day divided in 2 doses is ordered for an adult patient weighing 198 lbs. How many mg are needed for each dose?

________________ mg

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122

SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA

Solutions to Additional Practice Problems Convert the following body weight to kg or lb as directed below. Round weights to nearest tenth. 1.

25 lb = 25 ÷ 2.2 = 11.36 rounded to nearest tenth = 11.4 kg

2.

62.5 lb = 62.5 ÷ 2.2 = 28.40 rounded to nearest tenth = 28.4 kg

3.

115 lb = 115 ÷ 2.2 = 52.27 rounded to nearest tenth = 52.3 kg

4.

15.25 lb = 15.25 ÷ 2.2 = 6.93 rounded to nearest tenth = 6.9 kg

5.

156 lb = 156 ÷ 2.2 = 70.90 rounded to nearest tenth = 70.9 kg

6.

252 lb = 252 ÷ 2.2 = 114.54 rounded to nearest tenth = 114.5 kg

7.

58.2 kg = 58.2 × 2.2 = 128.04 rounded to nearest tenth = 128 lb

8.

13.5 kg = 13.5 × 2.2 = 29.7 lb

9.

5.4 kg = 5.4 × 2.2 = 11.88 rounded to nearest tenth = 11.9 lb

10.

23.6 kg = 23.6 × 2.2 = 51.92 rounded to nearest tenth = 51.9 lb

Determine the amount of medication needed based upon physician orders and patient’s weight. When available, read the label to determine how the medication is supplied. Work problems using dimensional analysis when appropriate. Calculate doses to nearest tenth. Calculate safe dose when requested. 11.

kg = 145 ÷ 2.2 = 65.9 kg mg = 1 mg × 65.9 kg = 65.9 mg/day divided into 2 equal doses = 32.95 rounded to nearest tenth = 33 mg/dose 1

12.

mL =

1 mL × 8 mg

2 mg kg

×

1 kg 2.2 lb

×

45 lb 45 = = 5.1 mL 1 8.8

4 3

13.

1 mg 15 kg 15 mcg 1 mL 45 × mL = × × = = 2.25 rounded to nearest 1 20 0.1 mg 1 kg 1000 mcg 200

tenth = 2.3 mL

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CHAPTER 13 ADULT AND PEDIATRIC DOSAGES BASED ON BODY WEIGHT

14.

123

First convert 6 lb 4 oz to a decimal = 4 oz/16 oz in a lb = 0.25 + 6 lb = 6.25 lb. mL =

1 mL 100 mg 1 kg 6.25 lb × × × remove the decimals 150 mg 1 kg 2.2 lb 1 2

1 mL × mL = 150 mg

125

100 mg 1 kg

1 kg

625 lb 250 × × = = 1.89 mL/day 220 lb 1 132 44

3

1.89 mL daily dose ÷ 2 = 0.94 rounded to nearest tenth = 0.9 mL/dose 15.

Safe dose = 6 mg × 20 kg = 120 mg/day 7.5 mg × 20 kg = 150 mg/day Safe dosage range is 120–150 mg/day. Ordered is 40 mg every 8 hrs or 3 times a day, therefore daily dose = 40 mg × 3 = 120. Yes, the ordered dose is within the safe dose range for this patient. 1

16.

5 kg 20 mg 5 1 mL × mL = = = 0.2 mL × 500 mg 1 25 1 kg 25

1

17.

250 mg 5 5 mL = = 5 mL/dose × mL = 1 1 250 mg 1

Ordered daily dose is 250 mg every 8 hours = 250 × 3 = 750 mg/day Child’s weight in kg = 35 ÷ 2.2 = 15.9 kg Safe dose = 20 mg × 15.9 kg = 318 mg/day 40 mg × 15.9 kg = 636 mg/day The ordered 750 mg/day exceeds the safe dose range, therefore, the ordered dose is not safe for this patient. 18.

kg = 70 lbs ÷ 2.2 = 31.8 kg. Daily dose = 5 mg × 31.8 kg = 159 mg/day divided in 4 doses = 159 ÷ 4 = 39.75 = 39.8

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124

19.

SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA

kg = 198 lbs ÷ 2.2 = 90 kg. mg = 0.5 mg × 90 kg = 45 mg/day divided in 2 doses = 45 ÷ 2 = 22.5 mg/dose

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