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CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 125 CHAPTER 14 Adult and Pediatric Dosages Based on Body Surface Area Objectives The learner will: 1. calculate BSA using formulas for weight and height. 2. use BSA to calculate dosages. 3. assess the accuracy of dosages prescribed on the basis of BSA. Suggested Review Questions 1. Why is body surface area (BSA) a factor considered by the prescriber when ordering medication dosages? 2. What are the nurse’s responsibilities when medications are prescribed based on BSA? 3. What is the formula for calculating BSA? Answers to Review Questions 1. The BSA is the external surface of the body expressed in square meters (m2). It represents a relationship between height and weight, and thus provides a more precise guide to the maturity of body organs and metabolic rate, both important factors in the absorption, distribution, metabolism, and excretion of drugs. Using the BSA to determine drug dosages allows the prescriber to adapt doses to individual characteristics. 2. Specific nursing responsibilities related to using BSA to determine drug dosages can vary among health care institutions; however, the nurse always needs to be able to assess that the medication dose prescribed is safe for the patient. Therefore, the nurse needs to know how to calculate the BSA, how to calculate dosages based on BSA, and how to assess prescriber orders based on BSA. © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 126 3. SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA The formula for calculating BSA using metric measures (kilogram and centimeter) is BSA = wt (kg) × ht (cm) 3600 The formula for calculating BSA using household measures (pounds and inches) is BSA = wt (lb) × ht (in) 3131 Remember that a calculator capable of computing a square root is needed; remember also to express the final number, the BSA, to the nearest hundredth. The formulas are the same for children and adults. However, if a nomogram is used to determine BSA, be sure to use a pediatric nomogram when determining the BSA of children. © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 127 Name ____________________________________________ Date ____________________ Chapter 14: Adult and Pediatric Dosages Based on Body Surface Area Additional Practice Problems Calculate the following BSAs using the appropriate formula. Calculate the BSA to the nearest hundredth. 1. An adolescent who weighs 142 lbs and is 4 feet 10 inches __________________ 2. A child who weighs 13.6 kg and is 90 cm __________________ 3. An adult who weighs 165 lbs and is 5 feet 8 inches __________________ 4. A child who weighs 42 lbs and is 34 inches __________________ 5. A child who weighs 11.3 kg and is 75 cm __________________ 6. An adolescent who weighs 125 lbs and is 5 feet 2 inches __________________ 7. A child who weighs 13.6 kg and is 62.5 cm __________________ 8. An adult who weighs 200 lbs and is 6 feet 2 inches __________________ 9. An adult who weighs 183 lbs and is 5 feet 10 inches tall __________________ An adult who weighs 235 lbs and is 6 feet tall __________________ 10. Calculate doses for the drugs indicated below based on the patient’s BSA. Calculate the BSA to the nearest hundredth and medication dosages to the nearest tenth. 11. A child is to receive dactinomycin 2.5 mg/m2 IVP daily for 5 days as treatment for leukemia. The child weighs 48 lbs and is 4 feet 6 inches tall. Calculate the dose of dactinomycin to be given each day. 12. ________________ A child is ordered an infusion of cytarabine 200 mg/m2 in 0.9% NaCl to infuse over 24 hours. If the child weighs 22.7 kg and is 120 cm, how many mg of cytarabine would you add to the 9% NaCl? ________________ © 2010 Cengage Learning. All Rights Reserved. Permission to reproduce granted for classroom use only. 128 13. SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA An adult is ordered vinblastine 25 mg IVP weekly. The recommended dose is 3.7 to 18.5 mg/m2. The patient weighs 195 lbs and is 5 feet 11 inches tall. Is the ordered dose within the recommended range? 14. ________________ A child is ordered didanosine 120 mg/m2 twice a day orally for 2 weeks. The child weighs 75 lbs and is 4 feet 3 inches tall. Didanosine is available in 25 mg, 50 mg, and 100 mg tablets. What tablet combination would you use to administer the prescribed dose? ________________ 15. The physician ordered zidovudine 250 mg/m2 orally daily divided into 4 equal doses for a 5-year-old child who is HIV+. The child weighs 38 lbs and is 36 inches tall. The recommended single dose of zidovudine is 100 mg/m2 to 180 mg/m2. Is the prescribed dose within the recommended single dose range? 16. ________________ What is the single dose of phenytoin for a child who is 112 cm in length and weighs 25.3 kg if the recommended dose is 250 mg/m2 divided in 2 doses? ________________ Phenytoin is available as 125 mg/5mL. How much would you give for a single dose? ________________ 17. A child who is 60 inches and 75 lbs has prednisone ordered. The recommended daily dose for immunosuppressive therapy is 6 to 30 mg/m2 in 2 divided doses. What is the daily dose range for this child? ________________ If prednisone is available as 5 mg/5 mL syrup, what is a single dose for this child if 6 mg/m2 a day is ordered? 18. ________________ An adult who weighs 130 lbs and is 5 feet 6 inches needs methotrexate for leukemia. The recommended dose is 3.3 mg/m2/day. What is the daily dose for this patient? ________________ Methotrexate is available as 2.5 mg/mL to be further diluted for IV infusion. How many mL would be added to the IV infusion for this patient per dose? ________________ © 2010 Cengage Learning. All Rights Reserved. Permission to reproduce granted for classroom use only. CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 19. 129 Ritalin® (methylphenidate) is ordered for a child who is 14.2 kg and 62 cm in length. The recommendation is 10 mg/m2 each dose. How many mg does this child need for each dose? ________________ The Ritalin is supplied as 5 mg/tablet. How many tablets should this child receive with each dose? (The tablet is scored.) 20. ________________ An adult who is 68.8 kg and 165 cm needs monthly IV doses of cyclophosphamide for rheumatoid arthritis. The recommended monthly dose is 0.5 to 1 g/m2. What is the dose range for this patient? ________________ If the lowest dose is ordered and cyclophosphamide for IV administration is available as 100 mg/5 mL, how many mL of medication would be added to IV fluid for infusion each month? ________________ © 2010 Cengage Learning. All Rights Reserved. Permission to reproduce granted for classroom use only. 130 SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA Solutions to Additional Practice Problems Calculate the following BSAs using the appropriate formula. Calculate the BSA to the nearest hundredth. 1. 4 feet 10 inches = 58 inches BSA = 142 × 58 = 3131 8236 = 3131 2.630 = 1.621 Answer: rounded to nearest hundredth = 1.62 m2 2. BSA = 13.6 × 90 = 3600 1224 = 3600 0.34 = 0.583 Answer: rounded to nearest hundredth = 0.58 m2 3. 5 feet 8 inches = 68 inches BSA = 165 × 68 = 3131 11, 220 = 3131 3.583 = 1.893 Answer: rounded to nearest hundredth = 1.89 m2 4. BSA = 42 × 34 = 3131 1428 = 3131 0.456 = 0.675 Answer: rounded to nearest hundredth = 0.68 m2 5. BSA = 11.3 × 75 = 3600 847.5 = 3600 0.2354 = 0.485 Answer: rounded to nearest hundredth = 0.49 m2 6. 5 feet 2 inches = 62 inches BSA = 125 × 62 = 3131 7750 = 3131 2.475 = 1.573 Answer: rounded to nearest hundredth = 1.57 m2 © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 7. BSA = 13.6 × 62.5 = 3600 850 = 3600 131 0.236 = 0.485 Answer: rounded to nearest hundredth = 0.49 m2 8. 6 feet 2 inches = 74 inches BSA = 200 × 74 = 3131 14800 = 3131 4.726 = 2.173 Answer: rounded to nearest hundredth = 2.17 m2 9. 5 feet 10 inches = 70 inches BSA = 183 × 70 = 3131 12810 = 3131 4.091 = 2.022 Answer: rounded to nearest hundredth = 2.02 m2 10. 6 feet = 72 inches BSA = 235 × 72 = 3131 16920 = 3131 5.404 = 2.324 Answer: rounded to nearest hundredth = 2.32 m2 Calculate doses for the drugs indicated below based on the patient’s BSA. Calculate the BSA to the nearest hundredth and medication dosages to the nearest tenth. 11. Calculate the BSA: 4 feet 6 inches = 54 inches BSA = 48 × 54 = 3131 2592 = 3131 0.827 = 0.909 = rounded to nearest hundredth = 0.91 m2 Calculate dose ordered: 2.5 mg × 0.91 m2 = 2.275 = rounded to nearest tenth = 2.3 mg © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 132 12. SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA Calculate the BSA: BSA = 22.7 × 120 = 3600 2724 = 3600 0.756 = 0.869 = rounded to nearest hundredth = 0.87 m2 Calculate dose ordered: 200 mg × 0.87 m2 = 174 mg 13. Calculate the BSA: 5 feet 11 inches = 71 inches BSA = 195 × 71 = 3131 13845 = 3131 4.421 = 2.102 = rounded to nearest hundredth; the trailing zero is eliminated = 2.1 m2 Calculate dose range: 3.7 mg × 2.1 m2 = 7.77 mg = rounded to nearest tenth = 7.8 mg 18.5 mg × 2.1 m2 = 38.85 mg = rounded to nearest tenth = 38.9 mg Compare ordered dose with recommended range. The ordered dose of 25 mg is within recommended dosage range. The correct answer is Yes. 14. Calculate the BSA: 4 feet 3 inches = 51 inches BSA = 75 × 51 = 3131 3825 = 3131 1.221 = 1.104 = rounded to nearest hundredth; the trailing zero is eliminated = 1.1 m2 Calculate ordered dose: 120 mg × 1.1 m2 = 132 mg to be given twice daily. One 100 mg tablet and one 25 mg tablet would be the best tablet combination to give the amount of didanosine closest to the amount ordered. © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 15. 133 Calculate the BSA: BSA = 38 × 36 = 3131 1368 = 3131 0.436 = 0.660 = the trailing zero is eliminated = 0.66 m2 Calculate the ordered dose: 250 mg × 0.66 m2 = 165 mg daily divided in 4 equal doses = 41.25 or 41.3 mg per dose (rounded to nearest tenth). Calculate the dose range: 0.66 m2 × 100 mg/m2 = 66 mg lower single dose 0.66 m2 × 180 mg/m2 = 118.8 mg upper single dose Compare the ordered single dose (41.3 mg) with the recommended single dose range (66 to 118.8 mg). The ordered single dose is less than the lower dose of the recommended single dose range. Therefore, the correct answer is No, the ordered dose is not within the recommended single dose range. 16. Calculate the BSA: BSA = 25.3 × 112 = 3600 2833.6 = 3600 0.787 = 0.887 = rounded to nearest hundredth = 0.89 m2 Calculate ordered single dose: 250 mg × 0.89 m2 = 222.5 mg ÷ 2 = 111.25 mg/dose (Do not round to nearest tenth until determining the final volume of drug to be given.) Determine amount of drug to give based on dose ordered. Use dimensional analysis. mL 5 mL 111.25 mg 556.25 = × = = 4.45 mL (rounded to nearest tenth) = 4.5 mL mg 125 mg 1 125 © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 134 17. SECTION 5 DOSAGE CALCULATION FROM BODY WEIGHT AND BODY SURFACE AREA Calculate the BSA: BSA = 75 × 60 = 3131 4500 = 3131 1.437 = 1.198 (rounded to nearest hundredth) = 1.2 m2 Determine dose range: 6 mg × 1.2 m2 = 7.2 mg/day 30 mg × 1.2 m2 = 36 mg/day The dosage range is 7.2 to 36 mg/day. Determine amount of drug to give based on dose ordered. Use dimensional analysis. mL 5 mL 7.2 mg 36 = × = = 7.2 mL/day ÷ 2 = 3.6 mL/dose mg 5 mg 1 5 18. Calculate the BSA: 5 feet 6 inches = 66 inches BSA = 130 × 66 = 3131 8580 = 3131 2.74 = 1.655 (rounded to nearest hundredth) = 1.66 m2 Determine daily dose based upon recommended dose: 3.3 mg × 1.66 m2 = 5.47 mg (Do not round to nearest tenth until determining the final volume of drug to be given.) Determine amount of drug to give based on dose ordered. Use dimensional analysis. mL 1 mL 5.47 mg 5.47 = × = = 2.188 (rounded to nearest tenth) = 2.2 mL. mg 2.5 mg 1 2.5 © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. CHAPTER 14 ADULT AND PEDIATRIC DOSAGES BASED ON BODY SURFACE AREA 135 19. Calculate the BSA: BSA = 14.2 × 62 = 3600 880.4 = 3600 0.244 = 0.494 (rounded to nearest hundredth) = 0.49 m2 Determine the dose needed: 10 mg × 0.49 m2 = 4.9 mg needed for each dose Determine amount of drug to give based on dose ordered. Drug is supplied in 5 mg tablets. 4.9 mg is very close to 5 mg; therefore one tablet would be given. 20. Calculate the BSA: BSA = 68.8 × 165 = 3600 11,352 = 3600 3.153 = 1.775 (rounded to nearest hundredth) = 1.78 m2 Determine the dose range; transform g to mg: 0.5 g = 500 mg × 1.78 m2 = 890 mg 1 g = 1000 mg × 1.78 m2 = 1780 mg The dosage range is 890 to 1780 mg. Calculate the dose to be given. Use dimensional analysis. mL 5 mL 890 mg 4450 = × = = 44.5 mL mg 100 mg 1 100 © 2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.