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Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI) Introduction In order to be safe and effective, some medications are prescribed with consideration for the size of the patient, rather than relying on a fixed dosage. In addition to patient size, certain drugs and indications for their use may require a prescriber to take the patient’s body weight into account. In this module, you will learn how to perform body weight-based dosage calculations, in order to confirm that a prescribed dosage is safe for a particular client. You will also be introduced to continuous bladder irrigation (CBI), a procedure used to flush out the contents of a patient’s bladder. While performing CBI, certain calculations are required to ensure that this therapy is proceeding safely.

Overview x x x x x

Purpose of Body Weight-based Dosage Calculations Safe Dosage Ranges Body Weight-based Dosage Calculations Purpose of CBI CBI Calculations

Module Outcomes x x x x x

Calculate a patient’s safe dosage based on literature values. Calculate a safe dosage range for administration per day and per dose. Determine if a prescribed dosage is safe for administration. Calculate the inflow and outflow of CBI irrigation fluid. Calculate urinary output for a patient undergoing CBI treatment.

Purpose of Body Weight-based Dosage Calculations Many medications are prescribed based on a fixed dosage that is appropriate for all patients who require the drug. Ambien, for instance, is a sedative used to treat insomnia, with a standard dosage of 5 mg before bedtime. Other drugs are indicated for several uses and may have different dosing regimens for each purpose.

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

For example, methotrexate is prescribed to treat psoriasis with a recommended dosage of 2.5 mg q12h. This drug is also used in the treatment of acute lymphoblastic leukemia, with a dosage of 2.5 mg per kg every 14 days. In the latter case, the prescriber must use the patient’s body weight to determine the appropriate dosage. Here, patients with greater body mass require more medication than those with less mass. This is often referred to as a patient’s metabolic mass. Quite often, medications will have separate dosing requirements for adult and paediatric patients. Since children and infants vary greatly in metabolic mass, usual paediatric doses are often stated with respect to the patient’s body mass. For example, the recommended adult dosage for vancomycin, a commonly prescribed antibiotic, is 500 mg q6h. On the other hand, the recommended paediatric dosage is 10 mg/kg q6h. It is not uncommon for drugs with a standard adult dose to have paediatric doses based on body weight. Safe dosages are often expressed as a range with parameters that define the therapeutically effective dosage. Digoxin, for instance, is indicated for congestive heart failure with a recommended dosage of 8 mcg/kg to 12 mcg/kg. When receiving a prescription based on a patient’s body weight, it is the nurse’s responsibility to ensure that the prescribed dosage is safe. The recommended dosage for a medication can either be found on the drug label, or from reputable literature, such as the Physician’s Desk Reference (PDR). Overdose: If the patient has been prescribed more medication than necessary, this constitutes an overdose and should be reported to the prescriber. Although this treatment will be effective, it may introduce undue side effects that harm the patient. Underdose: Likewise, an underdose of medication is inadequate for treating an illness. This is also considered unsafe and is to be reported.

Dosage Calculations Dosage Calculations: Example Timentin is prescribed to treat numerous bacterial infections. It is indicated for pneumonia with a recommended paediatric dosage of 75 mg/kg BID. The physician has ordered 900 mg of Timentin BID for a patient weighing 34.87 lbs. In order to determine if this dosage is appropriate, the recommended dosage must be tailored to your patient.

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

First, convert your patient’s body weight from pounds to kilograms. 1 kg 34.87 lbs × = 15 .85 kg 1 2.2 lbs Remember: Paediatric doses are rounded to the nearest hundredth (2 decimal places). For precision, all preliminary calculations should also be expressed to the nearest hundredth. Now, determine the recommended dosage for your patient. 15.85 kg 75 mg × = 1 188.75 mg 1 1 kg The prescribed dosage is considerably less than the recommended dosage based on your patient’s body weight. This underdose is considered unsafe and should be reported to the prescriber.

Dosage Calculations: Activity Strattera (atomoxetine HCl) is prescribed for the treatment of attention deficit hyperactivity disorder (ADHD). The usual paediatric dose is 0.5 mg/kg daily. Your patient weighs 132 lbs, and the physician’s order reads, “Give 40 mg Strattera PO q.a.m.” 1. What is the safe dosage for your patient? Answer: 30 mg Solution: 1 kg 132 lbs × = 60 kg 1 2.2 lbs 60 kg 0.5 mg × = 30 mg 1 1 kg 2. The physician’s order is: a. an overdose. b. an underdose. c. a safe dose. Answer: a. an overdose 3. The medication order is reported as unsafe and is adjusted to read, “Give 30 mg Strattera PO q.a.m.” Use the drug label below to determine how many capsules you will administer to satisfy the revised medication order. Answer: 3 cap

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

Solution: 30 mg 1 cap × = 3 cap 1 10 mg

Safe Dosage Ranges Medications that are prescribed based on body weight often have recommended doses expressed as a range of safe values. For example, the safe dosage for oxcarbazepine, an anticonvulsant, is expressed using a minimum and maximum daily dose: 8 mg/kg/day to 10 mg/kg/day. It is recommended to administer this drug in two divided doses per day, resulting in a safe single dosage range of 4 mg/kg/dose to 5 mg/kg/dose. When considering a particular patient’s body weight, this introduces two additional safe ranges, and it quickly becomes difficult to track which figures are relevant. Consider a patient who weighs 15 kg. The safe ranges can be characterized as follows. General per day

8 – 10 mg/kg/day This range is stated in the literature. General ranges can be applied to any patient and are expressed per kg.

General per dose

4 – 5 mg/kg/day

The daily allotment for this drug is recommended to be split among two doses.

Specific per day

120 – 150 mg/day

Specific ranges are tailored to a particular patient. This range represents your patient’s daily allotment.

Specific per dose

60 – 75 mg/dose

This is the safe range for your patient at each of the two doses.

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

Safe Dosage Ranges: Activity 1 It is helpful to pay particular attention to the units of measure when considering the different types of safe ranges. In this case, the recommendation to administer oxcarbazepine in two divided doses is cited in trusted literary sources (eg. PDR). Frequencies of administration can also be determined by medication orders. It is important to be aware that a frequency such as q12h is virtually equivalent to BID. Similarly, q8h

_________

q6h

_________

q4h

_________ times per day.

Answer: TID, QID, and 6

Safe Dosage Ranges: Activity 2 A physician has prescribed phenobarbital to a patient at your paediatric (Remember to round values to the nearest hundredth for paediatric calculations. You should also watch for terms such as adolescent, child, infant and neonate.) unit. The safe daily dosage range for this drug is 3 to 8 mg/kg/day (This is the ‘general per day’ safe range.), and your patient weighs 37.4 lbs. The prescription calls for 25 mg of phenobarbital q6h. a) What is your patient’s weight in kilograms? Answer: 17 kg Solution: 37.4 lbs 1 kg × = 17 kg 1 2.2 lbs b) What is the safe daily dosage range for your patient (mg/day)? Answer: 51 to 136 mg/day Solution: Minimum:

3 mg

Maximum:

1 kg 8 mg 1 kg

per day ×

17 kg

per day ×

1 17 kg 1

= 51

mg day

= 136

mg day

Range: 51 to 136 mg/day (specific per day)

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

c) What is the safe single dosage range for your patient (mg/dose)? Answer: 12.75 to 34 mg/dose Solution: q6h → QID (four doses per day) mg

mg

Minimum: 51 day ÷ 4 = 12.75 dose mg

mg

Maximum: 136 day ÷ 4 = 34 dose Range: 12.75 to 34 mg/dose (specific per dose) d) The physician’s order is: a. an overdose. b. an underdose. c. a safe dose. Answer: c. a safe dose. e) Use the drug label below to determine the volume of phenobarbital that you will administer.

Answer: 6.25 mL Solution: 25 mg 5 mL × = 6.25 mL 1 20 mg

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

Safe Dosage Ranges: Activity 3 A physician has prescribed Principen for an infant weighing 8 lbs, 5 oz. Use the drug label to determine the recommended volume to administer at each dose. This problem requires several intermediate steps in order to arrive at the final answer

Solution: 5 oz 1 lbs = 0.3125 lbs × 16 oz 1 8 lbs + 0.3125 lbs = 8.3125 lbs

Mathematics for Medication Administration 8.3125 lbs

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

1 kg  kg × 2.2 lbs = 3.77840909

1 “Children weighing 20 kg or less: 100 mg/kg/day q.i.d. in equally spaced doses.” This phrase implies 25 mg/kg/dose.  kg 3.77840909 25 mg  mg per dose per dose × = 94.40622727 1 kg 1  mg per dose 94.40622727 5 mL  mg = 1.889204545 × 250 mg 1 ≈ 1.89 mL x

Remember: Do not round any values off until you reach the final answer. It is helpful if you don’t clear your calculator. Notice that each calculated value is used in the following step of the solution.

Continuous Bladder Irrigation Continuous bladder irrigation (CBI) is a procedure used to flush out the contents of the bladder. This is often performed after a patient has undergone a surgical procedure known as transurethral resection of the prostate (TURP). This operation is used to cauterize sections of the prostate gland, which results in bleeding and a build-up of dead tissue. CBI is then prescribed to clear the bladder using N/S. CBI is performed using a three-way Foley catheter, which is inserted through the urethra and into the bladder. The irrigation port is used to deliver N/S to the bladder from an irrigation bag. As the volume of fluid builds up in the bladder, fluid is removed through the drainage port and collected in a drainage bag. Once the catheter is in position, the inflation port is used to inflate the balloon with 10 to 30 mL of sterile water or N/S. This balloon holds the catheter in place throughout the procedure.

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

The irrigation port is used to deliver N/S to the bladder from an irrigation bag. As the volume of fluid builds up in the bladder, fluid is removed through the drainage port and collected in a drainage bag. Typically, the flow rate for irrigation fluid is not explicitly stated in the order; rather, this fluid is run quickly enough to result in drainage fluid that has a light yellow-pink colour. CBI flow rates vary widely, ranging from 100 mL/hr to upwards of 1 500 mL/hr. Such high flow rates do not pose a problem, since all fluids that are collected in the bladder are quickly removed. This is unlike IV therapy, in which fluids infuse directly into the client’s bloodstream.

CBI Calculations While irrigation fluid consists only of N/S, drainage fluid is composed of an equal volume of N/S, in addition to blood and urine. The collective volume of these drainage fluids is referred to as outflow. The volume of irrigation fluid delivered to the bladder is termed inflow. If the procedure is running smoothly, then the outflow will outweigh the inflow. The difference in these volumes represents the urinary output—fluids produced by the patient. The purpose of CBI calculations is to determine the patient’s urinary output, which is then charted in order to monitor fluid balance.

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

To summarize, the three values used in CBI calculations are: x x x

inflow (volume delivered); outflow (volume drained); and urinary output (volume produced by the patient).

Urinary output is calculated by subtracting inflow from outflow. urinary output = outflow − inflow

CBI Calculations: Example You begin your shift at 0900 with a patient undergoing CBI treatment. There is 500 mL of irrigation fluid TBA, and the urinary drainage bag is empty. The irrigation bag goes empty at 1300, and you find 560 mL of fluid in the urinary drainage ba g. What is the patient’s urinary output from 0900 to 1300? urinary output = outflow – inflow urinary output = 560 mL – 500 mL

Mathematics for Medication Administration

Body Weight-based Dosage Calculations and Continuous Bladder Irrigation (CBI)

urinary output = 60 mL As these problems become more complex, it is helpful to calculate the inflow and outflow separately, before determining urinary output.

CBI Calculations: Example You are working a shift from 0800 to 1600 with a patient receiving CBI treatment. At the beginning of your shift, the urinary drainage bag is empty, and 475 mL of irrigation fluid is left TBA. Over the course of your shift, the irrigation bag runs empty and you hang a new, 2 L bag of N/S. At 1400, the new bag runs empty, and a total of 2 725 mL has been collected as drainage. a) How much irrigation fluid has been administered? Answer: 2 475 mL Solution: 475 mL + 2 000 mL = 2 475 mL 475 mL of irrigation fluid is left TBA. “The irrigation bag runs empty and you hang a new, 2 L bag of N/S.” b) How much fluid has been drained? Answer: 2 725 mL Solution: “a total of 2 725 mL has been collected as drainage.” c) What is the patient’s urinary output? Answer: 250 mL Solution: urinary output = outflow − inflow = 2 725 mL − 2 475 mL = 250 𝑚𝐿...