5 Paediatric dosages

In this chapter dosages of medications for children will be considered. The exercises focus on the arithmetic of oral medications and injections specifically for children. Intravenous infusion calculations have been covered in Chapter 4.

The skills covered in this chapter will include medication calculations relating to both body weight and body surface area.

Great care must be taken when administering medication to children; the smallest error is potentially life-threatening.

What you need to know

Children grow at different rates.

There are wide variations in the actual weight of a child of a given age compared to the average weight for a child of that age. Consequently dosages are usually calculated according to body weight. In more complex situations dosages are based on body surface area, for example in chemotherapy.

Body surface area can be determined using the body weight and height of a child. When dealing with an infant (a child aged less than 1 year) body weight and length are used.

Body surface area determines the loss of fluid from the body by evaporation. This fluid loss is critical in the case of some medications and this is when body surface area is used in a calculation, rather than weight.

The prescription should specify whether to use weight or body surface area.

Important: If in any doubt about the answer to a calculation, then ask a supervisor to check your working.

Refer to Inside front cover page for explanations of abbreviations.

Calculating a single dose based on body weight

Calculating the size of a single dose is based on the recommended dosage (in milligrams per kilogram per day) and a child’s weight. The nurse is not usually required to do this calculation. However, it is important that you understand how a single dose prescription is determined.

Note: There is no particular formula required for these calculations

Example 1

A child is prescribed erythromycin. The recommended dosage is 40 mg/kg/day, 4 doses daily. If the child’s weight is 15 kg, calculate the size of a single dose.

15 kg × 40 mg/kg/day = 600 mg/day

Then

600 mg ÷ 4 doses = 150 mg/dose

Calculating a single dose based on body weight

Example 2

A child is to be given ampicillin. The recommended dosage is 80 mg/kg/day, 4 doses per day. Calculate the size of a single dose if the child’s weight is 27 kg.

27 kg × 80 mg/kg/day = 2160 mg/day

Then

2160 mg ÷ 4 doses = 540 mg/dose

Exercise 5A

Calculate the size of a single dose for a child weighing 12 kg:

1 Erythromycin, 40 mg/kg/day, 4 doses per day

2 Penicillin V, 50 mg/kg/day, 4 doses per day

3 Cefalexin, 30 mg/kg/day, 4 doses per day

Calculate the size of a single dose for a child weighing 20 kg:

4 Cloxacillin, 50 mg/kg/day, 4 doses per day

5 Chloramphenicol, 40 mg/kg/day, 4 doses per day

6 Amoxicillin, 45 mg/kg/day, 4 doses per day

Calculate the size of a single dose for a child weighing 36 kg:

7 Flucloxacillin, 100 mg/kg/day, 4 doses per day

8 Capreomycin sulphate, 20 mg/kg/day, 3 doses per day

9 Cephalothin, 60 mg/kg/day, 4 doses per day

Check your answers on page 153.

Calculating volumes for injection in paediatric patients

Example 1

A boy is prescribed pethidine 35 mg, IM. Stock ampoules contain 50 mg in 1 mL. What volume must be withdrawn for injection?

The formula can be abbreviated to:

Example 2

A child is prescribed digoxin 40 micrograms, IV. Paediatric ampoules contain 50 micrograms/2 mL. Calculate the amount to be drawn up in a syringe.

If you are having difficulty simplifying the fractions in these examples then refer to Review exercises 1H and 1I in Chapter 1.

Exercise 5B

For each of these paediatric dosages calculate the volume to be drawn up in a syringe for injection.

Always check that each answer makes sense.

Should the volume you calculated be equal to, or less than, or more than the volume of the stock ampoule?

Check your answers on page 153.

Calculating paediatric dosages for oral medications in liquid form

Children are frequently prescribed oral medication in liquid form. Liquid medication allows a greater range of specific dosages compared to solid form (tablets and capsules). This is especially relevant for children as there is a great range of weights for a child of a given age. Generally a child finds a liquid medication easier to take than a tablet or capsule. The liquid medication may be a syrup, an elixir, a solution or a suspension. Suspensions must be shaken thoroughly before measuring the volume.

Example 1

A child is prescribed 80 mg of paracetamol elixir. Stock on hand is 100 mg in 5 mL. Calculate the volume to be given.

The formula can be abbreviated to:

Example 2

A child is to be given 175 micrograms of digoxin, orally. Paediatric mixture contains 50 micrograms per mL. Calculate the required volume.

The formula can be abbreviated to:

Exercise 5C

Calculate the volume to be given orally for these paediatric dosages. (The strength of the stock is given in brackets.)

1 70 mg of paracetamol elixir

(100 mg/5 mL)

2 300 mg of paracetamol elixir

(120 mg/5 mL)

3 12.5 mg of promethazine elixir

(5 mg/5 mL)

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Related

Related posts:

1. Dosages of oral medications
2. Dosages of medications for injection
3. Intravenous infusion
4. Answers

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