CHAPTER 8 Calculating in paediatrics
Introduction
Children are not small adults, and the delivery of safe and effective drug therapy presents challenges for all those involved in their care. Great progress has been made over the years to help to ensure that children (in the developed world) have access to the medicines best suited to their care. Team working is always the key to achieving good outcomes for patients. Nurses are central to all the processes needed to ensure effective medicines management in paediatric practice. Accurate calculation of drug doses is a prerequisite.
Key aspects
Key aspects in the safe and effective use of medicines in paediatric practice can be summarised as follows.
Response to medicines
Children and particularly neonates (up to 28 days old) respond to medicines differently from adults. Great care must be exercised in calculating doses, particularly in the neonatal period as a reduced rate of clearance of the drug or metabolites increases the risk of toxicity. Where required, the total daily dose of a medicine can be reduced by either reducing individual doses or increasing the interval between doses. For certain medicines, young children may need a higher dose per kilogram or body surface area (m2) than adults because of their higher metabolic rate. In such cases, the dose should never exceed the usual adult dose.
Renal impairment
A detailed discussion of the issues involved here is beyond the scope of this publication. The following principles should be borne in mind.
Table 8.1 Development of renal function
| Age | GFR (mL/min) approx. |
|---|---|
| < 37 weeks | <25 |
| Neonate | 15–35 |
| 1–2 weeks | 35–60 |
| 2–4 months | 60–80 |
| 6–12 months | 80–110 |
| 12 months → adult | 85–150 |
Paediatric medicines
Oral route
The oral route is the most common route of administration in children and should be used where available. It is associated with less pain and anxiety, and is more cost effective than a parenteral route. It is also more convenient and less time-consuming, it requires less equipment and the need to undertake calculations may be reduced. Many children can swallow tablets and capsules but a liquid preparation may be preferred. Prescriptions for liquid preparations should include the dose in milligrams, micrograms or nanograms and not just the volume to be given. The choice between liquid and solid oral dose will depend on product availability or personal preferences. Crushing tablets or opening capsules not specifically designed for this purpose renders their use unlicensed (NMC 2008). On some occasions, it may be necessary to use a medicine in an unorthodox way, for example, administering an injectable product orally. Further advice should be sought from the pharmacy. Some products have a high osmolality and must be diluted before administration (e.g. vitamin E suspension).
Other routes of administration
Where the oral route is contraindicated or not available, the rectal route may be used, ensuring compliance with local policies. Otherwise, parenteral therapy will be required, often by the intravenous route. Intramuscular injections should be avoided where possible as children find these distressing.
Parenteral administration is more complex and as a result presents greater risk than non-parenteral drug administration. The process involves selection of the appropriate administration device, calculating and setting the rate of infusion, and monitoring delivery. It is essential that the nurse is trained in and fully conversant with the use of the equipment.
All parenteral drug calculations should be carefully checked by a second member of staff and attention paid to the rate of administration. Where possible, it is good practice to record details of the calculation and the check. Where the contents of a vial are being reconstituted, consideration of the displacement volume is essential. If an injection requires further dilution, the appropriate diluent must be used. It is good practice to attach a label to the syringe (where a syringe driver is being used), the infusion bag or the burette chamber. Details on the label include patient’s name, ward number, name of drug additive and infusion fluid, volume, time of reconstitution, and date of expiry.
Cytotoxic therapy will normally be in accordance with national protocols. Cytotoxic preparations should always be prepared in the pharmacy. Similarly, to ensure controlled aseptic conditions, safety and accuracy, special precautions must be taken at all stages in the preparation and use of intrathecal injections.
Calculating a paediatric dose
Many medicine doses for children are determined by body weight. To determine the dose for an individual child, use the current body weight in kilograms. Care must be exercised in calculating by body weight in the obese child as higher doses than necessary may result. It is advisable in these cases to calculate from an ideal body weight related to height and age. Using aminophylline as an example:
| Normal dose | 5 mg/kg over 20 minutes |
| 5-year-old Height 108 cm | Ideal body weight 18 kg |
| 5-year-old Height 108 cm | Obese child weighs 21 kg |
| Using child’s ideal body weight | Dose 5 x 18 = 90 mg |
| Using child’s actual weight | Dose 5 x 21 = 105 mg |
It is clear that there is a significant difference between the two doses. In practice, it would be a matter for the prescriber to decide on the actual dose to prescribe.
Occasionally, doses are standardised by body surface area in square metres (m2). Paediatric doses calculated from body surface area generally reflect more accurately the patient’s needs than doses based on body weight. When a child is being treated for a chronic condition, the dose of the medicine should be reviewed at regular intervals to take account of changing age and weight.
Oral (enteral) syringes
A syringe exclusively for oral use is supplied when oral liquid medicines are prescribed in doses less than 5 mL (see also p. 60). Oral syringes are available in 1 mL, 3 mL and 5 mL, and supplied with a protective cap and adaptor, and a guidance leaflet. After shaking the medicine bottle, the bottle adaptor is inserted into the open neck of the bottle. The required dose is then drawn up into the oral syringe. The tip of the oral syringe is cautiously put into the child’s mouth and the plunger gently squeezed to run a small amount into the child’s mouth. The child is allowed to swallow before pushing the plunger again. The child is then given a drink to wash down the medicine and the syringe washed in warm soapy water, rinsed and dried. Alternatively, a graduated pipette may be used (see also p. 60).
Examples: Demonstration and practice
The demonstration examples that follow are provided as in the rest of the book so that the steps in each can be followed and understood.
The practice examples provide the opportunity to gain competence in doing calculations and thus increase confidence. Each practice example raises a question for the reader to consider. Answers are on pp. 196–197.
Whether or not a calculation is needed, care must be taken to measure the precise dose.
Level I Demonstration
Demonstration 8.3
Commentary
Level I Further exercises
Level II Demonstration
Demonstration 8.6
The medicine
Amoxicillin (Amoxil) belongs to the penicillin group. For paediatric use, it is available as a suspension (when reconstituted) containing 125 mg in 1.25 mL.
Commentary
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