Clinical Pharmacology and Therapeutics. Группа авторов
Prescribing for the young and the elderly
Prescribing for the young
A 4‐year‐old girl presents with her mother to the emergency department with shortness of breath, wheeze and a temperature. Her past medical history includes premature birth at 34 weeks gestation and asthma. She is prescribed salbutamol inhaler as required at home, which she has been using more frequently in the last 2 days. On examination, she is breathless at rest, is pyrexial, tachycardic and has bilateral rhonchi. She weighs 15 kg. A diagnosis of moderate acute exacerbation of asthma is made. You are asked to prescribe an anti‐pyretic medication, salbutamol and prednisolone.
Introduction
Prescribing in children (neonate to 16 years old) should always be undertaken with care. There are significant differences between children and adults in terms of pharmacokinetics, pharmacodynamics and practical aspects of drug treatment. Children should not be regarded simply as small adults as dosing requires calculation based on weight, age, surface area and renal function. Prescribing for children comes with significant risks of errors and potential harm and therefore should not be carried out if inexperienced without senior medical or specialist pharmacy support.
Pharmacokinetics
A variety of factors influence pharmacokinetics in the young. Drug absorption may be affected by relatively high gastric pH in infants, and topical preparations are better absorbed because the skin is relatively thin. Low body fat content may reduce the volume of distribution of lipophilic drugs, and lower plasma albumin concentration reduces plasma protein binding. Immaturity of neonatal hepatic enzyme systems and renal excretion mechanisms may impair drug elimination, necessitating dosage reduction in the first months of life.
Pharmacodynamics
The differential effects of some drugs in children compared with adults are well recognised, an example being the sedative effect of stimulant amphetamine derivatives in children, a paradoxical effect used in the management of ADHD. Although effects such as this are recognised, the mechanisms are poorly understood.
Practical aspects of prescribing in children
Medicine has no benefit if the child refuses to take prescribed substances due to unpleasant taste, bulky formulation or lack of understanding of the necessity to take the prescription. Many children's medicines contain excipients, aiming to make them more palatable. However, less used medicines may not. Discussion with parents regarding reward style encouragement or offering advice as to food/drinks which the medicine can be mixed with to improve the taste may help.
Physical and developmental stages should also be considered. For example, the management of common childhood complaints like asthma can be complicated by the need to acquire an effective inhaler technique, which younger children find difficult.
For children, social aspects of taking medicine are also important. Being ‘different’ from classmates or friends may impact on adherence to medication, particularly prevalent in children with diabetes requiring insulin. Taking time to explore the child's concerns may prevent potentially life‐threatening complications.
Safety
Safety is paramount when prescribing for children as overdosing and adverse effects can be serious. Abbreviations such as mcg, mg, ng and mL should be avoided and the full instruction written (e.g. micrograms, nanograms or milligrams). This helps to avoid errors, as doses are as likely to be ng as they are mg depending on the weight of the child and can be easily misread. One vial of drug may contain enough active product to overdose a child by 10 or 100 fold. For example, a vial of morphine sulphate contains 10 mg (10 mL) of substance. In a neonate, the recommended dose is 5–20 μg/kg/h, which in a normal 3 kg baby would only equate to a 15–60 μg (0.15–0.6 mL) dose. Therefore, one vial contains enough solution to significantly overdose the child.
Failure to calculate maximum daily allowances of drugs in children is another common reason for errors. After calculating the individual dose for a child, the daily dose should be correlated against the maximum daily allowance to ensure it does not exceed the allowance for the child's age and weight.
Development and regulation of medicines for children
Young children lack capacity to provide informed consent making trials of medicines in this age group ethically not possible. The majority of medicines available to children are unlicenced or their use is ‘off label’ (i.e. used for a purpose not indicated by the manufacturing label). Most paediatric units have developed their own guidelines and monographs for dosing, taking into account the factors discussed above. Valuable resources such as the BNF for Children are validated against available evidence, and are also helpful in the safe and appropriate management of diseases in childhood.
Prescribing for the elderly
A previously well 78‐year‐old woman is admitted with confusion and urinary frequency.