The Pharmacology of the Older Child

Abstract

The age-group which will now be discussed is a wide one — from the age of 2 months until adolescence. The choice is arbitrary, and is based upon the consideration that babies under the age of two months have been included in the perinatal section (Chapter 4), and that adolescents will still in some way differ from adults. There is a continuum of growth between these ages and, within limits, a similar continuum of ability to metabolise and excrete drugs. Generally speaking, the infant who is six months old has an adult’s basic ability to transform and excrete drugs. However, these mechanisms during the first year of life are much more readily saturated than in the adult. Therefore the child in general has an adult’s tolerance to most drugs. Casual observation tells us that children vary in size with age, but that size may also vary within the same age-group. Weight variations, for example, will assume a normal frequency distribution. This is shown in Figure 5.1, which is a percentile table. This illustrates the limits of normal weight distribution, which may be important in prescribing a drug dose. Table 5.1 shows that two normal children of the same age can vary greatly in weight. Thus, the one in the 97th percentile weighs 26 kg, the other in the third percentile weighs 16 kg, and each is regarded as a normal healthy child. These differences explain why it is impossible accurately to prescribe drugs on an age basis. The result could be under-treatment for the heavy child who is in the 97th percentile, and too high a dose for his lighter peer in the 3rd percentile. Thus, we tend to prescribe for children in terms of their body weight, or by surface area, which is generally derived from weight.