CLINICAL PHARMACOKINETICS AND PHARMACODYNAMICS OF OPIOID ANALGESICS IN INFANTS AND CHILDREN

Pain in childhood has not always been managed as actively as that in a dults because of the limited amount of research available to provide g uidelines for the management of paediatric pain. However, for many yea rs now the pharmacokinetics and pharmacodynamics of opioid analgesics in infants and children have been studied intensively. Morphine is the standard for opioid analgesics and its pharmacology is the best studi ed in paediatric patients. During the neonatal period, the volume of d istribution (Vd) appears to be smaller in neonates than in adults, but adult values are reached soon after the neonatal period. Although mor phine is absorbed both orally and rectally, there is little informatio n on the pharmacokinetics of morphine administered by these routes. Th e bioavailability of morphine after rectal administration appears to b e highly variable. For all the opioid analgesics studied, the eliminat ion of the opioids is slower in neonates than in adults. However, the rate of elimination usually reaches and even exceeds adult values with in the first year of life. The high rate of drug metabolism means high er dosage requirements. In regard to the pharmacodynamics of opioid an algesics, infants and children do not appear to be more sensitive to t he effects of opioids than adults. Thus, except for the neonatal perio d, the pharmacokinetics and pharmacodynamics of opioid analgesics are not markedly different from those of adults, and the risk of using opi oids in infants and children is not higher.