Paediatric anaesthesia: pharmacological considerations.

The young infant differs from the adult in his quantitative responses to many anaesthetic drugs and adjuncts. In the neonate, the larger extracellular fluid volume and blood volume, the smaller muscle mass and fat stores, and presumable greater blood flow to the central organs, not only influence the distribution of drugs to their active site but also secondary redistribution. The neonatal hepatic anzyme systems responsible for the metabolism of drugs are incompletely developed or absent. Glomerular filtration, important for drug excretion, is inefficient by adult standards. The neonate has increased toxicity and sensitivity to a variety of sedative-hypnotics, narcotics, and local anaesthetics. On the other hand, the infant requires more suxamethonium (succinylcholine) and ketamine on a weight basis that does the adult. The response of some infants to non-depolarising muscle relaxants resembles that of the myasthenic patients. The rate of uptake of alveolar levels of inhalation anesthetics is more rapid in infants and children than in adults. In addition, the neonate requires more anaesthetic than the adult for a given surgical stimulus. Biotransformation of inhalation anaesthetics is limited in neonates. Awareness of these pharmacological differences and their probable explanations allows one to provide rational, safer anaesthesia to infants.