Anatomy, physiology, and pharmacology in paediatric anaesthesia

Anaesthesia for children is tempered by changes that occur during both growth and development. Drug dose is affected by size and clearance maturation processes as well as the changing body composition that occurs with age. All organ systems undergo these maturation changes and most are complete within the first few years of life. Normal physiological variables in infancy and childhood are quite different from adults. The central nervous, cardiovascular, and respiratory systems are particularly important. Cerebral immaturity and plasticity impacts sensitivity to drugs, pain responses, and behaviour and increases potential harm from apoptosis with anaesthesia. The heart undergoes a transition from fetal to adult circulation during the first few weeks of life. Undiagnosed congenital defects are not uncommon. The neonate is very susceptible to conditions that trigger an increase in pulmonary vascular resistance, with reversion to fetal circulatory patterns. Respiratory anatomy and mechanics affect the propensity to apnoea, airway maintenance, artificial ventilation modalities, uptake of inhalational agents, and tracheal tube sizes. Metabolic rate and oxygen requirements increase with decreasing age. This physiology influences diverse aspects that include the rate of desaturation during apnoea, hypoglycaemia during starvation, cardiac output, drug metabolism, fluid requirements, and heat production or loss.