Children are not just "little adults." They have physiologic differences which change how anesthesia is delivered and how they tolerate it. First, a few definitions: Newborn is defined as the first 24 hours after birth, neonate is the first 30 days after birth, infant is 1 to 12 months of age, and child is 1 year of age to puberty.
Children use more oxygen than adults, nearly double in the case of neonates. Neonates also have about a 30-60% increase in cardiac output to meet their increased oxygen requirements, and both increased cardiac output and high hemoglobin level (about 17 g/dl) are needed to compensate for the diminished release of oxygen by fetal hemoglobin to tissues. Because of limited distensibility of the left ventricle, cardiac output in neonates is heart rate dependent. The oxyhemoglobin curve begins to approximate that of adults by four to six months of age.
Other physiological differences in neonates and infants include increased alveolar ventilation, increased proportion of extracellular fluid volume to body weight, more difficulty maintaining body temperature, and decreased ability to concentrate urine. Neonates and infants aren't able to compensate for extreme shifts in fluid balance as well as adults.
These physiological differences, along with differences in skeletal muscle mass, metabolic rate, and receptor maturity affect how neonates, infants, and children respond to anesthetics and other drugs
