Pediatric age groups are divided into neonates (birth to 1 month), infants (1 month to 1 year), toddlers (1 to 3 years), school-aged children (3 to 12 years), and adolescents (12 to 18 years). Significant developmental and physiologic changes occur across these age groups; Table 106-1 summarizes the developmental milestones as they relate to the ED evaluation and approach, and Table 106-2 lists the age-dependent vital signs.
Neonates undergo the most profound changes as they transition from metabolic and respiratory dependence on the placenta to independence as air-breathing beings. The cardiovascular and respiratory systems switch from near complete shunting of blood flow away from the lungs to typical adult circuitry and dependence on the lungs for oxygenation as the ductus arteriosus closes (see chapter 126, "Congenital and Acquired Pediatric Heart Disease"). Oxygen-avid fetal hemoglobin changes to adult hemoglobin with predictable changes in hemoglobin levels throughout the first years of life. The neonatal and infant immune systems depend on passive maternal humoral protection transferred through the placenta and breast milk until cellular and humoral defenses mature. Immunologic immaturity predisposes to bacterial and viral systemic infections early in life. The neurologic system is characterized by rapid growth, differentiation, and myelination and changes in the balance of excitatory and inhibitory neurotransmitters, which account for susceptibility to seizures.
Anatomically, growth and development of every organ system characterizes infancy and childhood and affects emergency care across the life span. A relatively large occiput, small jaw, high and anterior larynx, narrow cricoid cartilage, and large tongue require unique considerations in airway management (see chapter 111, Intubation and Ventilation in Infants and Children). A soft, compliant chest wall, obligate nose breathing, and gastric inflation from swallowed air alter the mechanics, symptoms, and signs of respiratory distress in young children. Neonates, infants, and children increase cardiac output through an increase in heart rate rather than stroke volume. Tachypnea, an increased rate of breathing, not hyperpnea, an increased depth of respiration, is the primary respiratory compensatory mechanism. The musculoskeletal system of young children differs from that of older children and adults not only in its proportions (e.g., relatively large head), but also because ligaments are stronger than bones, predisposing to fractures rather than sprains. Linear growth of long bones occurs from specialized cartilaginous end plates (physes), which results in unique fracture patterns (see chapter 140, "Musculoskeletal Disorders in Children").