Toddlers, infants, and neonates are all at significant risk for intraoperative hypoxemic events. Remember that babies are not just small adults; they have important differences in both anatomy and physiology, particularly in the circulatory and respiratory systems. Compared with adults, babies have higher rates of oxygen consumption (6 mL/kg/min in babies, 3 mL/kg/min in adults), as well as lower oxygen reserves or functional residual capacity (FRC). This higher ratio of oxygen consumption to FRC is one of the primary factors affecting the increased rate of desaturation when a baby is apneic or has upper-airway obstruction. Neonates and infants also have fewer alveoli and reduced elastic recoil and compliance of the lung. Coupled with greater chest-wall compliance, these factors create an increased risk of atelectasis and intrapulmonary shunt. A patent foramen ovale (PFO) or a patent ductus arteriosus (PDA) in a newborn creates anatomic shunt, which may cause more rapid desaturation. Also, a newborn can revert to a transitional circulation during the first day or so of life. Therefore, anything that causes an increase in pulmonary vascular resistance (PVR) can result in right-to-left shunting via a PDA or PFO.

Anatomic differences, including a relatively larger tongue, pharyngeal hypotonia, and less rigid supraglottic structures, are present in the airway as well. These contribute to a greater chance of airway obstruction. Negative airway pressure results in pharyngeal obstruction from the tongue sticking to the palate and pharyngeal soft-tissue collapse.