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Neonatal hypoxia: physiology

Neonatal hypoxemia is an inadequate oxygen level in the neonate to meet its metabolic demands. This may be different levels of oxygen depending on the age of the neonate. In utero, a normal partial pressure of oxygen from the umbilical artery is 20 mmhg (O2 saturation 40%) and the umbilical vein is 31 mmhg (O2 saturation 72%). With delivery and the transition from fetal circulation to neonatal circulation the oxygen saturation and PaO2 rise. This transition may take several hours. Five minutes after delivery, the PaO2 is approximately 35-40 and the oxygen saturation is in the mid 80’s. One hour after delivery the PaO2 increases to 60-65 mmhg and the oxygen saturation rises to 90-95%. It will take a few weeks for the neonatal PaO2 to approximate the adult values (90-100 mmhg).

The response to hypoxia varies on the age of the neonate. During the first 2 to 3 weeks of life, hypoxia during the neonatal period will cause a transient hyperventilation that is then followed by ventilator depression. The transient hyperventilation will be abolished if the neonate is hypothermic. Once the neonate is older than 3 weeks, hypoxemia induces a sustained hyperventilation which is the response observed in older children and adults.

There are several causes of hypoxia during the neonatal period. These include congenital heart disease, pulmonary disease (bronchopulmonary dysplasia), pulmonary hypertension, airway obstruction, and sepsis.