Regulation of breathing at birth.

The factors which regulate the transition to lung gas exchange in the newborn are not well understood. The transition begins within seconds of birth with the newborn's first breath and is largely complete by 30 min of age at which time breathing is continuous, and arterial blood gas tensions and pH approach stable newborn values. Experiments indicate that sensory stimulation caused by cutaneous cooling or sciatic nerve stimulation can result in the initiation of breathing within seconds. Thus, massive sensory stimulation of the newborn caused by labour and delivery probably plays an important role in promoting the rapid onset of lung ventilation. Any delay in the onset of lung gas exchange causes a rise in arterial PCO2 and fall in pH which would stimulate breathing probably via stimulation of the central chemoreceptors. Since an impairment of CO2 elimination is usually observed after birth, a rise in arterial PCO2 likely stimulates breathing in the newborn. However, this impairment is transient and is usually corrected within 30 min to 2 h of age. Recent experiments suggest that placental perfusion inhibits the fetal central respiratory system and that this effect may be mediated by a placentally-produced respiratory inhibitor. Thus, withdrawal of a respiratory inhibitor from the circulation may play an important role in maintaining breathing in the newborn after sensory stimulation wanes and arterial PCO2 returns to normal fetal levels.