Neonatal Respiratory Adaptation Processes
The major and first essential task of a neonate at birth is completing the initiation of respiration. If respiration is halted at birth it can lead to severe and fatal conditions to the neonate. Before proceeding to the mechanisms on how a neonate’s respiration is accomplished let us first take a look on the physiology of the fetal lung.
During the intrauterine life, fetal lung fluid is produced by the alveoli to expand it and to promote normal lung development. Some of this fluid empties into the amniotic fluid. This fluid is continuously produced at a rate of 4-6 ml/hg/hr with an overall volume of 20-30 ml/kg (the approximate amount of residual capacity after delivery). Another purpose of the fluid is to increase the pulmonary resistance to prevent a large amount of blood from the entering the lungs, since exchange of gases takes place in the placenta not the fetal lung.
Surfactant, a slippery, detergent-like lipoprotein is produced by the fetal lungs at about 22 weeks age of gestation. The main purpose of surfactant is to reduce the surface tension within the alveoli which allows the alveoli to remain partially open when the neonate begins to breathe after birth. At 34-36 weeks age of gestation adequate surfactant is already present in the fetal lungs to allow a neonate born at that time to breathe without difficulty.
Respirations at birth
As the pregnancy advances, the fetal lung fluid production decreases leaving only about 35% of the original amount. During labor the fetal lung fluid moves to the interstitial spaces around the alveoli, where it is absorbed, so that air can enter the respiratory tract. The process of absorption is escalated by the presence of fetal epinephrine secreted. Removal of fluid in the alveoli decreases pulmonary resistance that allows flow of air in the lungs.
Throughout labor, delivery and after birth the production of surfactant increases to adjust to the indifferent extrauterine life. The first breath forces the fetal lung fluid into the interstitial spaces to enhance the advent of breathing in air.
To push more fetal lung fluid on the interstitial spaces a much larger negative pressure is done by suctioning. There are several factors that contribute to the initiation of respiration in neonates. These factors are triggered by both external and internal stimuli.
Chemical Factors (internal stimuli)
Stimuli: Changes in blood chemistry (decrease O2, increase CO2, decrease pH)
At birth, a decrease oxygen level is circulating in a neonate’s body. The carbon dioxide then increases leading to an acidic pH. These events cause impulses of the chemoreceptors of carotid artery and aorta to stimulate the respiratory center in the medulla. In response to this, the diaphragm is triggered to contract causing the air to enter the lungs.
Mechanical Factor (external stimuli)
During vaginal delivery the compression of the fetal chest by the narrow birth canal forces small amount of fetal lung fluid to be pushed out of the lungs to the upper airway channels. This fluid is then suctioned in the mouth and nose of a neonate as the head emerges from the vagina. When the pressure from the chest is released the chest draws in air into the lungs.
Thermal Factor (external stimuli)
Intrauterine temperature is warm. At birth, when the neonate moves into a cooler environment, sensors of the skin transmit impulses to the brain to stimulate the respiratory center, thus, breathing is initiated.
Sensory Factor (external stimuli)
This pertains to the tactile stimulation on the neonate at birth. Holding, drying, wrapping of neonates in blankets further stimulates the skin sensors to transmit impulses to the brain to stimulate the respiratory center and breathing.
Sustaining of Respirations
Because the alveoli remain partially expanded (work of surfactant), following breaths of the neonate is already done with less effort than the firth one. As newborn cries, more fetal lung fluid is forced out into the interstitial spaces where it is absorbed by the pulmonary and lymphatic circulation.
image from commons.wikimedia.org