Role of Aeration in the Physiological Adaptation of the Lung to Air- Breathing at Birth

Abstract

At birth, the successful transition to extra-uterine life is dependent upon the ability of the lung to take over the role of respiratory gas-exchange. Before birth, the future airways of the lungs are filled with a liquid that is produced by the lung and leaves the lungs by flowing out of the trachea. At rest, adduction of the larynx restricts lung liquid efflux and promotes its retention within the airways, thereby generating a 1-2mmHg internal distending pressure on the lung. At birth the airways must be cleared of liquid to allow the entry of air, but a thin film must remain to protect the epithelium from desiccation. This creates an air/liquid interface across the large internal surface of the lung which increases lung recoil, despite the presence of surfactant. Thus, after birth, the loss of the distending influence of lung liquid, and the increase in lung recoil associated with lung aeration greatly alters the distribution of forces exerted within lung tissue. As a result, lung luminal volumes decrease and the tendency for the lung to collapse away from the chest wall increases, leading to the creation of a sub-atmospheric intra-pleural pressure. Until recently, the role of lung aeration and the resultant changes in physical forces, have been over-looked as factors regulating the normal changes in lung physiology that are required for the lung to adopt the role of respiratory gas exchange at birth. As these changes in the respiratory system are essential for the independent survival of the newborn infant after birth, it is important to understand the mechanisms involved so that these changes are not impeded in the sick, ventilated very preterm infant.At birth numerous changes in lung physiology occur which transform the respiratory system into an efficient gas-exchange system. For instance, the number of surfactant secreting type-II epithelial cells increases, chest wall compliance decreases and pulmonary blood flow (PBF) markedly increases. One of the most remarkable changes in lung physiology that occurs at birth is the large increase in PBF, resulting from a marked decrease in pulmonary vascular resistance (PVR). Before birth, most (~88%) of right ventricular output by-passes the lungs and passes through the ductus arteriosus (DA) into the systemic circulation. This is primarily attributed to the high PVR in the fetus which is also responsible for the unique PBF waveform contour, which is mainly characterized by a large retrograde, flow of blood away from the lungs during diastole; this blood exits the pulmonary circulation via the DA. At birth, the rapid decrease in PVR causes marked changes in the contour of the PBF waveform with the disappearance of the negative or retrograde component. As a result, forward flow occurs throughout the cardiac cycle. Although many factors are thought to contribute to this decrease in PVR at birth, the accumulating evidence indicates that a change in the distribution of forces within lung tissue is a major contributing factor.Other changes in the lung that occur at birth...

Antenatal Corticosteroids Increase Fetal, But Not Postnatal, Pulmonary Blood Flow in Sheep

“…These include the release of potent vasodilators such as NO and prostacyclin (30), increased oxygen tension, and the mechanical effect of ventilation (10,18). The latter likely results from changes in the distribution of force within perialveolar tissue caused by lung aeration, the formation of surface tension, and the resulting increase in lung recoil (8,32,33). Our finding, that antenatal corticosteroids increase prenatal PBF but have minimal affect on the increase in PBF at birth, indicates that antenatal corticosteroids may activate similar mechanisms as those normally activated at birth in the immature lung.…”

Fully automated predictive intelligent control of oxygenation (PRICO) in resuscitation and ventilation of preterm lambs

“…The average number of adjustments during resuscitation was similar in both groups (median 27, IQR (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) vs. 28 (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36); P = 0.796). (Figure 1b), however the oxygen need was highly variable in both the automated (IQR (0.52-0.98)) and the manual group (IQR (0.37-0.72)).…”

Cardiovascular transition at birth: a physiological sequence

“…Although lung aeration is accepted as the primary trigger for the increase in PBF at birth (Figure 1) (4), what remain less certain are the mechanisms by which aeration exerts this effect. A number of mechanisms are likely to be involved including increased oxygenation, enhanced activity of vasodilator agents, and a variety of mechanical effects associated with lung aeration (3,13).…”

“…In addition, the introduction of air into the lungs causes surface tension to form and increase lung recoil in a locally dependent manner (15,17). The increase in alveolar wall recoil increases capillary/alveolar wall transmural pressures, which increases capillary caliber in aerated regions (4). Alterations in alveolar/capillary wall transmural pressures are a well-established determinant of pulmonary capillary flow and are a major determinant of vascular resistance within the mature lung (18,19).…”

“…In particular, separating the infant from the placenta (the fetal organ of gas exchange) by clamping the umbilical cord necessitates the rapid switch to pulmonary gas exchange within minutes of birth. This switch not only involves aeration of the airways and gas-exchange regions of the lung, but also includes a major reorganization of the fetal cardiovascular system (2)(3)(4). Specifically, PVR must decrease rapidly so that PBF can increase and become the sole recipient of RVO.…”