Lung aeration at birth

During fetal life, the lung airways are filled with liquid that must be cleared at birth to allow air to enter the lungs. This initiates gas exchange and other cardiovascular changes that are vital for life after birth. Inadequate airway liquid clearance can quickly lead to respiratory failure and so assisted ventilation is frequently required to facilitate lung aeration and mitigate the respiratory failure. It is difficult to develop mechanical ventilation strategies that most effectively increase lung aeration because we do not understand the factors that influence airway liquid clearance at birth. This thesis aimed to determine the factors that regulate lung aeration and airway liquid clearance at birth and determine how these factors could be utilised in mechanical ventilation strategies to improve lung aeration in an immature lung. The experiments used a combination of traditional and new analytical techniques, including plethysmography and phase contrast (PC) X-ray imaging, to image and measure lung aeration at birth, thereby providing a unique perspective on this process. The first aim of this thesis was to determine the contribution of respiratory activity and expiratory braking manoeuvres (EBMs) to the formation and maintenance of a functional residual capacity (FRC) at birth in spontaneously breathing near-term rabbit pups (Chapter 3). Newborn rabbit pups develop most (~95%) of their FRC in association with inspiration and the FRC rarely increases in the absence of breathing. This is emphasised in PC X-ray images that clearly show lung aeration increasing after individual breaths. These results highlighted the importance of inspiration for lung aeration and suggest that inspiration is a prominent airway liquid clearance mechanism at the time of birth. Newborn rabbit pups were found to utilise EBMs during the immediate newborn period, but these were associated with both increases and decreases in FRC, although overall, they resulted in net gain of FRC per pup. EBMs were not common when FRC was low and were most frequently observed after ≥80% of the FRC had accumulated. In this experiment, I propose that FRC is developed by inspiration whereas FRC is maintained by EBMs during the early neonatal period. Findings from the first aim of this thesis implied that inspiration and its generation of a trans-pulmonary hydrostatic pressure gradient promotes rapid airway liquid clearance at birth. This challenges the currently supported hypothesis for airway liquid removal at birth; that adrenaline activates epithelial sodium channels (ENaCs) to generate an osmotic gradient in favour of airway liquid clearance. In light of this, the second aim of this thesis (Chapter 4) investigated the relative role of trans-pulmonary hydrostatic pressure gradients and activation of ENaCs in airway liquid clearance at birth. My findings demonstrated that FRC was similar in mechanically ventilated near-term rabbit pups with and without ENaC inhibition. ENaC inhibition, however, did increase the rate of lung gas volume loss in between inflations, indicating that the rate of liquid reflux may have increased. These findings further support the role of trans-pulmonary hydrostatic pressure gradients as a primary mechanism for rapid airway liquid clearance at birth. ENaCs, may help to maintain FRC in between inflations when the lung is at rest and trans-pulmonary hydrostatic pressure gradients are low. My findings, that trans-pulmonary hydrostatic pressure gradients significantly influence airway liquid clearance and lung aeration at birth, led me to apply these principles to facilitate lung aeration in mechanically ventilated premature pups. Specifically, my third aim (Chapter 5) investigated the role of positive end-expiratory pressure (PEEP) on the formation and maintenance of an FRC and the spatial and temporal pattern of lung ventilation in the preterm newborn rabbit pup ventilated from birth. Pups ventilated with 5 cmH2O of PEEP easily developed and maintained an FRC throughout the respiratory cycle well above the anatomical dead space (ADS) volume. The PC X-ray images clearly demonstrate the effect of PEEP on lung aeration, which followed a very similar temporal pattern as many of the near-term spontaneously breathing pups. In contrast, pups ventilated without PEEP were unable to aerate their lungs appropriately as the airways either collapsed and/or re-filled with liquid at the end of expiration. Therefore, gas filled the distal air sacs for less than half of the respiratory cycle. This form of ventilation would certainly impair gas exchange and cause atelectrauma leading to further lung injury in the newborn. My third study clearly demonstrated that PEEP increased FRC from birth but PC X-ray images indicate that PEEP did not promote uniform distribution of the gas. Non-uniform ventilation may result from high surface tension caused by the lack of surfactant within the immature lung. Therefore, my fourth aim (Chapter 6) was to investigate the role of surfactant with and without 3 cmH2O PEEP on the formation and maintenance of an FRC and the spatial and temporal pattern of lung aeration in the preterm newborn rabbit pup ventilated from birth. This study demonstrated that ventilation with 3 cmH2O of PEEP caused an ~6-fold increase in FRC whereas ventilation with surfactant only caused an ~2-fold increase in FRC. Although surfactant had a limited ability to increase FRC, from the onset of ventilation, surfactant increased respiratory system compliance (CRS) and markedly improved the uniformity of gas distribution within the lung ventilation at both FRC and at peak inflation. PEEP also increased CRS and promoted uniform lung ventilation however, this was secondary to the increase in lung gas volume with ventilation. When immature lungs were administered surfactant and mechanically ventilated with PEEP, large FRCs were formed and CRS was high, and demonstrated further improvement with increasing inflation number. In addition, at ventilation onset, aeration was so uniformly distributed at peak inflation and at FRC that there was little capacity for further improvement. Overall, this study demonstrated the different roles of PEEP and surfactant in lung aeration; PEEP increases lung gas volumes whereas surfactant promotes the even distribution of gas throughout the lung. When these techniques are used in combination, the effect is additive. In conclusion, the studies contained in this thesis increase our understanding of the factors that promote lung aeration at birth. Trans-pulmonary hydrostatic pressure gradients rapidly increased lung aeration at birth, independent of ENaC mechanisms of airway liquid clearance. In immature lungs, PEEP was highly effective at increasing FRC whereas surfactant promoted uniform lung aeration. Therefore, trans-pulmonary hydrostatic pressures increase airway liquid clearance and lung aeration whereas surface tension strongly influences the distribution of gas throughout the lung. The knowledge of how various factors influence lung aeration will benefit the future design of ventilation strategies that best aerate the lung at birth.