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Introduction
As the future airways of the fetal lungs are liquid-filled and gas exchange occurs across the placenta before birth, the site of gas exchange must immediately switch to the lungs after birth as soon as the umbilical cord is clamped. 1-3 To achieve this, the airways must be rapidly cleared of liquid so that the lungs can aerate. However, lung aeration is not only critical for pulmonary gas exchange as it is also responsible for initiating the cardiovascular changes at birth, which together underpin the transition to newborn life. 1 3 As such, lung aeration is the critical central event that initiates a sequence of interdependent physiological changes that enable the infant to transition to independent life after birth.
Failure to adequately clear the airways of lung liquid at birth is a major cause of neonatal morbidity and mortality, particularly in very preterm infants. 4 5 As a result, these infants usually require some form of respiratory support, which commonly involves the application of positive pressure ventilation, applied either non-invasively (via a face mask) or following intubation of the trachea. 6-8 However, there is still considerable debate as to how this respiratory support should be delivered. 8 Recent advances in our understanding of the mechanisms regulating lung aeration at birth 9 10 have provided new insights as to how this process can be facilitated in newborn infants. However, the current focus is almost entirely on gas exchange with little or no attention applied to facilitating airway liquid clearance, uniform lung aeration or preventing alveolar re-flooding during the immediate newborn period. While gas exchange is the primary consideration, these additional factors should not be overlooked as they greatly impact on the capacity of the lung to exchange respiratory gases. In this review, we will discuss recent advances in our understanding of the factors driving airway liquid clearance and propose a new approach for supporting pulmonary ventilation at birth. Specifically, we suggest that the respiratory transition occurs in three phases and that any respiratory support provided requires different approaches to be effective in each phase ( figure 1 ).
We propose that: The first phase involves airway liquid clearance and principally involves the movement of liquid through the airways and across the distal airway wall....