We propose that the respiratory transition at birth passes through three distinct, but overlapping phases, which reflect different physiological states of the lung. Accordingly, respiratory support given to infants should be optimised to suit the underlying physiological state of the lung as it passes through each phase. During the first phase, the airways are liquid-filled and so no pulmonary gas exchange can occur. Respiratory support should, therefore, be focused on clearing the gas exchange regions of liquid. In the absence of gas exchange, little or no CO2 will accumulate within the airways and, therefore, interrupting inflation pressures to allow the lung to deflate and exhale CO2 is unnecessary. This is the primary rationale for administering a sustained inflation at birth. During the second phase, the gas exchange regions are mostly cleared of liquid, allowing pulmonary gas exchange to commence. However, the liquid cleared from the airways resides within the tissue during this phase, which increases perialveolar interstitial tissue pressures and the risk of liquid re-entry back into the airways. As a result, respiratory support should be optimised to minimise alveolar re-flooding during expiration, which can be achieved by applying an end-expiratory pressure. The third and final phase occurs when the liquid is eventually cleared from lung tissue. Although gas exchange may be restricted by lung immaturity, injury and inflammation during this phase, considerations of how fetal lung liquid can adversely affect lung function are no longer relevant.
- Fetal Medicine
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