Neonate exhales incompletely
You will recognize the breathing pattern of a neonate you saw earlier. At certain moments in time the end-expiratory level is higher than average. Also, now and then ventilation just seems to cease; during the expiratory pause the lung deflates to a low volume. In the latter case the neonate's muscles are quite relaxed. These phenomena give away that in the neonate end-expiratory lung volume is normally grater than that obtained when the muscles are relaxed, and thus different from the volume at elastic equilibrium. The elevated end-expiratory lung volume (the FRC) is achieved by 2 mechanisms:
- The neural stimulus to commence inspiration occurs before the (normally passive) expiration has ended.
- The muscles of the glottis contract during expiration, increasing resistance to air flow, so that lung emptying is delayed.
These are very useful mechanisms, because at higher end-expiratory lung volume there will be fewer lung compartments that do not contribute much to gas exchange due to occluded airways during (part of the) expiration.
In newborns hypoxemia due to breathing at a low lung volume at which airway closure occurs, is minimized by:
- End-expiratory breaking of the expiration by expiratory narrowing of the glottis
- Commencing inspiration before passive expiration has completed.