The first inflation and deflation with air of excised lungs of mature fetal lambs and rabbits from the natural liquid-filled state invariably produced foam which was apparent in the trachea when transpulmonary pressure was <0 (lambs) and in terminal lung units (TLU) when monitored by stereomicroscopy (rabbits). Similarly, examination of neonatal lungs of lambs and rabbits that had breathed spontaneously (1 breath to 15 min) revealed bubbles in small airways and in TLU. Volume-pressure diagrams of excised lungs (lambs) revealed relatively low apparent opening pressures, low pressures at maximal volume, and appearance of foam in large airways after withdrawal of a volume of air equivalent to 30% or more of functional residual capacity (FRC) during deflation. The latter is consonant with the presence of bubbles in TLU. Studies of excised lungs indicated that additional foam may be produced during the second inflation. The extent to which subsequent breaths may contribute to foam production could not be determined, but it was apparent that the early neonatal lung (up to 15 min in our studies) is a three-phase system of intrapulmonary foam, free air, and liquid. The phospholipids of fetal pulmonary fluid (FPF) from which foam is produced were incorporated preferentially (in relation to the proteins) into the bubble films.Speculation
Intrapulmonary foam at birth could play an important role in a number of vital adaptations that are known to occur. 1) Bubbles apposing the walls of TLU are air pockets within thin films across which gas exchange would be facilitated beginning with the first breath. 2) Intrapulmonary foam may play a role in maintenance of dilated airways during expiration and at end-expiration when transpulmonary pressures are high. 3) Oxygenation and distention of TLU by foam could be the earliest stimuli for rapid fall of pulmonary vascular resistance. 4) The phospholipids of bubble films could be immediate precursors of the alveolar lining layer that needs to be established quickly at birth. 5) The large thoracic gas volume to FRC ratio of neonatal lungs may be due, in part at least, to intrapulmonary foam. Whereas foam volume and foam production were not quantified in our studies, it appears that the duration of the “foam lung” state and the relative amounts of intrapulmonary foam and free air would depend on FPF volume at the time air breathing is begun and on the rate at which FPF is subsequently absorbed.