INTRAALVEOLAR BUBBLES AND BUBBLE-FILMS .1. FORMATION AND DEVELOPMENT DURING THE FIRST 48 HOURS OF EXTRAUTERINE LIFE IN RABBITS

Background: Aeration of mature lungs at birth depends on formation of intraalveolar bubbles and bubble films (Scarpelli 1978. Pediatr. Res., 12:1070-1076). Bubbles establish immediately structural stability and pulmonary gas exchange. Given that air spaces are cleared in minutes of fetal liquid (the initial substrate for bubble formation), is forma tion possible beyond this period? If so, is this related to early deve lopment of pulmonary function and structure? Methods: Mature, spontane ously breathing rabbit pups at 1-10 min and 1, 3, 8, 24, and 48 h afte r vaginal birth were anesthetized, trachea was occluded at ''resting v olume'' (approximate to functional residual capacity), and lungs were rapidly exposed to preserve in vivo intrapulmonary status. The entire lung was examined by stereomicroscopy. Other determinations included r esting volume, lung wet weight, volume-pressure curves, histological s ections, lung dry weight, tissue hydroxyproline (OH-Pro), and lavage p hospholipids (PL). Bubble mobility in situ was tested. Bubbles were re leased into bathing liquid by incision of peripheral units and monitor ed over time. Results: Pup activity and gross appearance of the lungs, together with septal thinning, secondary septal development, clearanc e of intraluminal Liquid, increasing tissue OH-Pro, and PL distributio n indicated normal postnatal development. Each aerated unit examined a t resting volume (all lobes, all ages) contained intraalveolar bubbles . Transition to free gas exclusively in conducting airways and bubbles /bubble films in peripheral gas exchange units occurred within 1 h. Bu bbles appeared to be exclusively within alveoli at 4 h and thereafter. Bubbles persisted and new bubbles were formed during subsequent infla tion to maximal volume and deflation to atmospheric pressure (PO). Vol ume of intact lungs at PO was maintained by the counterforce of rigid bubble films against tissue retraction. When bubbles were released eit her at resting volume or at PO, the bubble-free loci became airless. C onstant size and stability of released bubbles support preferential in corporation of surfactants into bubble films and constant ''near-zero surface tension'' (Scarpelli 1978. Pediatr. Res., 12:1070-1076). Concl usions: We show the ubiquitous presence of intraalveolar bubbles and b ubble films in vivo throughout the first 48 h of postnatal life. Bubbl e film rigidity sustains aeration and prevents collapse, while low sur face tension of the films facilitate liquid removal from the air space s. Bubbles in situ are stable and, within apparent limits, mobile; aft er birth they are quickly restricted to the alveolar spaces, leaving a irways bubble free. (C) 1996 Wiley-Liss, Inc.