Time-dependency and static mechanics of immature airways and saccules

Immature rabbit lungs were inflated, then deflated from the fetal pulmonary fluid (FPF)-filled state. Stereomicroscopic observation and measurement of volume change (delta V) during each pressure step and after 15 and 120 sec at each pressure revealed the following: (1) Only airways inflate from atmospheric pressure (P0) to P25. Significant time-dependency here is due to FPF flow through the narrowest airways, airways dilation and recruitment as functions of tissue and surface forces, and, perhaps, interfacial adsorption of surfactants. (2) Saccular recruitment and distention are the principal transformations from P30 to P35. Time-dependency here is the result of FPF flow and labile bubble production. (3) Time-dependency during deflation from P25 to P10 is due to diminishing influence of inflation processes and to decreasing radii of curvature at air/liquid interfaces as FPF refills the saccular air-spaces. Redistribution of air and hypophase liquid probably also play a role. (4) Deflation from P10 to P0 is determined by FPF flow through the smallest airways, interfacial forces, and recoil of previously distended airways as liquid locks are formed. Some implications are that FPF flow through the smallest airways is a gate to saccular ventilation; time-dependent processes place airspaces at risk to rupture; and different time constants of saccules and airways renders 120 sec pressure steps adequate for evaluation of the latter but not the former.