Functional anatomy and volume-pressure characteristics of immature lungs.

The mechanical behavior of immature rabbit fetal lungs in situ was assessed by air and saline volume-pressure diagrams. All lungs were in their natural fetal state, i.e., filled with fetal pulmonary fluid, prior to inflation. Anatomic correlates were determined by continuous stereomicroscopic monitoring of the lungs. We found the following to be characteristic of immature lungs: (1) Tissue retractive forces are similar to adults. (2) Fetal lungs are not 'plastic' above functional residual capacity. (3) Initial aeration is by 'axial filling' in which airways are distended several times their resting size. (4) Invariably, peripheral rather than central saccules are the first to be aerated and saccules are recruited by both pressure- and time-dependent processes. (5) Pressure-dependence is related to surface forces and terminal orifice size, while time-dependent processes include orifice enlargement, liquid flow through terminal conduits, and the formation of very short-lived, labile bubbles. (6) 'Opening pressure' inflection in the VP diagram is not coincidental with, but follows the onset of saccular aeration. (7) Negative compliance at the onset of deflation is due to saccular enlargement and recruitment. (8) Hysteresis is due to tissue conformational characteristics at high pressures and air entrapment at low pressures. (9) Surface tension cannot be measured reliably from the saline and air VP diagrams.