Maturation of interdependence between extra-alveolar arteries and lung parenchyma in piglets.

Mechanical interdependence between intrapulmonary structures and parenchyma has not been studied previously in immature postnatal lungs. To study these interactions, lung elastic moduli were measured by pressure-volume and punch indentation studies in lobes excised from 3-day-old (n = 6), 1-month-old (n = 6), and 3-month-old (n = 7) piglets. After extra-alveolar arteries were filled with a radiopaque fluid silicone compound, transpulmonary pressure and arterial pressure were varied independently as the lobar vein was occluded. Arterial diameters and lengths were measured from radiographs. Behavior of 3-month-old lungs was consistent with previous studies of adult lungs, but results were unique in 3-day-old lungs. That is, during stepwise deflation of the immature lungs 1) intravascular pressures fell when arteries were occluded experimentally, 2) arteries increased their diameters when kept at a constant intravascular pressure, and 3) arterial lengths decreased by less than 3%. Behavior of 1-month-old lungs was intermediate. A previous continuum mechanics analysis of pressure-diameter behavior was modified to account for compression by alveolar pressure as vascular diameters increase. It was concluded that 1) radial and axial dimensions of extra-alveolar arteries are virtually independent of parenchymal expansion in newborn piglet lungs and 2) periarterial interstitial pressures increase as these lungs are inflated. Our interpretation of these findings is that a mechanical association of the arteries to the parenchyma occurs gradually with postnatal maturation.

Elastic moduli of lungs during postnatal development in the piglet.

Several manifestations of lung disease during infancy suggest that mechanical interdependence can be relatively high in newborn lungs. To test this possibility, we measured elastic moduli and pleural membrane tension in lungs excised from piglets ranging in age from less than 12 h to 85 days. Near maximum inflation, newborn lungs (less than 12 h, n = 6) had no detectable pleural membrane tension, although 3- to 5-day-old lungs (n = 6) had tension greater than 5,000 dyn/cm. In contrast, parenchymal recoil was greater in the newborn lungs [19.3 +/- 3.0 (SD) vs. 14.3 +/- 2.4 cmH2O at 90% of maximum inflation volume, P less than 0.01]. Shear moduli were higher (13.5 +/- 4.6 vs. 9.2 +/- 1.5 cmH2O at 15 cmH2O transpulmonary pressure, P less than 0.05) and Poisson ratios were lower in the newborn lungs as compared with the 3- to 5-day-old lungs. Postnatal lung growth between 3 and 85 days was characterized by 1) a constant shear modulus (0.6 times transpulmonary pressure); 2) decrease in the bulk modulus (from 6.8 to 5.1 times transpulmonary pressure, P less than 0.005); and 3) evidence of gas trapping at progressively higher transpulmonary pressures. Therefore, growth of parenchyma in the piglet lung is associated with reduced stiffness to volume change but with no effect on overall stiffness to shape change. Nevertheless, a relatively great stiffness to shape change occurs transiently in newborn piglet lungs.