Comparison of the shear modulus of mature and immature rabbit lungs

Maximal airway narrowing during bronchoconstriction is greater in immature than in mature rabbits. At a given transpulmonary pressure (PL), the lung p arenchyma surrounding the airway resists local deformation and provides a l oad that opposes airway smooth muscle shortening. We hypothesized that the force required to produce lung parenchymal deformation, quantified by the s hear modulus, is lower in immature rabbit lungs. The shear modulus and the bulk modulus were measured in isolated mature (n = 8; 6 mo) and immature (n = 9; 3 wk) rabbit lungs at PL of 2, 4, 6, 8, and 10 cmH(2)O. The bulk modu lus increased with increasing PL for mature and immature lungs; however, th ere was no significant difference between the groups. The shear modulus was lower for the immature than the mature lungs (P < 0.025), progressively in creasing with increasing PL (P < 0.001) for both groups, and there was no d ifference between the slopes for shear modulus vs. PL for the mature and th e immature lungs. The mean value of the shear modulus for mature and immatu re rabbit lungs at PL = 6 cmH(2)O was 4.5 vs. 3.8 cmH(2)O. We conclude that the shear modulus is less in immature than mature rabbit lungs. This small maturational difference in the shear modulus probably does not account for the greater airway narrowing in the immature lung, unless its effect is co upled with a relatively thicker and more compliant airway wall in the immat ure animal.