MECHANICAL-PROPERTIES OF SMALL AIRWAYS IN EXCISED PONY LUNGS

We evaluated the pressure-flow relationships in collaterally ventilating segments of excised pony lungs by infusing N2, He, Ne, or SF6 at known flows (V) through a catheter wedged in a peripheral airway. Measurements were made at segment- (Ps) to-airway opening (Pao) pressure differentials of 3-15 cmH2O when the lungs were held at transpulmonary pressures of 5, 10, and 15 cmH2O. The data were analyzed both by calculating collateral resistance (Ps - Pao/V) and by constructing Moody-type plots of normalized pressure drop [(Ps - Pao)/(1/2-rho-U2, where rho is density and U is velocity)] against Reynolds number to assess the pattern of flow through the segment and the change in dimension of the flow channels as Ps and Pao were changed. The interpretations from these analyses were compared with radiographic measurements of the diameters of small airways within the collaterally ventilating lung segment at similar pressures. Collateral resistance increased as Ps - Pao increased at high Reynolds numbers, i.e., high flows or dense gas (SF6). Analysis of the Moody-type plots revealed that flow was density dependent at Reynolds number >100, which frequently occurred when N2 was the inflow gas. The radiographic data revealed that small airway diameter increased as Ps - Pao increased at all lung volumes. In addition, at 5 cmH2O Pao, small-airway diameter was smaller for a given Ps in the nonhomogeneous case (Ps > Pao) than small-airway diameter for the same Ps in the homogeneous case (Ps = Pao). We interpret these data to suggest that the surrounding lung prevented the segment from expanding in the nonhomogeneous case. Taken together, these data suggest that collateral resistance measures the properties of structures that behave like small airways in pony lungs.