An experimental model investigation of the reopening of a collapsed pulmonary airway
Description
Pulmonary airway closure occurs in the elderly, emphysemics, and premature neonates, resulting in impaired gas exchange. The impetus of this study was to investigate the mechanisms relevant to the reopening of a collapsed pulmonary airway, where the walls and surrounding parenchyma collapse and are held in apposition by the cohesive properties of the lining fluid. The airway can reopen by either a positive pressure that pushes a bubble of air through the collapsed region, or a negative suction pressure in the collapsed region that pulls the bubble through the collapsed region. Three benchtop models were designed to study the untethered and tethered behavior of the system. The objective of this investigation was to examine how the fluid properties (viscosity, $\mu$, and surface tension, $\gamma$) and the structural characteristics of the models (effective diameter, D, longitudinal tension, T, fluid film thickness, H, and foam stiffness, K) affected the reopening of the airway. The pressure (P$\sb{\rm AW}$) required to move the meniscus down the tube at a constant velocity (U) was measured. In the untethered model, D was the dominant structural characteristic, causing P$\sb{\rm AW}$ to increase with a decrease in D. The relevant dimensionless parameters were also investigated. The dimensionless post-startup pressure (P$\sb{\rm AW}/(\gamma$/D)) increased monotonically with the Capillary Number (Ca = $\mu$U/$\gamma$). A 'yield pressure' of 3.69 $\gamma$/D, must be exceeded to initiate reopening. From these results, the reopening times and pressures were predicted for collapsed pulmonary airways under normal and disease conditions Two tethered models were designed to evaluate the coupling between the internal forces created by the airway lining fluid and the external forces produced by the surrounding parenchyma. In these studies, open-cell foam was used to mimic parenchyma. P$\sb{\rm AW}$ still scaled with $\gamma$/D, even though D was a dependent variable. Reducing the downstream suction (P$\sb{\rm DN}$), through tethering, caused an equivalent reduction in P$\sb{\rm AW}$ required to reopen the airway. A physiological dimensionless parameter $\lambda$ was derived from the transmeniscus pressure $(\rm P\sb{TRANS} = P\sb{AW} - P\sb{\rm DN}$) that represented the ratio of fluid and tethering forces in a collapsed airway. $\lambda$ was used to investigate whether parenchymal tethering would reopen collapsed airways in various disease states