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1 Center for Life Science and Technology, Keio University, Yokohama, Japan; Life and Environmental Science Division, Japan Synchrotron Radiation Research Institute, Sayo, Japan
2 Biomedical Engineering Department, The University of Michigan, Ann Arbor, Michigan, USA
3 Advanced Computing Center, Institute of Physical and Chemical Research (RIKEN), Wako, Japan
4 Integrated V-CAD System Research Program, Institute of Physical and Chemical Research (RIKEN), Wako, Japan
5 Department of Bioengineering, Imperial College of Science, Technology and Medicine, London, United Kingdom
6 Department of System Design Engineering, Keio University, Yokohama, Japan
* To whom correspondence should be addressed. E-mail: sera{at}spring8.or.jp.
Airway compliance is a key factor in understanding lung mechanics and is used as a clinical diagnostic index. Understanding such mechanics in small airways both physiologically and clinically is critical. We have determined the "morphometic change" and "localized compliance" of small airways under "near" physiological conditions, namely, the airways were embedded in parenchyma without dehydration and fixation. Previously we developed a two-step method to visualize small airways in detail by staining the lung tissue with a radiopaque solution and then visualizing the tissue with a cone-beam microfocal X-ray computed tomography system (Sera et al. 2003). In this study, we used this technique to analyze the changes in the diameter and length of the same small airways (to as small as ~ 150 µm internal diameter) and then evaluated the localized compliance as a function of airway generation (Z). For the smaller airways (D < 300 µm), the diameter was 36% larger at end tidal inspiration (TV) and 89% larger at total lung capacity (TLC), (length was 18% larger at TV and 43% at TLC) compared with the values at functional residual capacity. The diameter, especially at smaller airways, did not behave linearly with V1/3. With increasing lung pressure, the diameter changed dramatically at a particular pressure and the length changed approximately linearly during both inflation and deflation. The percentage of airway volume for the smaller airways did not behave linearly with that of lung volume. Smaller airways were generally more compliant than the larger airways with increasing Z and exhibited hysteresis in their diameter behavior. Compared with airways at lower Z, those at higher Z deformed at a lower pressure. These results indicated that the smaller airways did not behave homogenously.
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