Dynamic Imaging of Airways During Bronchoconstriction and Deep Inspiration in Intact Rats

Airways are continuously subjected to cyclic mechanical stretch during normal respiratory movements, and large distensions occur periodically with deep sighs. However, there is limited information on the levels of mechanical stretch in vivo and how these might change in hyper-responsive airways. Using a unique microfocal X-ray imaging system to visualize airways coated with tantalum dust, we imaged dynamic changes in dimensions of rat airways in vivo following exposure to methacholine during mechanical ventilation. Circumferential strain and longitudinal strain were measured based upon changes in diameter and length of segments during a breath cycle relative to dimensions at end expiration. We simultaneously performed measurements of lung resistance and compliance using the Flexivent system. We found significant changes in the diameters of airways of all sizes (~0.2 mm to 2 mm), but there was substantially more heterogeneity in smaller airways. We also found that a deep inspiration caused significant relaxation of airways circumferentially, but deep inspiration had little effect on longitudinal dimensions. These measurements allow us to quantify changes in the levels of mechanical stretch in airways during bronchoconstriction and to assess the time-dependent response to a deep inspiration