Alveolar Dynamics in Live Intact Mice: Real-time In-vivo X-ray Imaging

Abstract

Lungs of mammals contain millions of small gas exchange units called alveoli. Proper control of alveolar volume for lung function is important in pulmonary medicine, in particular, in treating patients with acute lung injuries by mechanical ventilation. Improper use of mechanical ventilation may cause ventilator-induced lung injury. Understanding of alveolar dynamics, i.e. the dynamic changes in alveolar size and shape, is required for optimum ventilation management, but is largely lacking because of a difficulty in the visualization of real-time, in-vivo, alveolar dynamics in live intact mice. Real-time imaging of alveoli is essential to identify alveolar dynamics during ventilation.In this thesis, a tracking X-ray microscopy (TrXM) is first developed as a novel methodology that allows alveolar imaging in live intact mice during respiration. By enabling tracking of individual alveolar movements, TrXM identifies alveolar dynamics in live intact mice. The tested region for alveolar dynamics using TrXM is upper lung apices in live intact mice. Alveolar sacs and ducts are visualized in two (2-D) and three dimensions (3-D) during respiration. Alveolar dynamics in 2-D is identified: individual alveoli in the upper lung apices show a small size increment as 4.9±0.4% (mean±s.e.m.) with little change in their shapes look almost invariant. Each alveolus shows a closed-loop hysteresis between inspiration and expiration. In alveolar dynamics in 3-D, furthermore, two alveolar shapes are revealed: hyper-hemisphere in sacs and hypo-hemisphere in ducts, while showing little change in the overall shape of each alveolus during respiration.Second, regional alveolar dynamics in live intact mice is studied by improving the tracking X-ray microscopy (TrXM), specifically by increasing the field of view (FOV) to 1mm x 1mm and by enhancing time/spatial resolutions. The tested regions for individual alveolar dynamics are not only the upper lung apices, but also the lower lung bases that show significant lung displacements. By tracking individual alveolar movements, the TrXM identifies regional alveolar dynamics in 2-D: the size increment of individual alveoli in the lower lung bases is slightly larger as 8.7±1.2% (mean±s.e.m.) than that in the upper lung apices, 5.7±0.6% (mean±s.e.m.), during respiration. Remarkably, the lung displacement shows region-dependent behavior, specifically, an increasing tendency from 86±18 μm in the apices of right upper lungs, to 213±43 μm in apices of the left upper lungs, to 423±67 μm in the bases of the right lower lungs, and to 513±76 μm in the bases of the left lower lungs. Comparing to the large lung displacements (86±18 μm to 513±76 μm), the alveolar expansions are very small as 3 μm to 6 μm with a slightly increasing tendency in the lower lung bases but with little difference between the right and the left regions. The volume increment of each alveolus, estimated based on the average alveolar size increment (6.7±0.4% (mean±s.e.m.)), is low as 22%, implying that the small increment might be sufficient for lung function and stability. The TrXM for regional alveolar dynamics of respiratory lung would be a significant gateway for better understanding of alveolar-based diseases.Last, alveolar dynamics in lipopolysaccharide(LPS)-induced acute lung injured (ALI) mice is studied using the improved tracking X-ray microscopy (TrXM). For the LPS-induced ALI lungs, the real-time, in-vivo, measurements based on tracking individual alveoli show that the average alveolar size increment, which is the measure of alveolar instability, is increased to 8.8±0.7% (mean±s.e.m.), compared to that for normal lungs, 6.7±0.4% (mean±s.e.m.). In particular, the size increment is significantly increased to 13.8±1.6% (mean±s.e.m.) in the bases of the left lower regions, comparing to that for normal lungs, 8.7±1.2% (mean±s.e.m.). Alveolar dynamics under positive end-expiratory pressure (PEEP) reveals that the size increment is significantly reduced to 2.5±0.2% (mean±s.e.m.) by PEEP (6 cmH2O), implying an improved alveolar stability in the LPS-induced ALI lungs. TrXM analysis in alveolar dynamics would be a significant methodology for better understanding of alveolar-based diseases, for instance, ventilator induced lung injury (VILI) in acute respiratory distress syndrome (ARDS).