Ventilation and Perfusion at the Alveolar Level: Insights From Lung Intravital Microscopy

Abstract

Intravital microscopy (IVM) offers unique possibilities for the observation of biological processes and disease related mechanisms in vivo. Especially for anatomically complex and dynamic organs such as the lung and its main functional unit, the alveolus, IVM provides exclusive advantages in terms of spatial and temporal resolution. By the use of lung windows, which have advanced and improved over time, direct access to the lung surface is provided. In this review we will discuss two main topics, namely alveolar dynamics and perfusion from the perspective of IVM-based studies. Of special interest are unanswered questions regarding alveolar dynamics such as: What are physiologic alveolar dynamics? How do these dynamics change under pathologic conditions and how do those changes contribute to ventilator-induced lung injury? How can alveolar dynamics be targeted in a beneficial way? With respect to alveolar perfusion IVM has propelled our understanding of the pulmonary microcirculation and its perfusion, as well as pulmonary vasoreactivity, permeability and immunological aspects. Whereas the general mechanism behind these processes are understood, we still lack a proper understanding of the complex, multidimensional interplay between alveolar ventilation and microvascular perfusion, capillary recruitment, or vascular immune responses under physiologic and pathologic conditions. These are only part of the unanswered questions and problems, which we still have to overcome. IVM as the tool of choice might allow us to answer part of these questions within the next years or decades. As every method, IVM has advantages as well as limitations, which have to be taken into account for data analysis and interpretation, which will be addressed in this review.

Intravital Imaging of Pulmonary Immune Response in Inflammation and Infection

“…However, the effects of artificial ventilation on lung physiology must be acknowledged (Poobalasingam et al, 2017 ). Manipulations in mechanics of the thoracic wall and the interplay between intrapleural and transpulmonary pressures impact both alveolar dynamics and microvascular perfusion (Tabuchi et al, 2020 ). Therefore, although mechanical ventilation is essential for survival of the animal, it can cause permanent damage to the lung as ventilator induced lung injury (VILI) or ventilator-associated lung injury (VALI) (Amato et al, 1998 ).…”

“…However, it has been reported that certain lung windows may allow for long-term observations in dogs, rabbits (De Alva and Rainer, 1963 ), rats (Fingar et al, 1994 ) or mice (Kimura et al, 2010 ), without stabilization and at low spatial resolution. Entenberg et al recently developed a permanent lung window which may present a promising method for long-term imaging (Entenberg et al, 2018 ; Tabuchi et al, 2020 ).…”

“…Stabilizing the lung via thoracic chamber implantation is also a challenge in mice as a result of their smaller size and higher respiratory rate in comparison to larger animals (Frevert et al, 2014 ). Second, although classical fluorescence IVM provides adequate spatial resolution, its two-dimensional imaging provides little information about three-dimensional alveolar dynamics such as alveolar walls and alveolar diameter during different physiological and pathophysiological conditions (Tabuchi et al, 2020 ). Third, limited penetration depth of current microscopes allows only for superficial imaging of the lung.…”

“…Consequently, alveolar dynamics differ between subpleural alveoli and deeper lung regions. In addition, subpleural pulmonary microvasculature is less dense and consists of larger capillaries compared to the interior pulmonary microvascular network (Tabuchi et al, 2008 , 2020 ; Kuebler, 2011 ; Looney et al, 2011 ; Looney and Bhattacharya, 2014 ; Masterson et al, 2019 ). Fourth, creating a window to access the lung involves a surgical procedure which can cause baseline leukocyte activation.…”

Mechanical ventilation-induced alterations of intracellular surfactant pool and blood–gas barrier in healthy and pre-injured lungs

“…The pulmonary surfactant system plays a critical role in reducing end-expiratory surface tension at the alveolar air–liquid interface as well as in minimizing respiratory workload during inspiration (Bachofen and Schürch 2001 ). These physiological functions are essential for alveolar micromechanics, defined as the morphological changes of alveoli during the respiratory cycle (Matuszak et al 2020 ). The surface tension-lowering properties of surfactant stabilize alveolar dimensions even at low lung volumes so that the gas exchange surface area remains stable.…”