Morphometry of subpleural alveoli may be greatly biased by local pressure changes induced by the microscopic device

Microscopy of subpleural alveoli has become an important technique to analyze alveolar morphology during mechanical ventilation. Mere contact of a microscope with the lung, however, may alter the local pressure at the pleural surface and thus the transmural pressure of the alveoli under view. The effect of local pleural pressure changes on alveolar morphology during microscopy has not been systematically evaluated hitherto. We developed a new microscopic device enabling control of the pressure directly at the field of view. In 6 isolated rat lungs we systematically varied the transmural pressure of subpleural alveoli by varying both the local pleural pressure and the alveolar pressure. Results show fixation pressure, alveolar pressure and local pleural pressure significantly influenced alveolar size and the number of alveoli per field of view. Our study demonstrates the important impact of local pleural pressure on the morphology of subpleural alveoli. We conclude that local pressures need to be determined during microscopy of subpleural alveoli to avoid misinterpretation of changes in alveolar geometry.