Morphometric assessment of rat lungs insufflated with liquid fixative at different pressures close to total lung capacity

Liquid lung fixing through the trachea to a pressure of 25 cmH2O is currently accepted to be ideal. However, some studies do not seem to confirm that assumption. The lungs of Fischer rats were filled with fixing liquid to four different pressures: 20 cm, 25 cm, 30 cm and 35 cmH2O. The fixed lungs were processed for inspection under a light microscope for morphometric study. The following variables were recorded: lung volume, tissue volume, air volume, internal alveolar surface (IAS), alveolar chord to measure the size of the distal air space, and the number of alveoli. Statistical comparisons were performed. Lung volume increased with insufflation pressure, with significant differences related to pressure increases from 20 cm to 25 cm and from 30 cm to 35 cmH2O. Air volume did not change, although tissue volume changed when pressure increased from 20 cm to 30 cmH2O and from 30 cm to 35 cmH2O. The increase of tissue volume was related to extravasation of interstitial fixer. The number of alveoli increased with pressure from 20 to 30 cm and from 30 to 35 cmH2O. IAS increased with pressure from 20 cm and all the other pressures. Alveolar chord, which is related to size of alveoli, decreased significantly as pressure increased from 20 cm to 25 cm. A pressure of 25 cmH2O is ideal for liquid fixing of lung volumes. With lower pressures the lung is partially distended and with higher pressures the fluid can pass into the interstitial space.