Alveolar resistance to atelectasis

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Abstract
The mechanical behavior of the terminal respiratory air spaces at very low lung volumes was studied. Pressure-volume (PV) diagrams of the lungs of living rabbits, cats, and dogs were recorded on an X-Y recorder during progressive reduction of volume (measured plethysmo-graphically) occurring a) in oxygen-filled lungs during absorption of gas following clamping of the trachea and b) in air-filled lungs when the air was withdrawn from the trachea with a syringe. Comparison of the PV curves obtained by procedures a and b revealed that all distensible air spaces remained in continuity with the trachea until a transpulmonary pressure of about -2 cm H2O was reached. Some airways must have stayed open as volume became progressively reduced down to and including complete alveolar collapse (method a), at which time trans-pulmonary pressures of about -2 to -6 cm H2O were reached in open-chest animals, or -12 cm H2O (esophageal pressure corrected to give intrapleural pressure) in closed-chest animals. The finding oa a negative transpulmonary pressure while all distensible air spaces are in communication with the trachea means that the combined forces of elastic recoil and surface tension do not completely empty the lungs under these circumstances. Instead, it appears that the alveoli as well as the airways have elastic resistance to collapse. Despite this, reexpansion from the collapsed state required positive transpulmonary pressure, indicating that the spontaneous outward elastic force is insufficient to reopen alveoli or airways once they have been collapsed.