Trapped gas and lung hysteresis.

The amount of gas trapped in excised rat lungs was determined after four inflation-deflation cycles between total lung capacity (TLC) and several end-expiratory volumes or end-expiratory pressures. Lungs ventilated in this way exhibited pressure-volume curves that formed closed loops with varying degrees of hysteresis. The area of these loops was highly correlated with the amount of gas trapped in the lungs. Trapped gas volume and hysteresis increased with deflation to increasingly lower end-expiratory volumes or pressures. The processes responsible for lung hysteresis, however, seem to be primarily dependent upon end-expiratory pressure and only slightly dependent upon end-expiratory volume. A possible explanation of these findings is that menisci, formed in the small airways of the lung during deflation at low lung volumes, are responsible for both the trapped gas and the pressure-volume hysteresis of the lung.