[Analysis of pressure-volume-hysteresis in rabbit]

The pressure-volume (PV) relationships of respiratory, lung and chest wall (Prs-V, PL-V and Pcw-V) were studied in 11 white rabbits. The relaxation pressure induced by Hering-Breuer reflex was measured on inflating certain volume of air. Four groups of inflation experiments were performed in each rabbit. (1) Inflations of incremental tidal air with 10 ml in each increment were performed from functional residual capacity (FRC) to 70 ml, then inflations of decremental tidal air with 10 ml in each decrement were done from 70 ml to FRC. (2) 70 ml of air was inflated just before each incremental and decremental inflation. (3) 10 ml more air was inflated before each predicted incremental inflation. (4) The first group inflation was repeated.

The P-V curves showed a "S" shape in all 4 groups. There was an obvious hysteresis on the Prs-V and PL-V curves in the 1st group between the incremental and decremental inflations; it disappeared in the 2nd and 3rd groups. Hysteresis reappeared in the 4th group, but it was smaller than that in the 1st group. These results substantiate the theory proposed by Frazer that pulmonary hysteresis mainly comes from sequential opening and closing of lung units during inflation and deflation.