Hysteresis measurements were obtained on six anesthetized, positive pressure-ventilated, paralyzed rats. In order to assure a constant volume hystory, the respiratory system was inflated three times consecutively up to a static pressure of 20-25 cmH2O. The respiratory system was then inflated by five 1 ml steps with a precision glass syringe, and then deflated in a similar manner.

The pertinent static elastic pressures were measured, and the static inflation-deflation pressure-volume curves determined. The hysteresis surfaces (HY,rs) were measured by electronic integration and expressed in cmH2O x ml.

The thorax was open, and the measurements repeated on the lungs exposed to the athmospheric pressure with the same above reported technique (Hy,l).

Results were as follows: 

a)inflation and deflation static elastic pressures mean values were significantly different for a given volume for both respiratory system and lungs. This confirms the respiratory system and the lungs of the rat exhibit significant hysteretic properties.

b) the mean values of Hy,rs and Hy,l (4.6 ± 0.4 and 3.6 ± 0.5 cmH2O x ml respectively) were similar to those we previously reported.

c) the mean values of Hy,rs and Hy,l resulted significantly different, suggesting a significant contribution of chest wall hysteresis to the overall hysteretic properties of the respiratory system.