Volume-pressure cycles from air and liquid-filled intact rabbit lungs

Air-filled and liquid-filled VP loops cycled over 100-300% FRC were measured using rabbit lung preparations. Four different stages of lung confinement were used ranging from intact lungs with the rib cage immobilized (Stage I) to isolated lungs (Stage IV). Stage I preparations showed larger air-filled and liquid-filled hysteresis areas of 1338 +/- 214 units2(air) and 849 +/- 73 units2(liquid), respectively, compared to 797 +/- 210 units2(air) and 128 +/- 33 units2(liquid) for excised lungs. The average peak pressure for Stage I at 300% FRC was 27.3 +/- 3.5 cm H2O compared to 21.8 +/- 4.8 cm H2O for Stage IV. The tissue contribution of the total air-filled hysteresis area was 63.5% for the Stage I lung preparation and decreased to 23.7% for the Stage IV lung preparations. These observations suggest that the tissue contribution to VP hysteresis may be greater in the intact lung preparation than previously assumed based upon excised lung studies and that geometric irreversibility may be a contributing factor.