Adaptation of vascular pressure-flow-volume hysteresis in isolated rabbit lungs

Hysteresis within two pairs of variables describing the state of the lung vascular system [pulmonary arterial pressure (Ppa) and flow (Q) and Ppa and change in vascular volume (delta Vvasc)] was investigated in isolated plasma-perfused rabbit lungs. Q was increased and decreased stepwise, in series of five cycles each, while pulmonary venous pressure (Ppv) and lung volume were held constant. Changes in Vvasc were estimated from changes in fluid volume of the venous reservoir. The relationships within pairs of variables over each complete cycle were described by loops whose areas and widths were used to quantify the hysteresis. In successive cycles, these parameters decreased toward constant values (limit cycles), most of the change occurring by the second cycle. Areas of Ppa-delta Vvasc loops correlated closely with areas of Ppa-Q loops over all five cycles of a series. For Ppa-Q loops, the ratio of average pressure-width to total pressure excursion decreased from 0.15 initially to around 0.05 in the fifth cycle. It was concluded that the relationships between Ppa and Q and Ppa and delta Vvasc are markedly sensitive to vascular pressure or flow history.