Effect of parenchyma and length changes on vessel pressure-diameter behavior in pig lungs.

At several transpulmonary pressures (Ptp), the pressure-diameter (PD) behavior of the largest intraparenchymal arterial segment in the isolated pig lung was compared with the behavior of the segment after its excision from the parenchyma and its extension to lengths equivalent to those in the intact state. For physiological changes in length, as may occur with lung inflation during Ptp changes from 4 to 25 cmH2O, excised-vessel diameters did not change significantly at a constant transmural pressure. The excised-vessel PD behavior was not significantly different from the intact-vessel PD behavior at a Ptp of 4 cmH2O. At any constant arterial pressure, intact-vessel diameters became larger as Ptp increased. Estimates of the perivascular pressure (Px) obtained by directly comparing intact-vessel and excised-vessel PD curves were as follows: 1) Px was equal to pleural pressure at a Ptp approximating the functional residual capacity; 2) Px decreased almost linearly as Ptp increased; and 3) Px decreased with a fall in arterial pressure. These results are consistent with direct measurements of the perivascular interstitial fluid pressure.