Do transvascular forces in isolated lobe preparations equilibrate?

We used the stepwise pressure elevation technique to study the relationship between rate of constant weight gain (Qf) and microvascular pressure (Pc) in eight isolated canine left lower lobes. The slope of this relationship, which is assumed to represent lobar conductance to filtration (Kf) was 0.0022 +/- 0.003 ml.min-1.cmH2O-1.g dry wt-1. The intercept when Qf = 0, Pcrit, commonly interpreted as the Pc at which the balance of forces across the microvasculature is overwhelmed, was 9.53 +/- 1.18 cmH2O, a lower Pc than commonly used in weight transient experiments. Consequently, at Pc greater than 9.53 cmH2O, isogravimetric conditions were never achieved. In 12 additional experiments, the perfusate's colloid osmotic pressure (II) was increased from 12 to 37 mmHg with albumin. On average, Pcrit increased from 12.2 to 23 cmH2O. Using the changes in Pcrit and II, we estimated the microvascular drag reflection coefficient for albumin (sigma d) to be 0.67. After the addition of albumin, Pc less than Pcrit induced constant weight loss along the same Qf-to-Pc relationship. To control for time, five additional lobes were observed at constant Pc for 100-180 min. Slight acceleration of the rate of weight gain occurred after they increased their weight by 30-40%. The low Kf and high sigma d, as well as the stability of the preparation, suggest a well-preserved microvasculature. Qf at Pc between 12 and 24 cmH2O did not influence measurements of Kf using the weight transient method. The low Pcrit may reflect obliteration of lymphatic channels.