Transvascular fluid flux measured from intravascular water concentration changes

We have measured transvascular water flux in eight canine left lower lobes perfused in a zone 3 of West with indocyanine green-stained plasma. Transvascular flux of water was induced by a step change in the inflow pressure. Assuming the indocyanine green-labeled protein did not significantly cross the capillary membrane in one pass, the rate of transvascular fluid flux could be calculated from the change in outflow-to-inflow concentration of water. The method was validated against gravimetric measurements of water exchange. By comparing our direct measurements of transvascular flux with the time course of lobar weight change, we conclude that following changes in inflow pressure, vascular volume changes occur rapidly and slow changes in lung mass can be accounted for exclusively by water exchange. Our method shows transvascular flux follows a biphasic course, suggesting both a membrane and an interstitial resistance to fluid transudation.