Physiological interpretation of the skewness of indicator-dilution curves; theoretical considerations and a practical application.

Indicator-dilution curves can be interpreted and analysed by describing the system between injection- and sampling site with a physical model. Till now mainly compartmental and distributed models have been investigated. One feature of distributed models is the possibility to interpret skewness or asymmetry of the curve in terms of a parameter, proportional with the Peclet number, which is a measure of the relative contribution between convection and diffusion in indicator transport. In patients with and without pulmonary edema, we analyzed a number of curves obtained with an intravascular indicator (131I radioiodinated serum albumin, RISA) and a diffusing indicator (tritiated water, THO) over the pulmonary vascular bed. Edema was measured by indexed extravascular lung water and by critical pressure, defined as the difference between pulmonary capillary wedge pressure and plasma colloid osmotic pressure. The significant decrease of the symmetry of the RISA curves with increasing cardiac output we explained by an increasing labyrinth dispersion and increasing turbulence at higher flows. For normals we found all THO curves to be less skew than albumin curves. This difference diminished and even reversed when the degree of pulmonary edema increased. We suggest a hypothesis for this phenomenon by considering various mechanisms responsible for dispersion and capillary exchange of the indicator during transport from injection to sampling site. In normals the contribution of Taylor diffusion during laminar flow in parts of the circulatory system may be responsible for the greater symmetry of THO curves; with increasing pulmonary edema, transcapillary diffusion of THO causes THO curves to become more skew and even more asymmetric when compared with albumin curves.