FUNCTIONAL CAPILLARY DENSITY - AN INDICATOR OF TISSUE PERFUSION

Functional capillary density (FCD) is one of the parameters obtained b y intravital microscopy using epi-illumination of the tissue surface o r trans-illumination of thin tissue layers. FCD, defined as the length of red cell-perfused capillaries per observation area (cm(-1)), has b een used as an indicator of the quality of tissue perfusion in various animal models. Quantitative analysis of FCD in randomly selected regi ons of the tissue is performed by means of a computer-assisted video a nalysis system which allows calculation of the length of RBC-perfused capillaries. Basically, two different mathematical approaches can be e mployed: the first approach is based on the addition of the distances between two neighboring points (pixels) on the video screen (Pythagore an principle). The second approach uses the superimposition of a grid system that allows estimation of the capillary length by counting the number of intersections between the capillaries and the grid lines (st ereological approach), The immanent error has been calculated in our l aboratory to be +/- 1% with the Pythagorean and +/- 5% with the stereo logical method. Beside these systematic errors of computerized measure ment, the individual (user-dependent) errors occurring during recognit ion and redrawing of the capillaries on the video image with use of a digitizing tablet are in the range of +/- 10% (intraindividual) and +/ - 70% (interindividual) for the recognition and +/- 3% (interindividua l) for the redrawing procedure, Our studies indicate that the errors r esulting from the use of a computer-assisted calculation (Pythagorean or stereological approach) or the user-assisted redrawing of the capil laries are negligible when compared to the errors made during recognit ion of the capillaries on the video screen. The methods are applied fo r assessment of FCD in two different microcirculation models of skin m uscle and pancreas yielding highly reproducible, user-independent resu lts under physiologic conditions and the pathophysiologic conditions o f ischemia-reperfusion.