Evaluation of an on-line videodensitometric measurement of the red blood cell velocity in the capillaries of the human nailfold

The volume flow rate of blood in the microscopic vessels is unquestionably a significant parameter of microvascular function. Besides this, at known pressure conditions, it reflects the rheological properties cf the blood. Yet the quantification of this flow rate has so far been handicapped by the fact that it generally requires invasive approaches. \Vith the aid of television-microscopy it has however become possible to determine non-invasively the speed of red blood cells and plasma spaces in certain capillaries of man. This was demonstrated by Bollinger et al. [l] who for the first time succeecled in measuring the speed of the erythrocytes äs they passed through the capillaries of the nailfold of various patients. The method used to this end was a laborious frame-by-frame analysis of a video tape recording of the microscopic image of the capillary bed. By adapting video signal processing techniques to the so-called double slit method [5] frequently used to assess the flow in capillaries of animals Intaglietta et al. [3,4] recently prepared the way for on-line measurements of the speed of erythrocytes in certain microscopic vessels. Thanks to the development of a dual channel videophotometric analyzer) (videodensitometer) and a velocity-tracking correlator), the speed determination can now be accomplished online for one capillary. For other capillaries within the field of view of the microscope the speed of RBC may be evaluated off-line from the video tape recordings. The processing of the video data during playback is carried out in the same manner and at the same rate äs during the on-line measurements. We have made use of such a densitometer-correlator combination in a preliminary series of clinical investigations and carried out an evaluation of the limiting features of the methodology and of the measuring System. The essential results of this evaluation are presented here.