Measurement of the lineal density of red blood cells in capillaries in vivo, using a computerized frame-by-frame analysis of video images.

In order to study the oxygen supply system within a capillary bed it is desirable to know the lineal density of red blood cells (RBCs) in individual capillaries; viz number of cells per millimeter. We have developed a video computer method for "continuous" measurements of lineal density, based on frame-by-frame analysis of the spatial-average of blood opacity over a selected length of capillary. Each capillary is calibrated separately, in order to determine the relationship between mean opacity and lineal density for that vessel. Since the method does not attempt to detect individual RBCs, it can be applied to capillaries with RBCs which overlap each other to some extent. Also, since the opacity data is normalized with respect to "background" light intensity and "contrast," this technique can be applied to thick tissues such as skeletal muscle. The method has been tested on capillaries in frog sartorius muscle and is able to predict, on average, the number of frog RBCs in an 80-micron length of capillary to +/- one-quarter cell. At present, computation times limit "real time" measurement of lineal density to a sampling rate of 10 sec-1. The data may also be used (1) to compute red cell flux (cells X sec-1), if corresponding velocity measurements are available, and (2) to estimate capillary hematocrit.