Determination of volumetric flow in capillary tubes using an optical Doppler velocimeter

The purpose of this study was to use the optical Doppler velocimeter of Borders and Granger [(1984), Microvasc. Res. 27, 117-127] to determine the ratio of centerline red cell velocity to mean velocity of blood flowing in small glass tubes. Red blood cell suspensions were perfused at different rates through capillary tubes (15-100 microns, id) while measuring centerline velocity (Vmeas). Mean red cell velocity (Vmean) was calculated from measurements of volume flow in the tubes. With the effective slit width (sensor size) of the velocimeter set at 9.0 microns, the ratio of Vmeas/Vmean averaged 1.54 +/- 0.03 (mean +/- SEM), and was essentially independent of tube size. When the slit width was increased, the ratio of Vmeas/Vmean was significantly lower and appeared to vary as a function of tube diameter. These results are compared with previous measurements using other velocimeters, and the relationship between relative sensor size and ratio of Vmeas/Vmean is discussed.