Is the laser Doppler flow signal a measure of microcirculatory cell flux?

To assess oxygen transport as a function of hematocrit, microcirculatory red blood cell flux (microflux) was measured by laser Doppler flowmetry (LDF) in the isolated rat mesentery as well as in cat sartorius muscle. The hematocrit value was varied from 0.5 to 50% while the perfusion pressure ranged from about 2.7 to 13 kPa. Simultaneously macroflux which is the volume flow times the systemic hematocrit value was determined. The curves LDF versus pressure showed saturation at several hematocrit values conveying the impression of autoregulation of microflux. The macroflux curves, on the other hand, curved upward. To clarify whether this discrepancy had a physiological cause, pressure-microflux as well as pressure-macroflux curves were collected in a flow chamber model where regulatory effects are absent. In the model the same discrepancies between micro- and macroflux as in vivo were observed. These discrepancies are explained by maximum values in velocity of about 3 mm/s and in hematocrit value of less than or equal to 20% which the LDF can discern. The limit in velocity is explained by the low pass filter of 12 kHz of the LDF instrument, the limit in hematocrit measurement is probably caused by the light scattering properties of red blood cell suspensions. A complicated combination of effects appears to be responsible for the linearity of the LDF signal versus macroflux at normal hematocrit value.