Failure of laser Doppler signal to correlate with total flow in muscle: Isthis a question of vessel architecture?

The signal strength from LDF probes positioned in perfused muscle can be al tered by vasoconstrictors despite total flow being maintained constant. Apa rt from redistribution of flow via collateral channels outside the region o f measurement, the change in LDF signal may arise because the vasoconstrict ors have switched now to vessels of different architecture or altered the a rchitecture of the blood vessels being perfused. Thus we have examined the effect of tube architecture on LDF signal using polymer tubes of 250, 100, and 50 mum internal diameter. At 3% hematocrit the LDF signal was linear fo r each of the three tube sizes from 10 to 80 mul/h. The signal strength was greatest from the smallest tube and least from the largest tube. For a sin gle tube (100 mum) that doubled back on itself twice to cross the field of measurement three times, the LDF signal at any now (10-80 mul/h, hematocrit 3%) was approx threefold greater than that for the same tube crossing the field of measurement once. The effect of progressively switching flow (cons tant at 120 mul/h, hematocrit 9%) from five to one tube in a manifold of fi ve tubes (100 mum) gave rise to a progressive increase in signal. It is con cluded that LDF signal derives predominantly from nonvectorial cell speed a nd less from cell number. Thus any agent that alters the architecture has t he potential to alter the LDF signal. (C) 2000 Academic Press.