Confocal scanning laser Doppler flowmetry: Experiments in a model flow system

Purpose: We conducted this study toward validating confocal scanning laser Doppler flowmetry (SLDF), a new noninvasive technique for measuring retinal and optic nerve head hemodynamics. Methods: We designed a model flow system using a glass capillary coupled to a microlitre syringe driven by an infusion pump. Eleven capillaries with parallel walls (internal diameters ranging 705 to 25 mu m) were used. The capillaries were perfused with skim milk over a range of pump flow rates. At each flow rate, measurements were made with the Heidelberg Retina Flowmeter (HRF) to study the relationship between HRF-measured flow and actual flow. The initial experiments (n = 2) were conducted to establish the approximate velocity operating range of the HRF with single HRF measurements across a wide range of flow rates, whereas the subsequent experiments (n = 9) were concentrated within this operating range with five HRF measurements at each flow rate. Results: When pump flow rates were converted to actual velocity at the measurement point far the initial experiments, the velocity operating range of the HRF was similar to 0.08 to 1.0 mm/s. For velocities of >1 mm/s, HRF measured velocity was not linearly related to actual velocity. Within the operating range, there was a highly significant linear relationship between HRF-measured flow and actual flow (0.935 less than or equal to r less than or equal to 0.990, p < 0.001). When the curves of HRF-measured velocity versus actual velocity for the different experiments were plotted, they largely superimposed. The variability of the HRF measurements was between 3.57% and 4.05% and was independent of flow rate. Conclusions: SLDF measures reliably and linearly within a given operating range.