Measurement of Retinal Microvascular Blood Velocity Using Erythrocyte Mediated Velocimetry

Abstract

Changes in retinal blood flow may be involved in the pathogenesis of glaucoma and other ocular diseases. Erythrocyte mediated velocimetry (EMV) is a novel technique where indocyanine green (ICG) dye is sequestered in erythrocyte ghosts and autologously re-injected to allow direct visualization of erythrocytes for in vivo measurement of speed. The purpose of this study is to determine the mean erythrocyte speed in the retinal microvasculature, as well as the intravisit and intervisit variability of EMV. Data from 23 EMV sessions from control, glaucoma suspect, and glaucoma patients were included in this study. In arteries with an average diameter of 43.11 µm ± 6.62 µm, the mean speed was 7.17 mm/s ± 2.35 mm/s. In veins with an average diameter of 45.87 µm ± 12.04 µm, the mean speed was 6.05 mm/s ± 1.96 mm/s. Intravisit variability, as measured by the mean coefficient of variation, was 3.57% (range 0.44-9.68%). Intervisit variability was 4.85% (range 0.15-8.43%). EMV may represent reliable method for determination of retinal blood speed, potentially allowing insights into the effects of pharmacologic agents or pathogenesis of ocular diseases.

Figure 1

Flowchart showing inclusion and exclusion of patients.

Figure 2

Example of erythrocyte tracking from erythrocyte mediated velocimetry images. Red circles show an individual erythrocyte flowing in a small vein along the retina (a–e).

Figure 3

EMV image showing coordinates of many individual erythrocytes from an angiogram sequence (blue) overlaid on user-drawn vessel path (red).

Figure 4

Difference in velocity as a function of the mean velocity between angiograms acquired in the same session (a) or separate sessions. (b) The center line represents the mean difference in velocity between sessions. The outer lines represent the mean difference ±1.96(SD).