Measuring Temporal and Spatial Variability of Red Blood Cell Velocity in Human Retinal Vessels

Abstract

Purpose The retinal circulation regulates blood flow through various internal and external factors; however, it is unclear how locally these factors act within the retinal microcirculation. We measured the temporal and spatial variability of blood velocity in small retinal vessels using a dual-beam adaptive optics scanning laser ophthalmoscope. Methods In young healthy subjects ( n = 3), temporal blood velocity variability was measured in a local vascular region consisting of an arteriole, capillary, and venule repeatedly over 2 days. Data consisted of 10 imaging periods separated into two sessions: (1) five 6-minute image acquisition periods with 30-minute breaks, and (2) five 6-minute image acquisition periods with 10-minute breaks. In another group of young healthy subjects ( n = 5), spatial distribution of velocity variability was measured by imaging three capillary segments during three 2-minute conditions: (1) baseline imaging condition (no flicker), (2) full-field flicker, and (3) no flicker condition again. Results Blood velocities were measurable in all subjects with a reliability of about 2%. The coefficient of variation (CV) was used as an estimate of the physiological variability of each vessel. Over 2 days, the average CV in arterioles was 7% (±2%); in capillaries, it was 19% (±6%); and, in venules, it was 8% (±2%). During flicker stimulation, the average capillary CV was 16% during baseline, 15% during flicker stimulation, and 18% after flicker stimulation. Conclusions Capillaries in the human retina exhibit spatial and temporal variations in blood velocity. This inherent variation in blood velocity places limits on studying the vascular regulation of individual capillaries, and the study presented here serves as a foundation for future endeavors.

Flow Heterogeneity and Factors Contributing to the Variability in Retinal Capillary Blood Flow

“…The V ave ± SD (range) values of 1.18 ± 0.46 (0.26–2.30) found here are in close agreement to those reported previously: 1.33 ± 0.28 (1.30–1.80), 7 1.14 (0.30–2.26), 4 1.22 (0.58–1.98), 5 and 1.70 (0.93–3.32) mm/s. 6 A mean V max of 1.87 (range, 0.61–3.73) observed here matched very well with previously reported V max values of 1.88 (0.80–3.35) 4 and 1.88 (0.71–3.98). 5 Figure 5 shows that the average velocity is correlated to the maximum.…”

Variability in Capillary Perfusion Is Increased in Regions of Retinal Ischemia Due to Branch Retinal Vein Occlusion

“…We acknowledge that it would be important to validate this technique against technologies such as AOSLO and live in vivo microscopy in human and animal models of retinal ischemia. 8 We note, however, that our previously published CoV results using the same technique in normal participants are consistent with those obtained by Warner et al 24 using AOSLO. 15 Both techniques demonstrate that venules and arterioles have similar CoV values and that CoV values for capillaries are higher.…”

“…It also permits comparison between patients/participants. Warner et al 24 similarly used CoV to compare RBC flow, measured using an adaptive optics scanning laser ophthalmoscope, between human retinal arterioles, venules, and capillaries. A nearly unbiased estimate of the population CoV can be obtained by multiplying the sample CoV by (1 + 1/4 n ), where n is the number of observations.…”

Foveal avascular zone vessel density is associated with visual field progression in early-stage glaucoma eyes with central visual field damage

“…OCT-A signals rely on the red blood cell (RBC) density and velocity. The RBCs move at variable velocities according to the metabolic demand 36 , which may even pause occasionally 37 . This may have resulted in an inaccurate detection of single-layer capillary microcirculation around the FAZ in our present analyses.…”