In vivo characterization of erythrocyte supply in the human retinal capillaries

Abstract

Purpose : To evaluate the relationship among the velocity, cell flux, and cell lineal density of the erythrocytes in the human retinal capillaries.

Methods : We imaged the erythrocytes flowing in retinal capillaries of the human subjects in normal ocular and physical health, using a high-speed adaptive optics near-confocal ophthalmoscope at a frame rate of 800 Hz. We measured the erythrocyte velocity, lineal density (defined as the number of cells presenting within a certain length of the capillary), and cell flux (defined as the number of cells per second flowing through a capillary segment) within a cardiac cycle with custom software.

Results : In 29 macular capillaries of 7 eyes of 7 human subjects aged 20-29 years old, the linear density was 59.60 ± 10 cells/mm and cell flux was 60.16 ± 20.18 cells/s. In a single capillary, erythrocyte cell flux showed a distinctive fluctuation associated with the heartbeat, the lineal density of erythrocytes varied over time within a cardiac cycle, but the average lineal density over multiple cycles exhibited a fairly consistent distribution within a cardiac cycle under the normal cardiovascular and respiratory resting state (Figure).

Conclusions : The consistent erythrocyte lineal density over time in a retinal capillary disclosed the essential number of erythrocytes that are needed to provide a constant, uniform flux of oxygen to the local tissue thereby maintaining a normal retinal function. In vivo quantification of erythrocyte lineal density in human retinal capillaries under the natural rheological state may provide a potential useful biological index or a biomarker for assessing vascular function and pathophysiology.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Erythrocyte velocity, cell flux, and lineal density within a cardiac circle in a capillary. Top panel: Blue dots are erythrocyte lineal density measured in different cardiac cycles. Blue line is the average lineal density over 26 cardiac circles. Red line is the average erythrocyte velocity. The plots were drawn in the following steps. 1) The velocity and lineal density were measured in all cardiac cycles. 2) The periods of all cardiac cycles were normalized to 1. 3) Erythrocyte velocity and lineal density at the corresponding time points within the normalized cycle were averaged. The cardiac cycle was determined by the simultaneously recorded cardiac pulses.