Fluorescent vesicle system. A new technique for measuring blood flow in the retina

To measure blood flow in the retinal circulation and optic nerve head capillaries with an innovative fluorescent vesicle system.

Carboxyfluorescein was encapsulated into liposomes; the vesicles ranged from 0.1 to 2 microns in diameter. After intravenous injection of the liposome suspension, the fundus was viewed using a scanning laser ophthalmoscope. Images of the fundus showing circulating liposomes were stored on videotape. An image analyzing system was used to digitize the captured video frames and transfer them to a computer's hard disk for permanent storage. Software developed in the authors' laboratory allowed them to overlay multiple video frames to create a single image that provided a visible record of the path taken by a particular vesicle in a given time period. The information on this image was used to calculate the velocity of the vesicle, and hence the velocity of the blood flow in the vessel.

With this system, individual liposomes as small as 100 nm were visible in all retinal vessels (arteries, capillaries, and veins). Quantitative analysis of vesicle movement in the major retinal vessels of the cynomolgus monkey yielded an average velocity of 9.33 +/- 1.67 mm/second in a large vein (diameter, 130 microns) and 16.10 +/- 5.7 mm/second in a large retinal artery (diameter, 64 microns). The average velocity in the macular capillaries was 0.76 mm/second (range, 0.45-1.33 mm/second), whereas the average velocity in the optic nerve head capillaries was 1.39 mm/second (range, 0.96-2.25 mm/second).

The fluorescent vesicle system can be used for simultaneous measurement of blood flow in the retinal arteries, veins, and capillaries of the macula and optic nerve.