Kit Guan

Variability and repeatability of retinal blood flow measurements using the Canon laser blood flowmeter

The purpose of this work was to determine the within-session variability and between-session repeatability of the Canon Laser Blood Flowmeter (CLBF), Model 100, an instrument that permits the noninvasive measurement of retinal blood flow. The CLBF calculates flow in microl/min based on the Poiseuille principle. One eye of 20 normal subjects (mean age 36.5; SD 9.7 years) was randomly chosen. A minimum of five measurements was acquired of a temporal arteriole approximately 1 disc diameter from the optic nerve. Measurements were repeated within a 1-month period. Blood pressure and intraocular pressure were measured. The coefficient of variation (COV) and the coefficient of repeatability (COR) were calculated for each individual. The individual COVs for diameter, velocity, and flow ranged from 0.5 to 6.5% (median 2.0%), 4.8 to 39.7% (median 19.9%), and 4.8 to 37.3% (median 19.3%), respectively. The group mean CORs for diameter, velocity, and flow were 5.2 microm (relative to a mean effect of 104.6 microm), 8.8 mm/s (relative to a mean effect of 33.9 mm/s), and 2.6 microl/min (relative to a mean effect of 8.8 microl/min), respectively. The CLBF gave consistent and repeatable measurements of blood flow within retinal arterioles in normal subjects. Given the range of individual variation in the velocity measurement, and thus flow, confidence limits for retinal hemodynamics need to be determined on an individual basis.

Assessment of retinal hemodynamics with the Canon laser blood flowmeter after a single dose of 2 % dorzolamide hydrochloride eyedrops

BACKGROUND Dorzolamide hydrochloride is a carbonic anhydrase inhibitor that reduces intraocular pressure (IOP) by decreasing the production of aqueous humour in the ciliary body. Theoretically, topical use of this agent has the potential to directly affect retinal vasculature through local induced acidosis. We performed a study to determine whether there are changes in retinal arteriole hemodynamics, as assessed with the Canon laser blood flowmeter, in healthy subjects following topical administration of dorzolamide. METHODS We recruited 17 healthy volunteers, nine men and eight women aged 25 to 55 years (mean 31.4 [standard deviation (SD) 9.88] years). The inclusion criteria were Snellen visual acuity of 20/30 or better, normal anterior eye examination, IOP of 21 mm Hg or less, and a normal fundus appearance. One eye of each subject was randomly assigned to receive a drop of 2% dorzolamide. The contralateral eye of 10 of the subjects received a placebo drop (artificial tears). Before and 1 hour after drop administration, we obtained blood flow measurements from an inferotemporal arteriole approximately 1 disc diameter from the optic nerve head rim using the Canon laser blood flowmeter, model 100. The IOP was measured by means of Goldmann applanation tonometry before and 1 hour after drop administration. RESULTS The mean IOP was significantly reduced in the dorzolamide-treated eyes, from 14.4 mm Hg (SD 2.94 mm Hg) to 11.7 mm Hg (SD 2.50 mm Hg) (p < 0.001). The IOP was also reduced in the placebo group (15.6 mm Hg [SD 3.41 mm Hg] vs. 14.6 mm Hg [SD 3.28 mm Hg]), but the difference was not significant. There was no significant difference in mean arteriole diameter, mean blood velocity or mean blood flow after drug administration in the dorzolamide-treated eyes. INTERPRETATION Our results indicate that a single topical application of dorzolamide in healthy subjects has no effect on retinal arteriole diameter, blood velocity or blood flow, as measured with the Canon laser blood flowmeter. Longer-term studies of retinal hemodynamics in patients with glaucoma are warranted as evolving treatments aim to improve ocular blood flow as well as reduce IOP.