HEIDELBERG RETINAL FLOWMETRY - FACTORS AFFECTING BLOOD-FLOW MEASUREMENT

Aims-To evaluate factors affecting Heidelberg retinal flowmeter (HRF) measurements of retinal and optic nerve head blood flow in human subje cts. Methods-The angle of incidence between laser beam and fundus, and camera distance from the eye, were evaluated for their effect upon me asures of blood volume, velocity, and flow in a single 100 x 100 x 400 mu m volume of temporal peripapillary retinal tissue in normal volunt eers. Both intra and intersession reproducibility of these measures we re studied. Intersession data were obtained by taking one image per we ek for 4 weeks. Finally, the intersession haemodynamic data were exami ned in the entire image (640 x 2560 x 400 mu m), using histograms of p ixel by pixel blood how. Results-Measures of blood volume, velocity, a nd flow from a single anatomical site were unaffected by laser beam to fundus angle of incidence (n = 12). As camera distance from the eye w as increased (from 2 to 5 to 7 cm), flow measurements showed increasin g individual changes, despite unaltered measured vessel lengths and co nstant overall mean flow (n = 14). The coefficient of variation for tw o intrasession images of optic nerve head blood flow averaged 7% (n = 20); in contrast, the 4 week intersession coefficient of variation ave raged 30% (n = 15). Intersession reproducibility was increased by usin g how histograms from the entire image: the coefficients of variation averaged 16% for total flow and 17% for flow in the pixel of median ho w. Conclusion-HRF measures of flow are independent of the laser beam t o fundus angle of the incidence and dependent upon camera distance fro m the eye. Intersession reproducibility is best using pixel by pixel a nalysis of the entire image.