Mechanical fundus-perfusion model: blood-flow velocity determined with Heidelberg Retina Flowmetry (HRF) and digital ICG-angiography (HRA)

Background Heidelberg Retina Flowmetry (HRF) is now popularly, perhaps even indiscriminately applied in eye research, without apparent concern for the fact that the results are given numerically, but without physical units. Methods 1) HRF: To challenge the HRF-device with known values of blood-flow velocity, a perfusion chamber with input and output connections was constr ucted of acrylic plastic. Three serial segments were milled to provide cros s-sectional areas (1.93 mm(2), 3.33 mm(2), and 5.08 mm(2)) and accordingly decreasing, true, clinically representative flow-velocity values. Under a c onstant perfusion setting of a calibrated clinical infusion pump (Perfusor Secura FT, B. Braun Medical AG, Sempach, CH), heparinized human blood (P.H. ) was pumped through the chamber, and the HRF-parameter, "VELOCITY" was mea sured within one image encompassing the three chamber segments, using a 20 degrees x 5 degrees-field and a 20 x 20-pixel measuring "window". 2) HRA: I mmediately thereafter, our perfusion model was placed in front of the Heide lberg Retina Angiography device, the infusion pump started at the same cons tant level, and a Ice bolus of ICG dye was added to the blood. Digital ICG- angiography was then conducted, and the images analyzed on-screen. Results In the three segments of the perfusion chamber, flow velocities det ermined ICG-angiographically were 11.5 mm/s, 6.7 mm/s, and 4.4 mm/s, respec tively. The corresponding values for HRF-"VELOCITY" were 5.3, 4.2, and 3.4, respectively (no units). Conclusions Under identical perfusion conditions, the phenomenologically (I CG-angiographically) determined values of flow velocity in the 3 perfusion chamber segments ran similar to (but not numerically coincidental with) tho se determined for HRF-parameter "VELOCITY". Extrapolation of HRF-values to true physical units is, thus, feasible.