Photodetector sensitivity level and Heidelberg retina flowmeter measurements in humans

PURPOSE. In vitro models suggest that Heidelberg retina flowmeter (HRF) mea surements are affected by changes in photodetector sensitivity. We measured blood flow in a single volume of human retinal tissue in vivo at various s ensitivity (DC) levels. METHODS. The peripapillary retinal regions of 12 normal subjects were exami ned by HRF under Eve different sensitivity settings: (1) average DC range b elow 100; (2) average DC range below 125; (3) average DC range near 150 (no rmal sensitivity); (4) average DC range above 175; and (5) average DC range above 200 or extremely overexposed. The distributions of flow values were examined by pointwise analysis. All pixels from a common tissue location we re analyzed, and the effect of their brightness on the flow measurement was evaluated by ANOVA with Fisher's protected least significant difference mo del. RESULTS. ANOVA analysis of image DC level showed that significantly differe nt DC levels were achieved fur each of the ave sensitivity settings (P < 0. 0001). Flow values decreased with increasing DC for each of the 25th percen tile, 50th percentile (P < 0.0001 for each), 75th percentile (P = 0.0026), 90th percentile (P = 0.0216), and mean (P = 0.0001) flow values. The percen tage of pixels with values of zero (avascular tissue) increased with increa sing photodetector sensitivity (P < 0.0001). CONCLUSIONS. Improper sensitivity settings alter the detected percentage of avascular tissue and the blood flow measurements in tissue containing capi llaries. Consistent assessment of retinal blood flow requires consistent ph otodetector sensitivity settings between longitudinal images.