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Methods.: Forty-six healthy subjects were included in the present study, and data of 41 subjects could be evaluated. Retinal vessel diameters, as well as oxygen saturation in arteries and veins, were measured using the Dynamic Vessel Analyzer. In addition, retinal venous blood velocity was measured using bidirectional laser Doppler velocimetry, retinal blood flow was calculated, and oxygen and carbon dioxide partial pressures were measured from arterialized capillary blood from the earlobe. Measurements were done during normoxia and during 100% oxygen breathing.

Results.: Systemic hyperoxia caused a significant decrease in retinal venous diameter (− 13.0%±4.5%) and arterial diameter (− 12.1%±4.0%), in retinal blood velocity (− 43.4%±7.7%), and in retinal blood flow (− 57.0%±5.7%)(P< 0.001 for all). Oxygen saturation increased in retinal arteries (+ 4.4%±2.3%) and in retinal veins (+ 19.6%±6.2%), but the arteriovenous oxygen content difference significantly decreased (− 29.4%±19.5%)(P< 0.001 for all). Blood oxygen tension in arterialized blood showed a pronounced increase from 90.2±7.7 to 371.3±92.7 mm Hg (P< 0.001). Calculated oxygen extraction in the eye decreased by as much as 62.5%±9.5%(P< 0.001).

Conclusions.: Our data are compatible with the hypothesis that during 100% oxygen breathing a large amount of oxygen, consumed by the inner retina, comes from the choroid, which is supported by previous animal data. Interpretation of oxygen saturation data in retinal arteries and veins without quantifying blood flow is difficult.(ClinicalTrials. gov number, NCT01692821.)