Retinal Vascular Response to Breathing Increased Carbon Dioxide and Oxygen Concentrations.

The retina has a high rate of oxygen consumption, and the retinal vessels are knoion to constrict with increased arterial oxygen tension and dilate ivhen arterial oxygen falls. In 10 subjects, measurement of retinal venous oxygen saturation by a photographic technique has shown an increase from 55 ± 8 per cent saturation breathing air to 82 ± 9 per cent breathing 100 per cent Os. This increase was accompanied by a 12 ± 6 per cent decrease in retinal arterial size and a decrease of 15 ± 3 per cent in venous diameter. This 27 per cent increase in oxygen saturation is significantly greater than the expected maximum increase of 15 per cent. Unlike the cerebral vessels, the retinal vessels showed no significant vasodilatation following inhalation of 10 per cent COS21 per cent Ot. However, there was a significant increase (p < .005) in retinal venous oxygen saturation, suggesting that a flow change had occurred. The inhalation of 10 per cent COs90 per cent O» significantly reduced the vasoconstrictor effect of oxygen alone while significantly increasing the retinal venous oxygen saturation to 88 ± 9 per cent from 82 ± 9 per cent found with Ot inhalation alone. From the decrease in retinal arteriovenous O2 difference, it appears that retinal blood fiow can alter without changes in visible vessel diameter. It is possible that vessel diameter is changing in vessels which are too small to be measured by the present technique. Carbon dioxide appears to be capable of significantly reducing the vasocotistrictor response to oxygen.