Retinal Vascular Response to Breathing Increased Carbon Dioxide and Oxygen Concentrations

¹ Departments of Physiology and Medicine, University of Indiana Medical School, Indianapolis, Ind.

The retina has a high rate of oxygen consumption, and the retinal vessels are known to constrict with increased arterial oxygen tension and dilate when 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 O₂. 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 percent. Unlike the cerebral vessels, the retinal vessels showed no significant vasodilatation following inhalation of 10 per cent CO₂^- 21 per cent O₂. 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 CO₂^- 90 per cent O₂ 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 O₂ inhalation alone. From the decrease in retinal arteriovenous O₂ difference, it appears that retinal blood flow 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 vasoconstrictor response to oxygen.