Retinal vascular response to breathing increased carbon dioxide and oxygen concentrations Regina Frayser and John B. Hickam 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 1 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 1 per cent O s. 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 1 per cent CO S - 21 per cent O t. However, there was a significant increase (p <.5) in retinal venous oxygen saturation, suggesting that a flow change had occurred. The inhalation of 1 per cent COs- 9 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 O t inhalation alone. From the decrease in retinal arteriovenous O 2 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. T, he retina has a high rate of oxygen consumption. The retinal vessels normally change caliber in response to alterations in arterial blood oxygen tension. Vasoconstriction occurs when the arterial oxygen tension rises; vasodilatation follows a fall in the arterial oxygen tension. This response of the retinal vessels suggests that shifts in From the Departments of Physiology and Medicine, University of Indiana Medical School, Indianapolis, Ind. This work was supported in part by Grant NB- 4285 from the National Institute for Neurological Diseases and Blindness, United States Public Health Service, in part by a grant from the Life Insurance Medical Research Fund, and was conducted in part from facilities provided by Grant HE-638. 427 retinal oxygen tension are opposed by appropriate alterations in retinal blood flow. The suggestion is supported by the observation that a reduction in arterial oxygen saturation from 97 to 7 per cent caused by inhalation of 1 per cent O 2 brought about a decrease of only 16 per cent in retinal venous oxygen saturation. 1 This decrease is of the expected magnitude if the accompanying retinal vasodilatation and presumed increase in retinal blood flow are taken into account. However, the inhalation of 99.6 per cent O 2 has been reported to cause a 23 per cent increase in retinal venous oxygen saturation,- which is much larger than the maximum increase of 15 per cent to be expected from the increase in arterial blood oxygen content. The response of the retinal vessels to al-
428 Frayser and Hickam Investigative Ophthalmology August 1964 terations in blood carbon dioxide levels is not as clearly defined. Huerkamp 3 reported a marked vasodilatation following inhalation of increased carbon dioxide levels, while Hickam and co-workers 2 found that neither the inhalation of a mixture containing 5 per cent CO 2, 21 per cent O 2, and 74 per cent N 2 nor inhalation of 1 per cent CO 2, 21 per cent O 2, and 69 per cent N, produced significant dilatation of the visible retinal vessels. Because of the apparent disparity in the effect of CO 2 and the unusually large increase in venous O 2 saturation on O* breathing, it was felt that an investigation of the combined effects of CO 2 and O 2 on the retinal venous oxygen saturation and vessel diameter would be of interest. The results of such observations are reported. Methods All subjects were normal male medical or dental students. All photographs were made with a Zeiss fundus camera, with the subject in the seated position. Photographs were taken after the subject had been breathing the test gas for approximately 5 minutes with a 1 minute rest period between studies. Retinal vascular was measured as the percentage change in the retinal vessel size in going from air breathing to breathing the test gas for approximately 5 minutes. The visible diameter of the larger vessels near the disc was measured from the fundus photographs, with a low-power (9x) dissecting microscope having a scale in the ocular. In general, 6 to 12 measurements were made for both the arteries and veins, and the results averaged, as all of the vessels do not react to the same degree. Measurements of vessel size were made without prior knowledge of the test condition. Estimates of over-all caliber change are reproducible within 2 per cent. The retinal venous blood oxygen concentration was measured by the photographic method of Hickam, Frayser, and Ross. 1 This procedure consists of taking two simultaneous photographs of the retinal vessels as they emerge from the disc. One photograph is taken with, in front of the film, an interference filter having a transmission peak at 64 mfi, and the other with an interference filter having a transmission peak at 51 m/i. The ratio of the optical density of the retinal veins by red light to their optical density by green light was measured. The per cent oxygen saturation of the retinal venous blood is then given by the equation : % saturation = 118 88 r. where r is the ratio of the red to the green optical density. Standard deviation from regression is 4.2 per cent saturation. Table I. Comparative effect of 1% CO 2-21% O,, 99.6% O 2 and 1% CO 2-9% O 2 on retinal vascular * Subject S. S. H. E. C. E. N. V. R. C. M. J. K. T. M. B.C. J.E. R. H. Mean ± S.D. p compared to air 1% COr- 21% O 2-69 <7o N, Arterial - 3 +1-4 + 4-2 + 7 - +2 ± 5 N.S. Venous - 3 +11-2 +1 N.S. Arterial -1-22 -1-4 -21-1 -12 ± 6 <.1 99.6% O, P Venous -16-15 -15-2 -19-16 -11-11 -15 ± 3 <.1 <.5 1% CO.- 9% O* Arterial Venous -4-13 +1-3 -9 +4 i -3 ± 4 >.5-7 -12-1 - 8 ± 6 <.5 p <.1 "Expressed as per cent change from vessel diameter while breathing air.
Volume 3 Number 4 Retinal response to carbon dioxide and oxygen 429 Table II. Comparative effect of 1% CO,- 21% O,, 99.6% O,, and 1% CO,- 9% O, on retinal venous oxygen saturation Subject W. B. W. D. J.A. N. V. R. C. E. F. M. F. Z. S. S. J.E. Mean ± S. D. p value compared to cent Os saturation on air air 61 52 55 59 51 43 48 63 55 ± 8 Retinal venous oxygen saturation cent Ot saturation on 1% CO S - 21% Os9% N. 58 61 71 73 72 52 5 78 65 ± 1 p <.5 cent Os saturation on 99.6%, 89 88 87 9 84 83 69 82 82 ± 9 p <.1 P < Retinal cent Os on 1% 81i ± 9 P * C 1.5 venous per saturation CO S - 9% Os 96 91 87 9 81 68 Results Table I presents the data on the vascular of 11 subjects on going from air breathing to breathing either 1 per cent CO,, 21 per cent O,; 69 per cent Nj; 99.6 per cent O,; or 1 per cent CO,, 9 per cent O,. In Table II are the values for retinal venous oxygen saturation for 9 subjects breathing the same test gases. Discussion The cerebral vessels dilate in response to increased blood CO, levels, and constrict upon the inhalation of 85 to 1 per cent oxygen. These changes in vessel diameter are accompanied by appropriate alterations in cerebral blood flow; a mean reduction of flow of 13 per cent while breathing 85 to 1 per cent O,; 4 a mean increase in flow of 51 per cent on breathing 5 per cent CO,; and an increase of 12 per cent on breathing 7 per cent CO,. 5 In contradistinction to the effect on cerebral vessels, in the present study, although the individual response was variable (Table I), there was no significant change in diameter of the visible retinal vessels for the entire group on breathing 1 per cent CO,, 21 per cent O a as compared to breathing air. However, measurement of the retinal venous oxygen saturation showed a significant increase (p <.5) from a mean of 55 ± 8 per cent saturation when breathing air to 65 ± 1 per cent saturation on breathing 1 per cent COj in 21 per cent O, (Table II). All subjects showed an increase in saturation over the control level. A narrowing of the retinal arteriovenous oxygen difference suggests that there may be an increase in flow rate without changes in size of the visible retinal vessels. Such an increase in flow could be the result of dilatation of vessels smaller than those on which measurements can be made by the present technique. The inhalation of oxygen has been shown to produce both a significant constriction of the visible retinal vessels-' Gi 7 and a significant increase in retinal venous oxygen saturation. 1 ' 2 In the present series, the increase of 27 per cent in venous oxygen saturation during O, breathing is considerably greater than the maximum increase of 15 per cent, which might be expected on the basis of increased arterial blood oxygen content if retinal oxygen consumption and blood flow were unchanged. There was a 12 per cent decrease in arterial diameter
43 Frayser and Hickam Investigative Ophthalmology August 1964 as compared to air while breathing 99.6 per cent O 2, and venous diameter decreased 15 per cent during oxygen breathing. This decrease in AV O 2 difference which occurs in the face of visible vasoconstriction appears paradoxical, and the explanation is not yet clear. The retina is known to have a high rate of respiration. Most of the energy is supplied by glycolysis and oxidation of glucose. One possible explanation for the marked increase in retinal venous oxygen saturation on breathing oxygen is that there may be a partial inhibition of one of the enzyme systems which facilitates oxygen consumption by the retina. This would not necessarily cause a reduction in energy utilization, as increased glycolysis might compensate for the reduction in oxygen consumption. The inhalation of 1 per cent CO 2, 9 per cent O. produced a decrease, as compared to air, of 3 per cent in the arterial diameter and 7 per cent in venous diameter (Table I). This represents a significant (p <.5) change in venous size as compared to air. However, these values are both significantly less than the change in vessel size seen on breathing O 2 alone (arterial p <.5; venous p <.1). Previous observations 8 have shown no significant difference in the degree of constriction with 9 per cent O 2 or 99.6 per cent O 2, so the difference in responsiveness of the vessels to the inhalation of 1 per cent CO L >, 9 per cent O u. cannot be attributed to the difference in oxygen concentration. It is concluded that adding CO> to the gas mixture prevents the full constrictor effect of oxygen from taking place. This observation, in conjunction with the individual variability seen in response to 1 per cent CO.,, 21 per cent O, (Table I) inhalation, suggests that CO 2 does have some effect in regulating retinal vessel tone. Measurement of retinal venous oxygen saturation during the inhalation of 1 per cent COj, 9 per cent O 2 showed a significant increase (p <.1) from 55 ± 8 per cent saturation on air to 88 ± 9 per cent saturation on the high CO 2 in high O> mixture (Table II). This value for retinal venous oxygen saturation while breathing 1 per cent CO 2, 9 per cent O 2 is also significantly higher (p <.5) than the 82 ± 9 per cent saturation found during the inhalation of oxygen alone. From the observations reported here, it would appear that, in the eye, oxygen is more effective and carbon dioxide less effective in producing alterations in vessel diameter than in the brain as a whole. Carbon dioxide in either 21 per cent or 9 per cent oxygen produced a significant decrease in arteriovenous oxygen difference, suggesting that, as in the cerebral vessels, carbon dioxide does cause an increase in blood flow. This increase in flow could result from dilatation of resistance vessels smaller than those in which measurements can be made. Although blood pressure measurements were not made in the present subjects at the time photographs were taken, our previous experience has indicated that inhalation of these gas mixtures does not produce in normal subjects a change in mean arterial pressure which would be expected to influence significantly the retinal blood flow. At the present time, it is not possible to determine with exactitude changes in retinal blood flow in the human. Until such measurements are possible, one can only surmise changes in relative flow rates from alteration in the AV oxygen difference. REFERENCES 1. Hickam, J. B., Frayser, R., and Ross, J. C: A study of retinal venous blood oxygen saturation in human subjects by photographic means, Circulation 27: 375, 1963. 2. Hickam, J. B., Sieker, H, O., and Frayser, R.: Studies of retinal circulation and A-V oxygen difference in man, Tr. Am. Clin. & Climatol. A. 71: 34, 1959. 3. Huerkamp, B., and Rittinghaus, F. W.: Uber die blutversorgung der menschlichen Retina unter die Einwirkung veranderter Sauerstoffspannung, von Kohlensaure, Hyperventilation und Adrenalin, Pfliigers Arch. ges. Physiol. 252: 312, 195. 4. Kety, S. S., and Schmidt, C. F.: The effects of
me 3 bar 4 Retinal response to carbon dioxide and oxygen 431 altered arterial tensions of carbon dioxide and )xygen on cerebral blood flow and cerebral )xygen consumption of normal young men, f. Clin. Invest. 27: 484, 1948. Schieve, J. F., and Wilson, W. P.: The inluence of age, anesthesia, and cerebral arteriosclerosis on cerebral vascular activity to CO;, \m. J. Med. 15: 171, 1953. Sieker, H. O., and Hickam, J. B.: Normal and impaired retinal vascular, Circulation 7: 79, 1953. Cusick, P. L., Benson, O. O., Jr., and Boothby, W. M.: Effect of anoxia and of high concentrations of oxygen on the retinal vessels, Proc. Staff Meet. Mayo Clin. 15: 5, 194. Frayser, R.: Unpublished observations.