Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Thorunn Scheving Eliasdottir, David Bragason, Sveinn Hakon Hardarson, Charles Vacchiano, Thorarinn Gislason, Jona Valgerdur Kristjansdottir, Gudrun Kristjansdottir, Einar Stefánsson, Thomas Penzel.
Determination of the blood oxyhemoglobin saturation in the retinal vessels of the eye can be achieved through spectrophotometric retinal oximetry which provides access to the state of oxyhemoglobin saturation in the central nervous system circulation. The purpose of this study was to test the capability of the Oxymap T1 oximeter to detect systemic hypoxemia and the effect of supplemental oxygen on retinal vessel oxyhemoglobin saturation.
Oxygen saturation of hemoglobin in retinal arterioles and venules was measured in 11 subjects with severe chronic obstructive pulmonary disease (COPD) on long term oxygen therapy. Measurements were made with and without their daily supplemental oxygen. Eleven healthy age and gender matched subjects were measured during ambient air breathing for comparison of oxyhemoglobin saturation in retinal arterioles and venules. Retinal arteriolar oxyhemoglobin saturation in COPD subjects inspiring ambient air was compared with finger pulse oximetry and blood samples from radial artery.
COPD subjects had significantly lower oxyhemoglobin saturation during ambient air breathing than healthy controls in both retinal arterioles (87.2%±4.9% vs. 93.4%±4.3%, p = 0.02; n = 11) and venules (45.0%±10.3% vs. 55.2%±5.5%, p = 0.01). Administration of their prescribed supplemental oxygen increased oxyhemoglobin saturation in retinal arterioles (87.2%±4.9% to 89.5%±6.0%, p = 0.02) but not in venules (45.0%±10.3% to 46.7%±12.8%, p = 0.3). Retinal oximetry values were slightly lower than radial artery blood values (mean percentage points difference = -5.0±5.4, 95% CI: -15.68 to 5.67) and finger pulse oximetry values (-3.1±5.5, 95% CI: -14.05 to 7.84).
The noninvasive Oxymap T1 retinal oximetry detects hypoxemia in central nervous system vessels in patients with severe COPD compared with healthy controls. The instrument is sensitive to changes in oxygen breathing but displays slightly lower measures than finger pulse oximetry or radial artery measures. With further technological improvement, retinal oximetry may offer noninvasive “on-line” measurement of oxygen levels in central circulation in general anesthesia and critically ill patients.
The eye is a window to systemic and central nervous system circulation. Its transparent anatomical structure provides a unique opportunity for direct noninvasive observation of arterial blood oxyhemoglobin saturation of retinal vessels; part of the central nervous system circulation. The retina has two separate vascular systems which differ anatomically and physiologically: the retinal circulation which supplies the inner retina and the choroidal circulation which supplies the avascular outer layers of retina. Both these circulations derive from the ophthalmic artery which is the first branch of the internal carotid artery on its way carrying oxygen rich blood from the aorta to the brain. The central retinal artery derives from the ophthalmic artery and runs centrally within the optic nerve before it divides into four major arterioles, each supplying one quadrant of the inner retinal tissue. Retinal arterioles are embryologically, anatomically and physiologically similar to cerebral arterioles[3,4] but branches of the central retinal artery lack innervation. In the inner layers of the retina, vessel blood flow autoregulation is governed by myogenic and metabolic mechanisms and similar to the cerebral vascular response reacts within seconds to a reduction in perfusion pressure. In the presence of shock and severe hypoxia, cerebral and ocular[1,9] perfusion is preserved by peripheral vasoconstriction and redistribution of blood flow from lower priority organs to the central nervous system. As a result of this redistribution of blood flow, measurement of oxyhemoglobin saturation by peripheral pulse oximetry may be unreliable  whereas measurement in retinal vessels may be a more direct indicator of cerebral oxyhemoglobin saturation.
Eleven (11) Caucasian subjects (7 female, 4 male) with severe COPD were enrolled in the study. The mean age was 70.4 ± 5.4 years and the range was 66 to 82 years. The control group data consisted of 11 age and gender matched healthy subjects (age 69.6 ± 4.9 years, range 64 to 80 years) selected from a group of 120 patients who had undergone retinal vessel imaging and oxyhemoglobin saturation determination while breathing ambient air prior to the current investigation. Of the 11 COPD subjects, one was not breathing the prescribed supplemental oxygen on arrival and therefore there was no first baseline measurement for this subject and we were unable to obtain an arterial blood gas sample from another. Therefore, data from 10 subjects was included in the statistical analysis at each study time period for comparison of mean and standard deviation within the COPD group. In addition, one COPD subject was a mouth breather and therefore we were unable to acquire sufficient FiO2 and EtCO2 data from the dual nasal cannula at any of the three study periods. All 11 COPD subjects were included in the ambient air breathing comparisons with the healthy controls.
Noninvasive spectrophotometric retinal oximetry was able to detect reduced oxyhemoglobin saturation in patients with severe COPD breathing ambient air compared to healthy subjects. The oximeter also captured an increase in retinal arteriole oxyhemoglobin saturation when the COPD subjects inspired supplemental oxygen. Our findings of increased oxyhemoglobin saturation in retinal arterioles and unchanged AV-difference during supplemental oxygen breathing are in agreement with the study of Palkovits and associates on patients suffering from severe COPD in a stable condition. Supplemental oxygen therapy improves global oxygen delivery and therefore the oxyhemoglobin saturation of retinal arterioles as demonstrated in our COPD subjects. A high FiO2 amplifies the oxygen flux from choriocapillaries, not only to the outer retina but to the inner retina as well  which may have contributed to the improved arteriolar oxyhemoglobin saturation in the inner retinal tissue with supplemental oxygen breathing.