The Effect of Mild Hypoxia on Retinal Oxygen Saturation

Purpose To study the effect of mild hypoxia on retinal oxygen saturation using quantitative analysis of hyperspectral retinal images Methods Ten healthy volunteers (6 male, 4 female, aged (mean±SD) 26±5 years) were recruited. Retinal images were acquired under normoxia & hypoxia using a modified fundus camera (Topcon TRC 50 IA, Japan) fitted with IRIS (image replicating imaging spectrometer) system that acquires images in single snapshot at 8 different wavelengths [1]. Hypoxia was induced by changing the O2 concentration of inhaled air from 21% to 15% using a hypoxia generator (Hypoxico Inc. NY, U.S.). Subjects breathed the hypoxic gas mixture for 10 minutes. Oxygen saturation of the subjects was monitored using fingertip-pulse oximetry. Images were processed using custom algorithms to calculate oxygen saturation. Data are presented as mean±SD and were analysed using paired sample t-tests with significance accepted at P<0.05. Results The retinal arterial and venous oxygen saturation was 99.0±2.1% & 72.9±1.4% during normoxia. A reduction in the fraction of inspired oxygen resulted in a decline (P<0.001) in retinal arterial and venous oxygen saturation to 90.6±2.5% and 62.4±2.8%, respectively (Fig.1). The arterio-venous oxygen saturation difference in hypoxia (28.1±1.6%) was found to be 2% greater (P<0.001) than normoxia (26.0±1.3 %) (Fig.2). Conclusions The retinal arterial oxygen saturation recorded under normoxia closely approximates the values found using pulse oximetry (Data not shown). Acute inhalation of a hypoxic gas mixture resulted in a decline in both retinal arterial and venous saturation and an apparent widening of the a-vO2 difference. This result is unexpected since most studies report a reduction in the a-vO2 difference on exposure to hypoxia [2]. However, the transient nature of exposure may be too short for the reported increases in cerebral perfusion [2] and as a consequence we have observed a decrease in retinal venous saturation. [1] Mordant D. J. et al (2011) Spectral imaging of the retina, Eye (London, England), 25:3, 309-20 [2] Severinghaus, J.W. et al (1966) Cerebral Blood Flow In Man at High Altitude: Role of Cerebrospinal Fluid pH in Normalization of Flow in Chronic Hypocapnia, Circulation Research, 19:274-282