Non-invasive assessment of cerebral oxygenation: A comparison of retinal and transcranial oximetry

BACKGROUND: To investigate the correlation between cerebral (SO(2-transcranial)), retinal arterial (SaO(2-retinal)) and venous (SvO(2-retinal)) oxygen saturation as measured by near-infrared spectroscopy (NIRS) and retinal oximetry respectively. METHODS: Paired retinal and cerebral oxygen saturation measurements were performed in healthy volunteers. Arterial and venous retinal oxygen saturation and diameter were measured using a non-invasive spectrophotometric retinal oximeter. Cerebral oxygen saturation was measured using near-infrared spectroscopy. Correlations between SO(2-transcranial) and retinal oxygen saturation and diameter measurements were assessed using Pearson correlation coefficients. Lin’s concordance correlation coefficient (CCC) and Bland-Altman analysis were performed to evaluate the agreement between SO(2-transcranial) as measured by NIRS and as estimated using a fixed arterial:venous ratio as 0.3 x SaO(2-retinal) + 0.7 x SvO(2-retinal). The individual relative weight of SaO(2-retinal) and SvO(2-retinal) to obtain the measured SO(2-transcranial) was calculated for all subjects. RESULTS: Twenty-one healthy individuals aged 26.4 ± 2.2 years were analyzed. SO(2-transcranial) was positively correlated with both SaO(2-retinal) and SvO(2-retinal) (r = 0.44, p = 0.045 and r = 0.43, p = 0.049 respectively) and negatively correlated with retinal venous diameter (r = -0.51, p = 0.017). Estimated SO(2-transcranial) based on retinal oximetry showed a tolerance interval of (-13.70 to 14.72) and CCC of 0.46 (95% confidence interval: 0.05 to 0.73) with measured SO(2-transcranial). The average relative weights of SaO(2-retinal) and SvO(2-retinal) to obtain SO(2-transcranial) were 0.31 ± 0.11 and 0.69 ± 0.11, respectively. CONCLUSION: This is the first study to show the correlation between retinal and cerebral oxygen saturation, measured by NIRS and retinal oximetry. The average relative weight of arterial and venous retinal oxygen saturation to obtain the measured transcranial oxygen saturation as measured by NIRS, approximates the established arterial:venous ratio of 30:70 closely, but shows substantial inter-individual variation. These findings provide a proof of concept for the role of retinal oximetry in evaluating cerebral oxygenation.