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S. E. LeBlanc, R. Munger; In vitro Investigation of the Effect of Optical Absorbance Models and Hemoglobin Extinction Coefficients on Retinal Oxygen Saturation. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3574.
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The ability to accurately measure retinal oxygen saturation (O2s) has high potential clinical value. The only non invasive method of achieving this is by an optical absorbance technique. Oxygen saturation of retinal blood is usually determined by a Beer Lambert absorbance model of the eye that takes advantage of the different extinction coefficients of hemoglobin and oxy-hemoglobin at different wavelengths. Oximetry values differ however in the literature depending on what type of light / eye interaction model is used: does light undergo a single or double pass through the blood vessels before exiting the eye and should vessel thickness matter? The purpose of this project is to investigate in vitro the impact of optical path length for single and double pass on calculated O2s using different published hemoglobin extinction coefficients.
Whole blood absorbance spectra from human subjects were obtained in vitro in both single and double pass configurations with cuvette thicknesses of 100 µm and 250 µm (N = 3). Oxygen saturation was calculated using a multiple linear regression absorbance model fitted from 520 nm to 800 nm using two different sources for hemoglobin and oxy-hemoglobin extinction coefficients. Results were compared with clinical CO -oximetry measurements.
The absorbance spectra varied with sample thickness and between single and double pass configurations. In the 100 µm samples, absorbance peak ratios (541 nm and 576 nm) varied by as much as 4% between the single and double pass. In the < 600 nm range, 250 µm samples measured in double pass showed a spectral shape no longer conform to accepted shape for whole blood absorbance for O2 calculation. Calculated O2s values varied by as much as 45 % depending on the sample thickness, absorbance model and hemoglobin extinction coefficients used.
The spectrum of whole blood changes as a function of optical path length. Single or double pass absorbance data varies significantly leading to significant errors in measured O2s. Differences between single pass and double pass measurements cannot be explained by the difference in optical path length alone.
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