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The contribution of water to the quantitative determination of hemoglobin concentration and saturation by near-infrared spectroscopy in turbid media was investigated. The study consisted of in vitro measurements of an aqueous suspension containing Liposyn, bovine blood, and yeast buffered at pH 7.2. The optical coefficients of the medium (μa~0.03 to 0.08 cm-1, μs' ~ 6 cm-1 at wavelengths of 715 and 825 nm) were similar to those of biological tissue in the near-infrared, and the hemoglobin concentration was about 23 μM. It was possible to reversibly saturate and desaturate hemoglobin in the full range of 0 to 100% by flowing either oxygen or nitrogen through the suspension. In these experimental conditions, water absorption must be taken into account to obtain accurate oxyhemoglobin concentrations and low hemoglobin saturation values. By contrast, the water correction has a small effect on the determination of deoxyhemoglobin concentration and high hemoglobin saturation values. By extrapolating the result to physiological conditions, where water content is lower and hemoglobin content is higher than in the experimental conditions, it was concluded that water absorption should have a smaller effect on the determination of hemoglobin concentrations and saturation in tissues at the wavelengths used in this study. In particular, for hemoglobin concentrations larger than 100 μM, the water correction is less than 5% at saturation values higher than 50%.
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