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K.R. Denninghoff, R.A. Chipman, R.I. Park, D. Salyer, N. Beaudry, T. Karen; A New Spectral Oxyhemoglobin Saturation Method Applied to the Swine Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):492.
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Recent work by our group has demonstrated that using intra ocular illumination of the fundus can simplify the light paths associated with retinal imaging. Our group has also recently reported the use of a novel method to measure oxyhemoglobin saturation that is insensitive to changes in pH, path length or concentration and is more accurate than previous methods. In this abstract we report the use of these methods to measure oxygen saturation in the retinal arteries of swine during progressive hypoxia.
Five swine retinae were illuminated using intraocular placement of a fiberoptic light pipe that diffusely places a spot of light on the retina at an oblique angle removing the vessel apex glint. Multiple wavelengths of light were introduced through the light pipe and the image of the retina obtained through the cornea, lens and pupil using a CCD Camera. The spectra from 460 nm to 521 nm were measured at about 10 nm increments and the results fit to a parabola. Measurements were made on the retinal vessels while the anesthetized swine breathed progressively lower concentrations of oxygen. The systemic arterial oxygen saturation was measured using an ISTAT blood gas machine at the same time that optical images were obtained.
The spectral minima correlated strongly with oxygen saturation (r2 = 0.95) and behaved as predicted by our work on whole blood as well as hemoglobin. The standard deviation of calibration residuals was ±3.0% saturation with no correction for path length, hemoglobin concentration, fundus reflectivity, or animal studied.
The first in vivo use of the spectral minimum technique to measure retinal arterial oxygen saturation was reproducible in a small number of animals and the calibration was nearly identical to that seen when in vitro hemoglobin solutions were tested (2.9% saturation). The use of this technique appears promising and requires further study to determine if it will provide truly calibrated measurements in the retina during reflectance retinal spectroscopy.
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