Abstract
PurposeTo study the variation in intravascular oxygen saturation (oximetry) during an acute retinal vein occlusion (RVO) using hyperspectral computed tomographic spectroscopy based oximetry measurements.MethodsThirty rabbits were dilated and anesthetized for experiments. Baseline oximetry measurements were made using a custom-made hyperspectral computed tomographic imaging spectrometer coupled to a fundus camera. RVO were induced using argon green laser following an intravenous injection of Rose Bengal. RVO induction was confirmed by fluorescein angiography. Retinal oximetry measurements were repeated in arterial and venous branches one hour after RVO induction and up to 4 weeks afterwards. Comparison of retinal oximetry before and after vein occlusion was made using the Student T-test.ResultsOne hour after RVO induction, we observed statistically significant reductions in the intravascular oxygen saturation in temporal retinal arteries (85.1±6.1% vs. 80.6±6.6%; p<0.0001) and veins (71.4±5.5% vs. 64.0±4.7%; p<0.0001). This decrease was reversible in animals that spontaneously recannulated the vein occlusion. There were no statistically significant differences in oxygen saturation in the nasal control arteries and veins before and after temporal vein RVO induction.ConclusionsWe demonstrate, for the first time, acute changes in the intravascular oxygen content of retinal vessels 1 hour after RVO. These changes are reversible upon spontaneous recannulation of retinal vessels. This study demonstrates that hyperspectral computer tomographic spectroscopy based oximetry can detect physiological variations in intravascular retinal oxygen saturation. The study also provides the first qualitative and quantitative evidence of the variation in retinal vascular oxygen content directly attributable to an acute retinal vein occlusion.
Highlights
[1] The earliest studies demonstrating this technique showed a difference between arterial and venous retinal oxygen saturation using photographic emulsions and multiple filter systems. [2,3,4,5] More sophisticated methods were later developed to allow near simultaneous measurements of a few wavelengths to calculate oxygen saturation. [6,7,8,9,10,11,12,13] Other methods use rapid serial scanning with liquid tunable filters [14] or confocal scanning laser devices. [15,16] Sophisticated calibration methods are commonly required for all non-invasive, multi-wavelength oximetry systems since the measured light depends on hemoglobin saturation as well as hematocrit, vessel size, and light scattering. [8,17,18][19] An alternative method, phosphorescence quenching, measures the fluorescence of an oxygen-sensitive probe injected into the vitreous and has been used to demonstrated retinal tissue oxygen gradients in the rat
[21] This finding is supported by human oximetry measurements that show variations in retinal arteriovenous (AV) oxygen difference under physiological and pathological conditions. [23,24,25,26,27,28,29] For example, venous oxygen saturation in central retinal vein occlusions and branch retinal arterial occlusions is variably reduced [27,30,31]; measurements in humans can be confounded by retinal hemorrhages or nerve fiber layer edema in the early phase of disease and arteriovenous or venous-venous shunting in the later phases of disease
Movies S1 and S2 illustrate an example of the rabbit circulation at baseline and after retinal vein occlusion (RVO) induction, respectively
Summary
Noninvasive measurement of hemoglobin oxygen saturation (oximetry) has been demonstrated by analysis of oxygenated and deoxygenated hemoglobin absorption spectra. [1] The earliest studies demonstrating this technique showed a difference between arterial and venous retinal oxygen saturation using photographic emulsions and multiple filter systems. [2,3,4,5] More sophisticated methods were later developed to allow near simultaneous measurements of a few wavelengths (usually between 2–4) to calculate oxygen saturation. [6,7,8,9,10,11,12,13] Other methods use rapid serial scanning with liquid tunable filters [14] or confocal scanning laser devices. [15,16] Sophisticated calibration methods are commonly required for all non-invasive, multi-wavelength oximetry systems since the measured light depends on hemoglobin saturation as well as hematocrit, vessel size, and light scattering. [8,17,18][19] An alternative method, phosphorescence quenching, measures the fluorescence of an oxygen-sensitive probe injected into the vitreous and has been used to demonstrated retinal tissue oxygen gradients in the rat. [8,17,18][19] An alternative method, phosphorescence quenching, measures the fluorescence of an oxygen-sensitive probe injected into the vitreous and has been used to demonstrated retinal tissue oxygen gradients in the rat. [21] This finding is supported by human oximetry measurements that show variations in retinal arteriovenous (AV) oxygen difference under physiological and pathological conditions. [23,24,25,26,27,28,29] For example, venous oxygen saturation in central retinal vein occlusions and branch retinal arterial occlusions is variably reduced [27,30,31]; measurements in humans can be confounded by retinal hemorrhages or nerve fiber layer edema in the early phase of disease and arteriovenous or venous-venous shunting in the later phases of disease. The direct effect of the occlusive event on retinal vascular oxygen content is difficult to demonstrate and differentiate from possible secondary effects
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