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Pang-yu Teng, Justin Wanek, Norman P. Blair, Mahnaz Shahidi; Retinal Oxygen Extraction Fraction: the Ratio of Oxygen Consumption to Delivery. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2168.
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Oxygen extraction fraction (OEF) is the fraction of oxygen removed by the tissue from blood. By Fick’s principle, OEF equals the ratio of oxygen consumption to delivery. In the brain, increased OEF has been associated with cerebral ischemia and is considered a predictor of stroke. To our knowledge, OEF has not been previously measured in the retinal tissue. The purpose of this study is to report a method for quantitative measurement of retinal OEF in rat, and compare retinal OEF under systemic normoxic and hypoxic conditions.
Retinal vascular oxygen tension (PO2) was measured using our established optical section phosphorescence lifetime imaging system in 10 rats ventilated with 21% (normoxia) and 10% (hypoxia) oxygen. Systemic arterial blood gas tensions (PaO2, PaCO2), pH, hemoglobin concentration, mean blood pressure (BP) and heart rate (HR) were obtained prior to imaging. Retinal vascular PO2 measurements were converted to O2 contents. OEF was quantified in a retinal sector, a zone bounded by 2 major arteries with a major vein between them, in areas temporal and nasal to the optic disc. In each sector, retinal OEF was calculated as (m[O2]A - [O2]V)/m[O2]A, where m[O2]A was the mean arterial O2 content of 2 major arteries and [O2]v was the venous O2 content. Paired Student’s t-test was used for comparative statistical analysis. Statistical significance was accepted at p < 0.05.
During normoxia, PaO2, PaCO2, pH, BP and HR were 93 ± 8 mmHg, 40 ± 5 mmHg, 7.38 ± 0.05, 113 ± 17 mmHg and 214 ± 36 beats/min, respectively (mean ± SD; N = 10). During hypoxia, PaO2, PaCO2, pH, BP and HR were 34 ± 4 mmHg, 41 ± 5 mmHg, 7.31 ± 0.05, 78 ± 22 mmHg and 162 ± 45 beats/min, respectively. PaO2, pH, BP and HR under normoxia and hypoxia were significantly different (p ≤ 0.002; N = 10). Under both normoxia and hypoxia, retinal OEF measurements obtained in nasal and temporal areas were similar (p ≥ 0.18; N = 10). Retinal OEF under hypoxia (0.71 ± 0.17) was significantly higher than under normoxia (0.46 ± 0.13) (p < 0.001; N =10).
Quantitative measurement of retinal OEF was reported for the first time, demonstrating an increase in the fraction of oxygen extracted by retinal tissue during hypoxia. Retinal OEF can indicate compromised oxygen delivery and may become valuable for assessing retinal viability and ischemic injury.
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