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James M. Beach, Alon Harris, Brent A. Siesky, Yoel Arieli, Aaron Pickrell; Longitudinal Oxygen Gradients Affect Corrections for Vessel Diameter Sensitivity in Retinal Oximetry. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5035.
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© ARVO (1962-2015); The Authors (2016-present)
To determine if oxygen gradients along retinal arterioles may lead to different measured diameter sensitivities in saturation measurements from arterioles and venules.
A sensitivity of oxygen saturation (OS) measurements to the vessel diameter was previously shown for dual wavelength oximetry. Diameter sensitivity (DS), expressed as an apparent change in %OS/um, causes an artifactual increase in measured oxygen saturation with reduction in vessel diameter. DS was found to differ for arterioles and venules, so that a correction for the effects of vessel diameter used different sensitivity values in the two vessel types. Here we assume that DS is not dependent on vessel type, but rather that different oxygen gradients along the two vascular trees produce only an apparent difference in DS. We further assume that the saturation gradient along venular trees is small, so that venules yield a more accurate estimate of DS. Under these assumptions, a gradient in saturation along the arteriolar tree was determined from apparent diameter sensitivities of the two vessels types. Measurements were obtained from six normal subjects using the Oxymap retinal oximetry monitoring system. Optical densities and diameters were computed at contiguous points along vessel segments of arteriolar and venular trees, averaging 148 um in diameter near the disc to 44 um in diameter in third-order vessels. Vessel oxygen saturations were found from optical density ratios using the Oxymap Analyzer software. Regression between saturation and vessel diameter gave apparent values of DS in arterioles and venules.
DS (apparent change in %OS/um) was -0.1 ± 0.08 SD in arterioles and -0.27 ± 0.06 SD in venules. Assuming just the venous value for DS and an OS of 92.2% in the large arteriolar segments, OS was found to decrease to 74.3%, in the smallest measured arterioles. This amount of reduction is in agreement with simulations of oxygen transport reported in retinal arterioles.
Presence of saturation gradients along arterioles may explain why separate diameter sensitivities have been reported in arterioles and venules. A diameter sensitivity that is independent of vessel type will simplify the correction of vessel oximetry for effects of vessel size.
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