Spectrophotometric Retinal Oximetry in Pigs

Abstract

To assess the validity of spectrophotometric retinal oximetry by comparison to blood gas analysis and intravitreal measurements of partial pressure of oxygen (pO(2)). Female domestic pigs were used for all experiments (n = 8). Oxygen fraction in inspired air was changed using a mixture of room air, pure oxygen, and pure nitrogen, ranging from 5% to 100% oxygen. Femoral arterial blood gas analysis and retinal oximetry were performed at each level of inspiratory oxygen fraction. Retinal oximetry was performed using a commercial instrument, the Oxymap Retinal Oximeter T1. The device simultaneously acquires images at two wavelengths (570 nm and 600 nm), and specialized software automatically detects retinal blood vessels. In three pigs, invasive pO(2) measurements were performed after the initial noninvasive measurements. Comparison of femoral arterial oxygen saturation and the optical density ratio over retinal arteries revealed an approximately linear relationship (R(2) = 0.74, P = 3.4 × 10(-9)). In order to test the validity of applying the arterial calibration to veins, we compared noninvasive oximetry measurements to invasive pO2 measurements in three pigs. This relationship was approximately linear (R(2) = 0.45, P = 0.04). Noninvasive spectrophotometric oximetry is sensitive to changes in oxygen saturation in pigs and correlated with intravitreal pO(2) measurements and with femoral artery pO(2). Pigs present a higher intraindividual variability in retinal oxygen saturation and a lower overall saturation than do humans. The difference between porcine and human eyes makes direct comparisons of measurements difficult.