Spectrophotometric retinal oximetry in pigs.

PURPOSE: To assess the validity of spectrophotometric retinal oximetry by comparison to blood gas analysis and intravitreal measurements of partial pressure of oxygen (pO(2)). METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.

Fractal analysis of the normal human retinal fluorescein angiogram.

The fractal dimension of the retinal vasculature and isolated venous and arterial trees down to a caliber of 40 microns was estimated in 23 routine fluorescein angiograms of normal retinas. Fractal dimension was determined with a method based on the box counting theorem. This method is less susceptible to the radial architecture of the retinal vascular tree than those previously reported (mass-radius relation and density-density correlation function). Two scale ranges with different fractal dimension were consistently present. The estimated fractal dimensions showed no significant difference between isolated arterial and venous trees which is not supported by previous reports. This method was designed for simple application in a clinical setting.

Angiotensin II binding receptors in retinal and optic nerve head blood vessels. An autoradiographic approach.

Angiotensin II (AII) binding sites were identified in cross-sections of the cat retinal and optic nerve vasculatures. The authors used 3H-AII and 125I-saralasin, an agonist and a high-affinity antagonist of AII receptors, respectively, to generate light microscopy autoradiograms in resin-embedded tissues. With both radioligands the presence of AII binding sites was confirmed in retinal arterioles but not in the veins or capillaries of the retina. Additionally the presence of such binding sites in the capillaries of the optic nerve head was shown. These results support the hypothesis that microvascular tone and perhaps autoregulatory responses of optic nerve capillaries might be influenced by vasoactive substances, such as AII, either leaking from the choroid or locally synthetized.