Dual-wavelength phosphorimetry for determination of cortical and subcortical microvascular oxygenation in rat kidney

This study presents a dual-wavelength phosphorimeter developed to measure microvascular P-O2 (mu P-O2) in different depths in tissue and demonstrates its use in rat kidney. The used phosphorescent dye is Oxyphor G2 with excitation bands at 440 and 632 nm. The broad spectral gap between the excitation bands combined with a relatively low light absorption of 632 nm light by tissue results in a marked difference in penetration depths of both excitation wavelengths. In rat kidney, we determine the catchments depth of the 440-nm excitation to be 700 mu m, whereas the catchments depth of 632 nm is as much as 4 mm. Therefore, the measurements differentiate between cortex and outer medulla, respectively. In vitro, no difference in P-O2 readings between both channels was found. On the rat kidney in vivo, the measured cortical mu P-O2 was on average 20 Torr higher than the medullary mu P-O2 over a wide P-O2 range induced by variations in inspired oxygen fraction. Examples provided from endotoxemia and resuscitation show differences in responses of mean cortical and medullary P-O2 readings as well as in the shape of the P-O2 histograms. It can be concluded that oxygen-dependent quenching of phosphorescence of Oxyphor G2 allows quantitative measurement of mu P-O2 noninvasively in two different depths in vivo. Oxygen levels measured by this technique in the rat renal cortex and outer medulla are consistent with previously published values detected by Clark-type oxygen electrodes. Dual-wavelength phosphorimetry is excellently suited for monitoring mu P-O2 changes in two different anatomical layers under pathophysiological conditions with the characteristics of providing oxygen histograms from two depths and having a penetration depth of several millimeters.