Monitoring the dynamics of tissue oxygenation in vivo by phosphorescence quenching.

Tissue oxygen level is a critical determinant of both functionality and viability of cells and tissue in vivo. The oxygen level is highly regulated through complex, multilevel modulation of vascular resistance throughout the vascular tree. We have developed a method for oxygen measurement using oxygen dependent quenching of phosphorescence that is well suited for study of the regulation of tissue oxygenation in vivo. It is a minimally invasive optical method that makes it possible, in real time, to determine either mean oxygen pressure or entire histograms of the distribution of oxygen in the tissue microvasculature. When using near infrared phosphors, the measurements sample the blood volume throughout the tissue between the excitation and collection sites. By measuring phosphorescence lifetimes instead of intensity, interference by other pigments in the tissue that absorb or fluoresce at the measurement wavelengths is avoided. Since the strength of the signal is inversely related to the oxygen pressure, tissue regions with relatively low oxygen (hypoxia) can be readily identified. Calibration of the oxygen dependence of phosphorescence is absolute, eliminating the potential errors due to altered calibration, and the lifetime measurements do not «drift» over time of measurement.