The difficulties in comparing in vivo oxygen measurements: turning the problems into virtues

There has been rapid development of effective new tools that provide information on oxygenation in vivo and an increased recognition of how valuable such information can be. Consequently, there also has been considerable interest in comparing and evaluating the accuracy and usefulness of the different types of measurements. The various types of measurements usually do not measure the same thing. They may measure PO2 or [O2] or something less directly related, such as hemoglobin saturation. They may make measurements in different compartments (e.g. intracellular, extracellular, vascular) in the volume that they sample, the time span over which they average, the local perturbation that they may cause, etc. They also differ in their sensitivity, accuracy, ability to measure repetitively. However, these potentially confounding and confusing differences can be made into an outstanding virtue, if their nature is considered carefully. Then a proper model can relate them to each other. The ability to relate the various measurements to each other can be a powerful tool to test the validity of models that attempt to explain fully the distribution of oxygen in real systems and the factors that affect this. We then could have a major advancement in our understanding of oxygen transport in tissues, with an ability to determine accurately the effects of physiological and pathophysiological perturbations on oxygenation at all levels of cells and tissues in vivo.