In vivo photometric analysis of hemoglobin.

Since virtually all the oxygen carried by blood at normal hematocrit is reversibly bound to red blood cell hemoglobin, the distribution of oxygen within the microcirculation can be determined from measurements of hemoglobin concentration and hemoglobin oxygen saturation in vessels of the network. Photometric methods that rely on light absorption and scattering properties of blood are described. Criteria for selecting the wavelengths needed to analyze hemoglobin in the microcirculation are specified. Two theoretical descriptions of light absorption and scattering, multiple scattering theory and photon diffusion theory, are applied to the problem. Practical approaches to the determination of hemoglobin concentration and oxygen saturation in the microcirculation follow from these theoretical formulations. Technical aspects of microscope photometry including light sources, microscopy, and detection systems are described with special emphasis on the problem of glare. The importance of in vitro as well as in vivo calibrations is stressed, and several recent applications of a working system are discussed. Current problems as well as future developments of this methodology are delineated as a guide to future work in this area.