Noninvasive Assessment of Tissue Oxygenation

 

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Abstract

Adequate measurement of microcirculatory oxygen (O₂) concentrations in vivo is essential for further understanding of determinants of cellular respiration under normal and pathological conditions. Conventionally, such types of measurements have been made by the use of O₂ electrodes and spectrophotometry of the hemoglobin (Hb) molecule. Spectrophotometrically measured changes in the absorption spectrum of the Hb molecule determine the ratio of the amount of oxyhemoglobin and deoxyhemoglobin. Oxygen electrodes provide quantitative measurements but are too big to measure at the microcirculatory level and, if inserted into the tissue, disrupt the microcirculatory environment. These constraints have led to the development of alternative methods. One of the most promising techniques in this respect has been the use of O₂-dependent quenching of phosphorescent dyes. In such measurements the decay time of phosphorescence following excitation by a light pulse is O₂ dependent. This time-resolved technique could be very suitable for use in vivo because many of the problems encountered with intensity measurements, such as the absorption and scattering of light by tissue components, are eliminated. O₂-dependent quenching of phosphorescence has been used for the determination of O₂ concentrations in vitro. The recent introduction of palladium (Pd)-porphyrin phosphorescent dyes allows quantitative in vivo measurements and could be developed clinically as a valuable technique for continuous measurement of microvascular O₂ partial pressure at multiple locations.