Nearinfrared spectroscopy NIRS

Human tissue contains substances that can be qualified and quantified according to their well-defined absorption spectra at near-infrared wavelengths81. The tissue concentrations of oxygenated and deoxygenated haemoglobin as well as oxidised cytochrome oxidase are proportional to tissue oxygenation and metabolism. Jobsis82 was the first to demonstrate that measurements of transmitted light of near-infrared radiation could be used to monitor the extent of oxygenation of certain compounds. According to Lambert-Beer's Law, the absorbance is proportional to the concentration of the compound in the solution and the optical path length:

A = absorbance measured in optical densities Po = parallel beam of radiation of power Po entering the medium P = power of radiation transmitted through the medium by absorption a = specific extinction coefficient of the absorbing compound c = concentration of the absorbing compound in the solution d = distance between entrance and leaving points of the light.

Cytochrome c oxidase is located in the mitochondrial membrane and is the terminal enzyme of the respiratory chain. It contains four active redox groups: haem iron centres a and a3, and copper centres CuA and CuB. The determination of cytochrome oxidase is more difficult than that of oxygenated haemoglobin due to its low tissue concentration. Oxygen availability within the cell limits the rate of oxygen consumption by cytochrome oxidase, resulting in a higher degree of reduction of the copper centre. The absorbance of near-infrared light by cytochrome oxidase reflects oxygen availability at the cellular level.

A primary goal for monitoring tissue oxygenation is the early detection of cellular respiratory failure. A therapeutic measure to correct tissue hypoxia requires profound knowledge about critical oxygen-supply thresholds of the individual organs. Monitoring oxygenation at the tissue and cellular levels83 will be important to titrate and to guide the dosage of oxygen during HBO therapy. The quality of sampling, the tissue volume interrogated by any local probe, the localization of the probe, and the distribution of capillaries must be taken into account for the interpretation of results.

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