Non-invasive in vivo spectrophotometric monitoring of brain cytochrome aa3 revisited.

Cytochrome aa3 (cyt aa3) is the main catalyst of cellular oxygen consumption. The properties of cyt aa3 will define the tissue oxygen requirements and provide an insight into energy supply and demand. Currently dual-wavelength (605-590 or 605-620 nm) reflectance spectrophotometry is used to monitor cyt aa3 redox state in vivo. We have experimentally demonstrated that the compensation for blood contamination in the surveyed tissue by these wavelength pairs is less than optimal. An alternative approach, similar to spectrophotometric analysis of multicomponent systems used in vitro, is presented in the triple wavelength equation as follows: delta cyt aa3 = 1.000 (delta A605) - 0.662 (delta A586.1) + 0.316 (delta A580) Based on a series of experiments performed in cuvette in vitro, isolated perfused rat head in situ, and living rat head in vivo, we have demonstrated that the cyt aa3 equation fully compensates for changes in cerebral blood volume and saturation. This non-invasive method of in vivo monitoring of cyt aa3 can provide the means to reliably and accurately determine tissue oxygen delivery under physiological conditions.