Test-retest reliability of brain mitochondrial cytochrome-c-oxidase assessed by functional near-infrared spectroscopy.

Functional near-infrared spectroscopy (fNIRS) is a noninvasive method for measuring in vivo both hemodynamic and mitochondrial metabolic activities in brain cortical structures. Although the test-retest reliability of the hemodynamic measures, such as reflected by oxygenated (HbO2), deoxygenated (HHb) hemoglobin, and the tissue oxygenation index (TOI), has been previously reported to be good to excellent, the reliability of the metabolic signal indexed by oxidized cytochrome-c-oxidase (oxCCO) has not been reported. The present test-retest study compared the reliability of the metabolic and hemodynamic signals in 10 healthy participants undergoing hypo- and hypercapnia challenges. The primary reliability measure was the intraclass correlation coefficient (ICC). Results of both hypo- and hypercapnia showed that the oxCCO signal (ICC  =  0.876  /  0.757) had robust reliability comparable with that of the HbO2 (ICC  =  0.841  /  0.801), HHb (ICC  =  0.804  /  0.571), and TOI (ICC  =  0.574  /  0.614) signals. These findings show that the oxCCO signal can be assessed by fNIRS with comparable reliability to the hemodynamic measures. We discuss the results in light of current interest in a mitochondrial metabolic marker derived from fNIRS.