Muscle oxygenation by near-infrared-based tissue oximeters.

TO THE EDITOR: In a recent article, R. C. Davies et al. (2) presented the results of a study investigating the effect of eccentric exercise-induced muscle damage on the dynamics of pulmonary O2 uptake by near-infrared spectroscopy-based measurement of oxygenation in the vastus lateralis muscle. Although these results are of interest (1), we believe that certain points concerning the presentation of the data concerning muscle oxygenation need to be addressed further. For measuring muscle oxygenation, Davies et al. (2) used a multi-distance spatially resolved tissue oximeter (NIRO-300, Hamamatsu Photonics KK) without exploiting the advantage of quantifying muscle oxygenation directly as tissue oxyhemoglobin (O2Hb) saturation (namely tissue oxygenation index, TOI%). TOI reflects the dynamic balance between O2 supply and O2 consumption and is independent of near-infrared photons pathlength in muscle tissue (4, 9). Near-infrared spatially resolved spectroscopy implies that the light intensity is measured at several distances from the source detector (4). This noninvasive technique, therefore, allows measurement of the slope of light attenuation vs. distance, resulting in a high signal-to-noise ratio, without being as sensitive to optical coupling and superficial tissue layers (4). Davies et al. (2) made their physiological conclusions mainly on the basis of interpretation of the changes in deoxy-hemoglobin concentration [HHb]. The authors stated that “HHb signal is relatively insensitive to blood volume changes during exercise and thus reflects the balance between the delivery and utilization of O2.” This statement is incorrect and was erroneously attributed to the review published by Ferrari et al. (6), which makes no reference to this assumption. Indeed, [HHb] changes might represent tissue oxygenation changes only when tHb (tHb O2Hb HHb) is stable. Davies et al. (2), however, expressed HHb and tHb results as arbitrary units using different scales. The data should have been reported as changes in concentration expressed as micromoles per centimeters or micromoles [if a pathlength factor is used (5)] to make the data comparable with other studies. We would like to comment that, since the introduction of the tissue oximeters in 1995 (3), tissue O2Hb saturation ensures more accurate evaluation of changes in oxygenation in muscle and brain tissues (7, 9). Considering the potential physiological importance of the findings reported by Davies et al. (1), we would suggest that the results could be strengthened by including TOI data.