Possible Influences on the Interpretation of Functional Domain (FD) Near-Infrared Spectroscopy (NIRS): An Explorative Study

The influence of subcutaneous adipose tissue (ATT) and oxygen (O-2) delivery has been poorly defined in frequency domain (FD) near-infrared spectroscopy (NIRS). Therefore, the aim of this study was to investigate the possible influence of these variables on all FD NIRS responses using a reliable protocol. Moreover, these influences were also investigated when using relative oxy- and deoxyhemoglobin and -myoglobin (oxy[Hb+Mb] and deoxy[Hb+Mb]) values (in %). A regression analysis was carried out for ATT and maximal-minimum oxy[Hb+Mb], deoxy[Hb+Mb], oxygen saturation (SmO2), and total hemoglobin (totHb) amplitudes during an incremental cyclic contraction protocol (ICCP) in a group of 45 participants. Moreover, the same analysis was carried out between subcutaneous ATT and the relative oxy- and deoxy[Hb+Mb] values (in %). In the second part of this study, a regression analysis was performed for peak forearm blood flow (FBF) during ICCP and the absolute and relative NIRS values in a group of 37 participants. Significant exponential correlation coefficients were found between ATT and deoxy[Hb+Mb] (r = 0.53; P<0.001), oxy[Hb+Mb] (r = 0.57; P<0.001), and SmO2 amplitudes (r = 0.57; P<0.001). No significant relations were found between ATT and relative oxy[Hb+Mb] (r = 0.37; P = 0.07) and deoxy[Hb+Mb] (r = 0.09; P = 0.82). Significant positive correlation coefficients were found between force at exhaustion and maximal FBF (r = 0.66; P<0.001), maximal differences in deoxy[Hb+Mb] (r = 0.353; P = 0.032) and totHb (r = 0.512; P = 0.002) while no significant correlation coefficients were found between these maximal force values and maximal differences in oxy[Hb+Mb] (r = -0.267; P = 0.111) and SmO2 (r = -0.267; P = 0.111). Significant linear correlation coefficients were found between FBF and deoxy[Hb+Mb] (r = 0.51; P = 0.001), oxy[Hb+Mb] (r = -0.50; P = 0.001), SmO2 (r = -0.54; P = 0.001), and totHb amplitude (r = 0.61; P<0.001). No significant correlations were found when using relative oxy[Hb+Mb] (r = -0.01; P = 0.957) and deoxy[Hb+Mb] (r = -0.02; P = 0.895). Based on these findings, caution is advised when using NIRS values, as subcutaneous ATT and O-2 delivery significantly influence NIRS measurements. To eliminate these influences, use of relative deoxy[Hb+Mb] is advised, especially in clinical settings or in people with a higher subcutaneous ATT layer.