Author(s): NERI, M., MARCIANO, A., MOTTO, C., LACERENZA, M., PORCELLI, S., Institution: UNIVERSITY OF PAVIA, Country: ITALY, Abstract-ID: 2095

INTRODUCTION:
Near-infrared spectroscopy (NIRS) is a non-invasive technique that detects tissue concentration changes of heme and other chromophores through near-infrared light absorption and scattering (μ’s) [1,2]. In continuous-wave (CW) NIRS, μ’s is assumed to be constant and changes in chromophores concentrations are calculated from changes in light absorption. Alternatively, the tissue optical properties can be determined quantitatively by time-dependent (TD) devices. Since changes in microvascular blood volume, subcutaneous adipose thickness (ATT), and different muscle groups influence μ’s, NIRS derived parameters might be altered in an unpredictable manner when constant μ’s is assumed [3,4,5]. In this study, we investigated heme-related changes of two different knee extensor muscles by both CW and TD NIRS during vascular ischemia and reperfusion. We hypothesized that tissue saturation index (TSI) values and functional indexes during ischemia-reperfusion differ between muscles and devices.
METHODS:
Twenty-four healthy active male participants (26±4 years, 76±12 kg) were recruited. ATT of vastus lateralis (VL) and vastus medialis (VM) was measured by ultrasound. TSI of VL and VM muscles was assessed using CW (Train.Red Plus, Train.Red BV) and TD (NIRSBOX, PIONIRS srl) devices during a 5-min vascular occlusion test (VOT). Baseline (BAS), minimum (MIN), and maximum (MAX) values were recorded. Desaturation rate (Slope1) and reperfusion rate (Slope2) were also calculated. Statistical differences were evaluated by two-way RM ANOVA with muscle (VL vs VM) and device (CW vs TD) as main effects. Tukey’s post-hoc test was used to test differences when interactions were present.
RESULTS:
ATT was significantly lower in VL compared to VM (4±2mm vs 6±2mm, p<0.05). A muscle-effect for TSI was observed at BAS (VL: 72.1±7.8% vs VM: 73.4±7.4%, p<0.001) and MIN (VL: 48.9±16.3% vs VM: 52.2±16.9%, p<0.001). Slope1 was significantly higher in VL (-0.10±0.04%*s-1) than VM (-0.09±0.04%*s-1, p<0.01). At BAS, MIN and MAX, TSI was significantly lower when determined by CW, compared to TD. A device effect was observed for both Slope1 and Slope2 (all p<0.001). A significative interaction muscle x device was found for Slope2 (p<0.05).
CONCLUSION:
Our findings show significant differences in TSI values between CW and TD NIRS, as well as between VL and VM, suggesting caution when functional outcomes obtained with different devices or from different muscle sites are compared. Therefore, instrument- and site-specific reference values are essential when NIRS indices are used to identify endothelial dysfunction in ageing, disuse and clinical populations.

REFERENCES:
[1] Grassi et al, J Biomed Opt, 2016
[2] Barstow, J Appl Physiol, 2019
[3] Chance et al, Am J Physiol Cell Physiol, 1992
[4] McCully et al, J Appl Physiol, 1994
[5] Hammer et al, J App Physiol, 2019