Comparison of analysis strategies to assess sex differences in microvascular reperfusion using near-infrared spectroscopy

The near-infrared spectroscopy (NIRS) vascular occlusion test (VOT) assesses microvascular reperfusion. Two strategies have been used to quantify reperfusion following reactive hyperemia, but it is unclear whether both yield similar results when comparing biological sex. This study aimed to determine whether sex differences in NIRS-based microvascular reperfusion are similarly apparent using the 10-s reactive hyperemia slope of the tissue saturation index (StO₂) signal (slope 2) and the halftime to maximal reperfusion (T ½). Healthy, recreationally active males (<i>n</i> = 31) and females (<i>n</i> = 31) between 18 and 82 years took part in this study. A NIRS VOT was performed on the tibialis anterior muscle, and reperfusion was quantified using slope 2 (% s^-1) and T ½ (s). Adipose tissue thickness (ATT) was higher in females (<i>P</i> = 0.009), which was associated with a lower StO₂ (<i>P</i> = 0.001) and oxygenated hemoglobin (O₂Hb) (<i>P</i> = 0.05) signal range. The StO₂ slope 2 was significantly steeper in males versus females (<i>P</i> = 0.001) but not after correcting for ATT (<i>P</i> = 0.295). There were no sex differences in StO₂ T ½ (<i>P</i> = 0.067) or O₂Hb T ½ (<i>P</i> = 0.197). In a subset of males (<i>n</i> = 26) and females (<i>n</i> = 21) with similar ATT, there were no sex differences in StO₂ slope 2 (<i>P</i> = 0.068), StO₂ T ½ (<i>P</i> = 0.491), or O₂Hb T ½ (<i>P</i> = 0.899). An ATT-corrected StO₂ slope 2 or the T ½ approach is recommended for analysis of NIRS-based microvascular reperfusion when differences in ATT are present between sexes.<b>NEW & NOTEWORTHY</b> Sex differences in near-infrared spectroscopy (NIRS)-based microvascular reperfusion have been previously reported. We found that greater adipose tissue thickness in females reduces kinetic measures of NIRS-based microvascular reperfusion. Sex differences are eliminated when performing an adipose tissue thickness correction, when the NIRS signal range is accounted for, or when adipose tissue thickness is similar between sexes. This highlights the importance of considering factors that affect NIRS signals, such as adipose tissue thickness, when drawing comparisons between groups.