Microvascular oxygenation responses are associated with the reactive hyperemic stimulus but not brachial artery flow-mediated dilation

Microvascular responses contribute to the post-occlusive shear rate stimulus (quantified as the area under the curve [SR AUC]) that precedes flow-mediated dilation (FMD). While FMD is measured via ultrasonography, near-infrared spectroscopy (NIRS) characterizes aspects of microvascular function; however, the extent to which NIRS-derived indices relate to FMD remains unclear. Therefore, this investigation examined the associations between NIRS-derived metrics and peak hyperemic velocity (PHV), FMD, and SR AUC in 20 healthy adults. Skeletal muscle tissue oxygenation (StO2) was assessed using NIRS during a vascular occlusion test, while brachial artery diameter and blood velocity were measured using duplex ultrasound. NIRS-derived measures included the rate of resaturation (upslope), maximum resaturation (StO2max), and the area under the curve (AUC) of sustained hypersaturation. Pearson correlations and multiple linear regression were used to assess relationships between NIRS-derived measures, PHV, FMD, and SR AUC. There were no observed significant associations between NIRS-derived metrics and FMD (p > 0.08). In contrast, StO2max was significantly related to PHV (r = 0.530, p = 0.016) and SR AUC (r = 0.629; p = 0.004), and independently predicted SR AUC (β = 0.632, t = 3.893, p = 0.001). Baseline artery diameter (p = 0.346), SR AUC (p = 0.153), and upslope (p = 0.790) did not predict FMD (R = 0.668, adjusted R2 = 0.336, F(3,15) = 4.03, p = 0.027). These findings suggest that NIRS-derived metrics represent distinct yet complementary measures to FMD, while the association between StO2max and SR AUC suggests a relationship between microvascular function and the shear stimulus underlying FMD.