Impact of cutaneous blood flow on NIR-DCS measures of skeletal muscle blood flow index

Near-infrared diffuse correlation spectroscopy (NIR-DCS) is an optical technique for estimating relative changes in skeletal muscle perfusion during exercise but may be affected by changes in cutaneous blood flow, as photons emitted by the laser must first pass through the skin. Accordingly, the purpose of this investigation was to examine how increased cutaneous blood flow affects NIR-DCS blood flow index (BFI) at rest and during exercise using a passive whole body heating protocol that increases cutaneous, but not skeletal muscle, perfusion in the uncovered limb. BFI and cutaneous perfusion (laser-Doppler flowmetry) were assessed in 15 healthy young subjects before (e.g., rest) and during 5 min of moderate-intensity handgrip exercise in normothermic conditions and after cutaneous blood flow was elevated via whole body heating. Hyperthermia significantly increased both cutaneous perfusion (∼7.3-fold; P ≤ 0.001) and NIR-DCS BFI (∼4.5-fold; P ≤ 0.001). Although relative BFI (i.e., fold-change above baseline) exhibited a typical exponential increase in muscle perfusion during normothermic exercise (2.81 ± 0.95), there was almost no change in BFI during hyperthermic exercise (1.43 ± 0.44). A subset of eight subjects were subsequently treated with intradermal injection of botulinum toxin-A (Botox) to block heating-induced elevations in cutaneous blood flow, which 1) nearly abolished the hyperthermia-induced increase in BFI and 2) restored BFI kinetics during hyperthermic exercise to values that were not different from normothermic exercise (P = 0.091). Collectively, our results demonstrate that cutaneous blood flow can have a substantial, detrimental impact on NIR-DCS estimates of skeletal muscle perfusion and highlight the need for technical and/or pharmacological advancements to overcome this issue moving forward.NEW & NOTEWORTHY We used passive whole body heat stress, in combination with local intradermal botulinum toxin type A treatment, to experimentally manipulate cutaneous blood flow and investigate its impact on NIR-DCS measures of skeletal muscle BFI at rest and during exercise. Collectively, the results show that cutaneous blood flow, which was augmented in response to passive whole body heat stress, markedly affects NIR-DCS-derived BFI, such that the BFI signal becomes dominated by changes in cutaneous red blood cell flux.