Effects of increased skin blood flow on muscle oxygenation/deoxygenation: comparison of time-resolved and continuous-wave near-infrared spectroscopy signals

We quantified the contribution of skin blood flow (SkBF) to tissue oxygenation/deoxygenation of the flexor digitorum profundus muscle during cutaneous vasodilation. Time-resolved near-infrared spectroscopy (TRS-NIRS) was utilized to measure the potential influence of optical factors [mean optical pathlength (PL) and coefficients of absorption (mu(a)) and reduced scattering ()] on the NIRS-derived signals of eight male subjects. The approximately threefold elevation of SkBF during 1 h whole-body heating (increased internal temperature similar to 0.9 A degrees C) increased both mu(a) and without changing PL. Assuming that the coefficient remained constant, i.e., as with continuous-wave (CW) NIRS, resulted in a significant increase in the apparent oxygenation [oxy(Hb + Mb), from 113 +/- A 13 mu M (mean +/- A SD) for control to 126 +/- A 13 for the increased SkBF condition, P < 0.01]: this was in marked contrast to the unchanged TRS-derived values. The deoxygenation [deoxy(Hb + Mb)] also increased from control to elevated SkBF (CW-NIRS, from 39 +/- A 8 to 45 +/- A 7; TRS, from 38 +/- A 6 to 44 +/- A 7 mu M; P < 0.01 for both), but less than that seen for oxy(Hb + Mb) and not different between TRS- and CW-NIRS. Further, and in contrast to oxy(Hb + Mb), temporal profiles of deoxy(Hb + Mb) measured by the two NIRS methods were not different. These findings support use of either NIRS method to estimate local muscle fractional O-2 extraction, but not oxygenation, when SkBF is increased at rest.