Synchronization of sacral skin blood flow oscillations in response to local heating.

Local heating causes an increase in skin blood flow by activating sensory axon reflex and metabolic nitric oxide controls. It has been observed that the remote skin area without temperature changes also shows a slightly increase in blood flow. The responsible mechanism of this indirect vasodilation remains unclear. We hypothesized that the remote skin area will have enhanced synchronization of blood flow oscillations (BFO), thus inducing a vasodilatory response. We studied BFO in two sites separated 10 cm of the sacral skin in 12 healthy people. Ensemble empirical mode decomposition method was used to decompose blood flow signals into a set of intrinsic mode functions (IMFs), and an IMF was selected to quantify each of myogenic, neurogenic, and metabolic modes of BFO. Then the instantaneous phase of the mode was calculated using the Hilbert transform. From the time series of phase difference between a pair of characteristic modes, we detected the epochs of phase synchronization and estimated the level of statistical significance using surrogate time series. The results showed that phase synchronization between neurogenic BFO was significantly higher in the period of the maximal vasodilation. We also observed a weak synchronization between myogenic BFO of the two skin sites. Our results suggested that synchronization of BFO may be associated with the changes in skin blood flow at the non-heated site.