## 4 Discussion and Conclusions  
Our pilot study has revealed that the contactless IPPG method can be used for assessing the hemodynamic changes caused by local thermal impact. We have found that BPA is a very sensitive marker of changes in skin blood flow that are observed in response to local heating of the forehead skin. A distinctive feature of our study (achieved due to use of a transparent warm compress) is the coincidence of the locally heated region with the area in which the parameters of PPG waveforms are assessed with high spatial resolution. This feature increases the reliability of the processed experimental data, since BPA reflects the state of microcirculation variation due to local thermal exposure, while simultaneously controlling blood flow in areas in which this exposure was not applied. Using the IPPG method, it is possible to select a localization region and compensate for the spatially heterogeneous distribution of blood vessels. This is due to subsequent averaging of the signal over the chosen area, with reference to the BPA parameter. In contrast, the LDF fiber probe, which is widely used for microcirculation assessment, cannot be applied for measurements through the compress. Moreover, it does not allow the assessment of microcirculation parameters simultaneously in the large facial area with high spatial resolution. Our study has also revealed that an increase of the BPA parameter and duration of the vasomotor response, caused by a few degrees of local heating, vary substantially. However, several universal features were observed. (i) The vasomotor response is clearly distinguishable by the IPPG method and \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Subject**} & **Heating** & **Heating** & **Response** & **Mean BPA** & **Mean BPA** & **Mean BPA** \\  & **Grade, \({}^{\circ}\)C** & **Time, min.** & **Duration,** & **before** & **during** & **after Impact,** \\  & & & & **min.** & **Impact, \%** & **Impact, \%** & **\%** \\ \hline ZDV & 1.0 & 7.0 & 7.0 & 0.39 \(\pm\) 0.01 & 0.58 \(\pm\) 0.08 & 0.39 \(\pm\) 0.06 \\ BVE & 1.9 & 6.0 & 6.5 & 0.55 \(\pm\) 0.07 & 0.86 \(\pm\) 0.04 & 0.53 \(\pm\) 0.02 \\ CMA & 2.0 & 6.0 & 6.0 & 0.50 \(\pm\) 0.23 & 0.85 \(\pm\) 0.23 & 0.44 \(\pm\) 0.15 \\ VMA & 2.4 & 6.5 & 6.5 & 0.57 \(\pm\) 0.04 & 0.63 \(\pm\) 0.06 & 0.62 \(\pm\) 0.03 \\ KVO & 2.5 & 7.0 & 11.0 & 0.48 \(\pm\) 0.02 & 0.68 \(\pm\) 0.08 & 0.46 \(\pm\) 0.07 \\ EIA & 3.0 & 6.0 & 7.0 & 0.69 \(\pm\) 0.07 & 1.35 \(\pm\) 0.32 & 0.84 \(\pm\) 0.10 \\ RDI & 3.3 & 6.0 & 6.5 & 0.56 \(\pm\) 0.14 & 1.02 \(\pm\) 0.29 & 0.66 \(\pm\) 0.23 \\ GRD & 3.5 & 7.0 & 7.0 & 0.44 \(\pm\) 0.02 & 0.56 \(\pm\) 0.09 & 0.48 \(\pm\) 0.02 \\ FDA & 4.2 & 7.5 & 11.5 & 0.44 \(\pm\) 0.02 & 1.31 \(\pm\) 0.22 & 1.14 \(\pm\) 0.10 \\ KGA & 4.9 & 6.5 & 11.5 & 1.08 \(\pm\) 0.05 & 2.24 \(\pm\) 0.61 & 1.32 \(\pm\) 0.15 \\ SVS & 5.0 & 6.0 & 11.0 & 0.46 \(\pm\) 0.02 & 0.94 \(\pm\) 0.15 & 0.85 \(\pm\) 0.06 \\ MNB & 5.3 & 7.0 & 11.5 & 0.48 \(\pm\) 0.04 & 0.71 \(\pm\) 0.06 & 0.55 \(\pm\) 0.02 \\ GLU & 6.1 & 6.0 & 11.0 & 0.51 \(\pm\) 0.04 & 0.96 \(\pm\) 0.18 & 0.73 \(\pm\) 0.18 \\ KVS & 6.6 & 5.5 & 9.5 & 0.43 \(\pm\) 0.03 & 1.13 \(\pm\) 0.15 & 0.94 \(\pm\) 0.14 \\ AME & 6.7 & 6.0 & 13.0 & 0.73 \(\pm\) 0.10 & 1.58 \(\pm\) 0.19 & 1.28 \(\pm\) 0.12 \\ SAA & 7.6 & 6.5 & 11.0 & 0.87 \(\pm\) 0.09 & 1.02 \(\pm\) 0.25 & 1.09 \(\pm\) 0.18 \\ SKP & 8.6 & 5.5 & 9.5 & 0.78 \(\pm\) 0.04 & 1.36 \(\pm\) 0.22 & 1.09 \(\pm\) 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Heating grade, duration of vasomotor response, and mean BPA measured in the forehead before, during, and after the thermal impact in different subjects. characterized by a several-fold increase of BPA. (ii) The response is localized in the area of thermal impact and arises immediately with impact. (iii) In most cases, the increase in BPA continues after the cessation of heat exposure and the respective diminishing of the skin temperature. These features were observed in all healthy subjects who participated in the experiment.