Small nerve fibers regulate local skin blood flow in response to local thermal perturbations. Small nerve fiber function is difficult to assess with classical neurophysiological tests. In this study, a vasomotor response model in combination with a heating protocol was developed to quantitatively characterize the control mechanism of small nerve fibers in regulating skin blood flow in response to local thermal perturbation. The skin of healthy subjects’ hand dorsum (n = 8) was heated to 42 °C with an infrared lamp, and then naturally cooled down. The distance between the lamp and the hand was set to three different levels in order to change the irradiation intensity on the skin and implement three different skin temperature rise rates (0.03 °C/s, 0.02 °C/s and 0.01 °C/s). A laser Doppler imager (LDI) and a thermographic video camera recorded the temporal profile of the skin blood flow and the skin temperature, respectively. The relationship between the skin blood flow and the skin temperature was characterized by a vasomotor response model. The model fitted the skin blood flow response well with a variance accounted for (VAF) between 78% and 99%. The model parameters suggested a similar mechanism for the skin blood flow regulation with the thermal perturbations at 0.03 °C/s and 0.02 °C/s. But there was an accelerated skin vasoconstriction after a slow heating (0.01 °C/s) (p-value 

Introduction: Small nerve fiber dysfunction is an early feature of diabetic neuropathy. There is a strong clinical need for a non-invasive method to assess small nerve fiber function. Small nerve fibers mediate axon reflex-related vasodilation and play an important role in thermoregulation. Assessing the reflex vasodilation after local heating might elucidate some aspects of small fiber functioning. In this study, we determined the reproducibility of the reflex vasodilation after short local heating in healthy subjects, assessed with thermal imaging and laser Doppler imaging.
Methods: Healthy subjects underwent six heating rounds in one session (protocol I, N = 10) or spread over two visits (protocol II, N = 20). Reflex vasodilation was elicited by heating the skin to 42 °C with an infrared lamp. Skin temperature and skin blood flow were recorded during heating and recovery with a thermal imaging camera and a laser Doppler imager. Skin temperature curves were fitted with a mathematical model to describe the heating and recovery phase with time constant tau (tauHeat and tauCool1).
Results: The reproducibility of tau within a session was moderate to excellent (intra-class correlation coefficient 0.42–0.86) and good (0.71–0.72) between different sessions. Within one session the differences in tauHeat were small (bias ± SD − 1.3 ± 18.9 s); the bias between two visits was − 1.2 ± 12.2 s. For tauCool1 the differences were also small, 1.4 ± 6.6 s within a session and between visits − 1.4 ± 11.6 s.
Conclusions: The heat induced axon reflex-related vasodilation, assessed with thermal imaging and laser Doppler imaging, was reproducible both within a session and between different sessions. Tau describes the temporal profile in one parameter and represents the effects of all changes including blood flow and as such, is an indicator of the vasodilator function. TauHeat and tauCool1 can accurately describe the dynamics of the axon reflex-related vasodilator response in the heating and recovery phase respectively.