Increasing acclimation period improves the reproducibility of short-heating local thermal hyperemia.

Objectives: The axon reflex (AR) can be induced by the activation of afferent C-fibers during local skin heating. The previously used long-heating local thermal hyperemia (LTH) protocols tested AR flare by normalizing to endothelial-mediated maximal vasodilatation to adjust capillary heterogeneity when the recording sites were randomly selected. The AR flare induced by short local heating can be reproducible without using the data from subsequent longer heating for normalization when the recording sites were fixed with holders in the same session. The aim of this study was to investigate the effect of acclimation period on the intersession reproducibility of short-heating LTH when the recording sites were relocated after a longer interval of 1-3 days.

Methods: After 30 or 60 min acclimation, LTH with 5 min heating was assessed on bilateral human forearms using single-point laser Doppler flowmetry. The test was repeated at the same recording sites again at the same time 1-3 days later. Baseline and heating blood flux were analyzed and the data were expressed as different forms. Reproducibility of two tests was assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC) statistics.

Results: The intersession reproducibility of peak cutaneous vascular conductance (CVC) (CV=18.38%, ICC=0.82), peak CVC change (CV=20.38%, ICC=0.83) and 4 min area-under-the-curve (AUC) (CV=18.66%, ICC=0.75) of the right forearm and time to peak (CV=16.84%, ICC=0.52) of the left forearm were acceptable after 30 min acclimation. When the acclimation period was increased to 60 min, all of these data except 4 min AUC of both sides reached an acceptable level.

Conclusions: The AR flare induced by short local heating is reproducible when the recording sites are relocated by a predefined rule. The reproducibility of LTH on right forearm is different from that on left forearm, and increasing acclimation period improves the reproducibility.