Microcirculatory Response to Photobiomodulation—Why Some Respond and Others Do Not: A Randomized Controlled Study

Background and Objectives: Photobiomodulation (PBM), a non-ionizing, non-thermal irradiation, used clinically to accelerate wound healing and inhibit pain, was previously shown to increase blood flow. However, some individuals respond to PBM, but others do not. The purpose of this study was to investigate factors affecting this patient-specific response using advanced, noninvasive methods for monitoring microcirculatory activity. Study Design/Materials and Methods: In this prospective, randomized controlled clinical trial (NCT03357523), 20 healthy non-smoking volunteers (10:10 males:females, 30 ± 8 years old) were randomized to receive either red- (633 nm and 70 W/cm²) or near-infrared light (830 nm and 55 mW/cm²) over the wrist for 5 minutes. Photoplethysmography, laser Doppler flowmetry, and thermal imaging were used to monitor palm microcirculatory blood volume, blood flow, and skin temperature, respectively, before, during, and 20 minutes after irradiation. Participants with skin temperature change ≥0.5°C from baseline were considered “responders”. Results: Near-infrared PBM was found to induce a 27% increase in microcirculatory flow that increased to 54% during the 20-minute follow-up period (P = 0.049 and P = 0.004, respectively), but red light PBM did not increase the median flow. Only 10 of 20 participants were responders by thermal imaging (i.e., ≥0.5°C from baseline), and their initial skin temperature was between 33 and 37.5°C. The non-responders had either “hot” hands (≥37.5°C) or “cold” hands (≤33°C). In responders, the meantime to 20% increase in microcirculatory blood volume and blood flow was less than 2.5 minutes after initiation of PBM irradiation. Conclusions: We demonstrated that PBM induces arteriolar vasodilatation that results in both immediate and long-lasting increased capillary flow and tissue perfusion in healthy individuals. This response was wavelength-dependent and modified by skin temperature. These findings regarding physiological parameters associated with sensitivity or resistance to PBM provide information of direct relevance for patient-specific therapy. Lasers Surg. Med.