Reactive Hyperemia – new realities and rationalisation

Reactive Hyperemia (RH) and Post-Occlusive Reactive Hyperemia (PORH) likely represent the best known in vivo exploratory strategies with diagnostic purposes. PORH is believed to represent the local microcirculatory physiology measured by the quantification of the reperfusion response after occlusion of a major artery. A reduction of related endpoints has been linked to impairment of the local microcirculatory unit where measurements take place. We believe this oversimplified vision precludes other mechanisms representing a more complex reality. To prove our views, we analyzed the effects of hyperemia following PORH and ice-cold exposure (ICE) in two groups of participants of both sexes (total n=34). In each group, PORH or ICE were applied to one randomly chosen limb using a pressure cuff (applied to the arm) or an ice-frozen covered surface (in contact with the palm). Occlusion was maintained for 2 minutes with a 200 mmHg pressure and ice contact was maintained for 3 minutes. Throughout the procedure, involving baseline (Phase 1), challenge (Phase 2) and recovery (Phase 3), perfusion was measured in both hands by laser Doppler flowmetry (LDF) while microcirculation imaging was assessed in the middle contralateral forearm by photoacoustic tomography (PAT). Hemodynamics were followed by the Task Force platform. A consistent perfusion decrease (LDF) occurs immediately in both hands with PORH or ICE in Phase 2. PAT revealed a strong constriction of superficial skin plexus that keeps perfusion to a minimum during this phase. When PORH or ICE ceases, reperfusion occurs rapidly in both limbs, although with different intensities, with blood moving from deeper to superficial plexus. No other changes were observed. These data confirm that RH IS NOT A SIMPLE LOCAL PHENOMENA – in fact both of these challenges evoke a sequence of two reflexes in both limbs - a fast strong constrictor (sympathetic?) response followed by a longer duration vasodilation. Both reflexes are meant to preserve hemodynamics in each phase of this experiment. In addition, these results indicate/confirm that RH DOES NOT RELATE TO ISCHEMIA – in fact sudden changes in perfusion and oxygen seem to be the triggers of adaptive response to preserve hemodynamics. Therefore with this procedure RH IS NOT A MEASURE OF LOCAL MICROCIRCULATION since reperfusion triggers another reflex generating durable vasodilation adaptation to preserve hemodynamics. This new rationalization on RH points to new research directions such as the efficiency of the adaptive response to a sudden hypoperfusion and hypoxia in Phase 2, or the components of the recovery period in Phase 3. These will provide a wider view of the circulatory homeostatic reflexes involved in these adaptive responses. FCT—Foundation for Science and Technology, I.P. (Portugal), under the [UIDB/04567/2020] and [UIDP/04567/2020] programs and ALIES-COFAC. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.