Phase-resolved photoplethysmography imaging of microvascular responses to skin prick testing

Background Allergic skin reactions induce pronounced microvascular changes that are typically evaluated using wheal-and-flare responses. While photoplethysmography imaging (PPGI) can quantify perfusion amplitude, little is known about how local inflammation affects the temporal organization of pulsatile microvascular signals. Methods This study is based on a secondary, cross-sectional observational analysis of data acquired during a previous clinical skin prick testing (SPT) protocol. Forearm PPGI recordings were acquired using a monochromatic camera, with simultaneously recorded fingertip photoplethysmography serving as a systemic reference. Cardiac-frequency components were extracted using wavelet ridge reconstruction, and phase shift (Δϕ), phase locking value (PLV), and phase-derived temporal offset (Δt) were computed. Linear mixed-effects models were used to assess associations with allergic reactivity. Results Reactive SPT sites showed increased cardiac-band perfusion and a consistent phase advance relative to both references. Allergic sites exhibited phase shifts of approximately − 0.54 rad (fingertip) and − 0.52 rad (saline), corresponding to a temporal advance of about 60–65 ms, while synchronization remained high (PLV ≈ 0.81–0.91). Conclusion Allergic inflammation is associated with local phase and temporal advance of pulsatile microvascular signals, indicating locally modified pulsatile dynamics in the skin. Phase analysis of PPGI provides complementary information beyond perfusion amplitude and may enhance objective assessment of inflammatory skin responses.