Photoplethysmography imaging reveals frequency-specific microvascular responses to histamine in human skin

Skin prick testing (SPT) remains the gold standard for allergy diagnostics, yet it relies on subjective assessment of wheal size and provides limited insight into the underlying vascular mechanisms. In this study, the histamine positive control within the SPT framework was used as a standardized model to characterize frequency-specific microvascular responses using photoplethysmography imaging (PPGI).

PPGI data were analyzed from 16 healthy volunteers who underwent SPT with histamine and saline control sites. Vascular oscillations were analyzed in the cardiac (0.6-2.5 Hz), respiratory (0.15-0.6 Hz), and myogenic (0.05-0.15 Hz) frequency bands using 1-min segments to characterize time-resolved changes across the post-application stages.

A significant suppression of myogenic oscillations was detected at the histamine site, with an 80 % reduction observed 10-15 min post-application and a 60 % reduction at 15-20 min relative to baseline. Respiratory-band energy showed no significant changes during the experiment. No significant changes were observed at the negative control sites. Skin blood-volume-oscillation maps supported these observations, showing localized reduction of myogenic vasomotion in the affected areas.

Histamine-induced allergic responses produce localized, frequency-specific alterations in cutaneous blood flow, characterized by a sustained suppression of myogenic oscillations accompanied with flare-related hyperemia. These results support the hypothesis of "vascular locking," wherein vasodilation attenuates smooth muscle contractility, and highlight the potential of PPGI to enhance the diagnostic resolution of allergy testing by capturing vascular dynamics across multiple frequency bands.