Wavelet Components of Photoplethysmography During Reactive Hyperemia: Absolute vs. Relative Metrics

Photoplethysmography (PPG) is a non-invasive optical technique that quantifies blood volume pulsations and enables assessment of skin microvascular dynamics during vascular challenges. Its complex waveform can be decomposed by wavelet transform (WT) into physiological frequency bands reflecting cardiac, respiratory, myogenic, neurogenic, endothelial NO-dependent and endothelial NO-independent activity. Spectral activity may be expressed as either absolute power or relative contribution, which can capture different aspects of microvascular regulation. This study compared both metrics of PPG signals during post-occlusive reactive hyperemia (PORH). PPG was recorded bilaterally in twelve healthy adults (21.6 ± 1.9 years) during a 10-min baseline, 5-min occlusion, and 10-min recovery. Spectral power and percent contribution were calculated for each band and phase using nonparametric statistics (p < 0.05). Occlusion markedly suppressed higher-frequency oscillations (cardiac, respiratory, myogenic, neurogenic) while enhancing endothelial activity. Hyperemia produced a rebound of respiratory and endothelial oscillations above baseline. In the contralateral limb, occlusion induced a milder perfusion decrease, consistent with sympathetic vasoconstriction. Correlations between absolute and relative metrics were strong for cardiac, respiratory, and myogenic components (ρ > 0.80, p < 0.01), but weak or absent for endothelial bands. Absolute power primarily reflected perfusion magnitude, whereas relative contribution represented spectral redistribution among regulatory mechanisms. These metrics are complementary yet non-equivalent, underscoring the need for methodological consistency in the physiological interpretation of wavelet-derived PPG analyses.