Reactive Hyperemia Reveals Fractal Scaling and Multiscale Complexity in Photoplethysmography Waveforms

Area of Science:

• Physiology
• Biomedical Engineering
• Nonlinear Dynamics

Background:

• Post-occlusive reactive hyperemia (PORH) is a standard method for assessing microvascular function.
• Current assessment relies on amplitude-derived indices, which fail to capture the temporal dynamics of vascular regulation.
• Photoplethysmography (PPG) is a widely accessible technology suitable for nonlinear analysis.

Purpose of the Study:

• To investigate the nonlinear characteristics of PORH using PPG.
• To compare nonlinear metrics with traditional amplitude-based indices for microvascular function assessment.
• To explore the temporal organization of vascular responses during PORH.

Main Methods:

• A standardized PORH protocol involving occlusion and reperfusion was applied to 12 healthy adults.
• Bilateral reflective green-light PPG was used to record vascular signals.
• Analysis included pulse amplitude, detrended fluctuation analysis (DFA α exponent), and multiscale entropy (MSE Complexity Index, CI).

Main Results:

• Occlusion significantly reduced pulsatility and collapsed the multiscale complexity (CI), while fractal structure (α) showed minimal change.
• During reperfusion, α showed a trend toward increased fractal persistence, but CI recovered only partially.
• Contralateral limb responses were subtle, characterized by minor α reductions and higher CI compared to the occluded limb.

Conclusions:

• Nonlinear analysis (DFA and MSE) reveals crucial aspects of PORH dynamics not captured by amplitude metrics.
• Occlusion disrupts multiscale complexity, while reperfusion partially restores correlation structure and dynamical richness.
• PPG technology enables advanced nonlinear characterization of microvascular responses in PORH.