Optical Feature Modeling of Photoplethysmographic Signals—Coupling Mechanisms of Blood Volume, Red Blood Cell, and Vascular Wall Models

光电容积脉搏描计法(Photoplethysmography, PPG)是一种无创技术，广泛应用于心率、血氧饱和度等生理参数的监测。然而，PPG信号的生成机制尚未完全明确。现有的血容量模型、红细胞模型以及近年来提出的血管壁运动模型均无法全面解释在不同解剖部位和检测条件下观察到的PPG信号变化。为此，本研究提出并验证了一种综合光学模型，该模型结合了血容量、红细胞和血管壁的影响，能够更全面地解释PPG信号的生成机制。为验证该模型的有效性，本研究开发了一套定制的多光谱PPG信号采集平台，采集了20名健康参与者左手食指远节和中节指节区域的PPG信号。实验结果表明，不同解剖部位和光谱通道PPG信号的脉动分量存在显著差异。通过定量分析各模型因素对PPG信号的贡献，验证了综合光学模型在解释这些差异方面的有效性和普适性，表明其可用于不同解剖部位的PPG信号分析，并为生理参数监测提供了新的理论支持。Photoplethysmography (PPG) is a noninvasive technique widely used for monitoring physiological parameters such as heart rate and blood oxygen saturation. However, the signal generation mechanism of PPG remains incompletely understood. Existing models—including the blood volume model, red blood cell model, and the recently proposed vascular wall model—fail to comprehensively explain PPG signal variations observed across different anatomical sites and measurement conditions. To address this, we propose and validate a comprehensive optical model that integrates the effects of blood volume, red blood cells, and vascular wall dynamics, providing a more complete explanation of PPG signal generation. To validate the model, we developed a custom multi-spectral PPG acquisition platform and collected signals from the distal and middle phalanges of the left index fingers of 20 healthy participants. Experimental results demonstrate significant differences in the pulsatile components of PPG signals across anatomical sites and spectral channels. A quantitative analysis of the contributions of each factor in the model confirms its validity and generalizability in explaining these variations. This comprehensive model enhances the understanding of PPG signal generation across various anatomical regions and provides a stronger theoretical foundation for improving physiological parameter monitoring.