Investigating the physiological mechanisms of the photoplethysmogram features for blood pressure estimation

OBJECTIVE Nowadays, photoplethysmogram (PPG) signals have been widely used to estimate blood pressure (BP) cufflessly and continuously, in which a number of different PPG features have been proposed and extracted from PPG signals for an objective of accurate BP estimation. However, the underlying physiological mechanism for PPG-based BP estimation still remains unclear, particularly those corresponding various PPG features. In this study, the physiological mechanism of existing PPG features for BP estimation was investigated, which may provide an insight into the physiological mechanism. APPROACH Experiments with cold stimuli and exercise trial were designed to change the total peripheral vascular resistance (TPR) and cardiac output (CO), respectively. Instantaneous BP and continuous PPG signal from 12 healthy subjects were recorded throughout the experiments. A total of 65 PPG features were extracted from the original, the first derivative, and the second derivative waves of PPG. The significance of the change of PPG features in the cold stimuli phase and in the early exercise recovery period was compared with that in the baseline phase. RESULTS Intensity-specific PPG features changed significantly (p<0.05) in the cold stimuli phase than in the baseline phase, demonstrating they were TPR-correlated. Time-specific PPG features changed significantly (p<0.05) in the early exercise recovery period than in the baseline phase, suggesting they were CO-correlated. And most of the PPG features associated with slope and area changed obviously both in the cold stimuli phase and in the early exercise recovery period, indicating that they should be TPR-correlated and CO-correlated. SIGNIFICANCE The findings of this study explained the intrinsic physiological mechanism why the proposed PPG features could be applied to BP estimation, and provided insights for exploring more diagnostic applications of the PPG features.