Multi-Wavelength PPG Amplitude Ratio Features for Cuffless Blood Pressure Estimation: An Exploratory Study

Background: Cuffless blood pressure (BP) estimation from photoplethysmography (PPG) remains challenging. This exploratory study investigated whether curvature-derived waveform features from multiple PPG wavelengths, particularly amplitude ratios, are associated with cuff-based BP readings. Methods: Red, infrared, and green PPG signals were recorded for 90 s from 48 physician-screened volunteers (23 males and 25 females; mean age, 63.41 ± 14.20 years). A single automated upper-arm cuff BP measurement was initiated approximately 2 s after each PPG recording. A curvature-based procedure identified pulse valleys, systolic peaks, dicrotic notches, and diastolic peaks, from which timing, amplitude, and amplitude ratio features were extracted. Separate linear regression models for systolic BP (SBP) and diastolic BP (DBP) were evaluated using participant-level ten-fold cross-validation. In each iteration, feature selection and model fitting were performed using only the nine training folds, and the fitted model was evaluated on the held-out fold. Results: In the descriptive analysis across all 48 participants, amplitude ratio features involving the systolic peak and dicrotic notch showed stronger associations with cuff-based BP readings than most individual timing and amplitude features. For green-light PPG, the systolic-peak-to-dicrotic-notch amplitude ratio (RatioSN) had a correlation of approximately 0.56 with SBP and 0.48 with DBP. Based on out-of-fold predictions, the best combined-feature models achieved root mean square errors of 12.87 mmHg for SBP and 7.17 mmHg for DBP, with corresponding R2 values of 0.87 and 0.83. Conclusions: Multi-wavelength PPG amplitude ratio features, particularly those involving the dicrotic notch, warrant further investigation for cuffless BP estimation. The substantial SBP error, small cohort, and use of a single cuff measurement obtained after each PPG recording limit interpretation. These findings do not establish clinical measurement accuracy or continuous BP tracking.