Abstract
Utilizing mobile phone cameras for continuous blood pressure (BP) monitoring presents a cost-effective and accessible approach, yet it is challenged by limitations in accuracy and interpretability. This study introduces four innovative strategies to enhance smartphone-based photoplethysmography for BP estimation (SPW-BP), addressing the interpretability-accuracy dilemma. First, we employ often-neglected data-quality improvement techniques, such as height normalization, corrupt data removal, and boundary signal reconstruction. Second, we conduct a comprehensive analysis of thirty waveform indicators across three categories to identify the most predictive features. Third, we use SHapley Additive exPlanations (SHAP) analysis to ensure the transparency and explainability of machine learning outcomes. Fourth, we utilize Bland-Altman analysis alongside AAMI and BHS standards for comparative evaluation. Data from 127 participants demonstrated a significant correlation between smartphone-captured waveform features and those from standard BP monitoring devices. Employing multiple linear regression within a cross-validation framework, waveform variables predicted systolic blood pressure (SBP) with a mean absolute error (MAE) of 9.86 ± 6.78 mmHg and diastolic blood pressure (DBP) with an MAE of 8.01 ± 5.15 mmHg. Further application of Random Forest models significantly improved the prediction MAE for SBP to 8.91 ± 6.30 mmHg and for DBP to 6.68 ± 4.54 mmHg, indicating enhanced predictive accuracy. Correlation and SHAP analysis identified key features for improving BP estimation. However, Bland-Altman analysis revealed systematic biases, and MAE analysis showed that the results did not meet AAMI and BHS accuracy standards. Our findings highlight the potential of SPW-BP, yet suggest that smartphone PPG technology is not yet a viable alternative to traditional medical devices for BP measurement.