Multi-Wavelength Photoplethysmographic Assessment of Fingertip Vasculature for Auto-Calibrated Cuffless Blood Pressure Monitoring

Long-term cuffless monitoring of blood pressure (BP) is of immense value in the diagnosis and prevention of cardiovascular diseases (CVD). While there are critical needs for wearable cuffless BP monitoring devices in personalized CVD care and hypertension management, current technologies face challenges in long-term accuracy which limits significantly its clinical applicability. To maintain the accuracy of cuffless blood pressure forecasting, frequent calibration is a vital necessity. However, conventional BP calibration approaches are inherently inconvenient and time-consuming. In this work, we address the challenge by developing a novel auto-calibration system enabling long-term BP estimation. The innovation of this approach combines depth-resolved vascular characteristics extracted from multi-wavelength photoplethysmographic (MWPPG) measurements at fingertip, with an adaptive cross-modal knowledge distillation algorithm for continuous refinement of blood pressure estimation. We have evaluated the system on the CAS-BP database of over a thousand subjects using various calibration strategies. The results have shown that frequent calibration can effectively enhance the accuracy of long-term blood pressure estimation. Compared to traditional manual calibration, the proposed auto-calibration method can perform frequent calibration of the model without requiring the true BP values. Furthermore, under the condition of daily auto-calibration, the system attains an optimal Mean Absolute Error (MAE) of 3.77 mmHg and 2.62 mmHg for systolic BP (SBP) and diastolic BP (DBP) respectively, significantly outperforming the performance achieved through traditional calibration methods. This work provides a novel conceptual framework to guide the calibration of cuffless wearable devices for BP monitoring. © 2025 The Authors.