Optical methods for cuffless blood pressure measurements.

Significance: Accurate, noninvasive, and continuous blood pressure (BP) monitoring without a cuff is a long-standing goal in biomedical engineering, having the potential to revolutionize hypertension care. Significant effort has gone toward the development of an optical cuffless BP device, primarily using the finger photoplethysmography (PPG) signal. Despite this effort, optical-based BP monitors have yet to achieve widespread adoption primarily due to accuracy concerns. Recently, new optical techniques such as multiwavelength PPG, imaging PPG, and speckle contrast optical spectroscopy/speckle plethysmography (SCOS/SPG) have shown promise toward improved performance. Aim: We aim to review methods in optical cuffless BP estimation. Approach: We first review prior literature in finger PPG, wrist PPG, multiwavelength PPG, imaging PPG, and SCOS/SPG strategies for BP estimation. We then review the current state of commercial cuffless BP devices and accuracy standards for new devices and provide an outlook for future work. Results: Most of the effort toward the development of an optical cuffless BP monitor has centered on the use of the finger PPG signal. The methods used to quantify device performance vary widely, hindering cross-device evaluation. To mitigate this issue, future works should adhere to recently published consensus standards for performance evaluation. Emerging techniques such as imaging PPG, multiwavelength PPG, and SCOS/SPG increase the depth of physiological information used for BP monitoring and may improve cuffless BP estimation going forward. Conclusion: Optical methods for cuffless BP monitoring hold enormous promise for the healthcare and consumer spaces, but further improvements in both technology and device characterization are needed to achieve widespread adoption.