Wearable Photoplethysmography for Cardiovascular Monitoring This article summarizes the key literature on wearable photoplethysmography and points to future directions in this field. By PETER H. CHARLTON ,PANICOS A. KYRIACOU , Senior Member IEEE,J ONATHAN MANT , VAIDOTAS MAROZAS , Member IEEE,PHIL CHOWIENCZYK , AND J ORDI ALASTRUEY ABSTRACT | Smart wearables provide an opportunity to mon- itor health in daily life and are emerging as potential tools for detecting cardiovascular disease (CVD). Wearables such as fitness bands and smartwatches routinely monitor the photo- plethysmogram signal, an optical measure of the arterial pulse wave that is strongly influenced by the heart and blood vessels. Manuscript received October 3, 2021; revised January 6, 2022; accepted January 27, 2022. Date of current version March 9, 2022. This work was supported in part by the British Heart Foundation under Grant PG/15/104/31913 and Grant FS/20/20/34626, in part by the University of Cambridge EPSRC Impact Acceleration Account, in part by the Wellcome EPSRC Centre for Medical Engineering at King’s College London under Grant WT 203148/Z/16/Z, in part by the European COST ACTION-Network for Research in Vascular Ageing under Grant CA18216 supported by the European Cooperation in Science and Technology (COST), in part by the European Regional Development Fund under Project 01.2.2-LMT-K-718-01-0030 under a grant agreement with the Research Council of Lithuania (LMTLT), and in part by the Department of Health through the National Institute for Health Research Cardiovascular MedTech Co-Operative at Guy’s and St Thomas’ NHS Foundation Trust. (Corresponding author: Peter H. Charlton.) Peter H. Charlton is with the Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King’s College London, King’s Health Partners, London SE1 7EU, U.K., with the Research Centre for Biomedical Engineering, City, University of London, London EC1V 0HB, U.K., and also with the Department of Public Health and Primary Care, University of Cambridge, Cambridge CB1 8RN, U.K. (e-mail: pc657@medschl.cam.ac.uk). Panicos A. Kyriacou is with the Research Centre for Biomedical Engineering, City, University of London, London EC1V 0HB, U.K. (e-mail: p.kyriacou@city.ac.uk). Jonathan Mant is with the Department of Public Health and Primary Care, University of Cambridge, Cambridge CB1 8RN, U.K. (e-mail: jm677@medschl.cam.ac.uk). Vaidotas Marozas is with the Department of Electronics Engineering and the Biomedical Engineering Institute, Kaunas University of Technology, 44249 Kaunas, Lithuania (e-mail: vaidotas.marozas@ktu.lt). Phil Chowienczyk is with the Department of Clinical Pharmacology, King’s College London, London SE1 7EH, U.K. (e-mail: phil.chowienczyk@kcl.ac.uk). Jordi Alastruey is with the Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King’s College London, King’s Health Partners, London SE1 7EU, U.K. (e-mail: jordi.alastruey-arimon@kcl.ac.uk). This article has supplementary downloadable material available at https://doi.org/10.1109/JPROC.2022.3149785, provided by the authors. Digital Object Identifier 10.1109/JPROC.2022.3149785 In this survey, we summarize the fundamentals of wearable photoplethysmography and its analysis, identify its potential clinical applications, and outline pressing directions for future research in order to realize its full potential for tackling CVD. KEYWORDS | Cardiovascular (CV); photoplethysmogram (PPG); pulse wave; sensor; signal processing; smartwatch. I. INTRODUCTION Cardiovascular disease (CVD) is a major burden on indi- viduals and societies worldwide. In 2015, there were an estimated 422 million cases of CVD and 18 million deaths due to CVD [1]. Several effective strategies have been iden- tified to reduce cardiovascular (CV) risk, including drugs, such as antihypertensives, lipid-lowering agents, and anti- coagulants, and lifestyle changes, such as regular exercise, improved diet, and weight control [2]. Approaches to identify individuals at risk of CVD could prompt these interventions and help reduce CVD-associated mortality and morbidity. The proliferation of smart wearables equipped with photoplethysmography sensors provides an opportunity to monitor CV health in daily life. Photoplethysmography has already had a profound impact on clinical care through its use in pulse oximeters, which are routinely used to assess blood oxygen saturation in a wide range of clin- ical settings. The photoplethysmogram (PPG) signal is a measure of arterial blood volume, which fluctuates with each heartbeat and is used by many wearables to monitor heart rate (HR). The PPG also contains information on the cardiac, vascular, respiratory, and autonomic nervous systems. Consequently, signal processing techniques have been developed to extract additional physiological parame- ters from the PPG. If these techniques could be refined and This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ Vol. 110, No. 3, March 2022 |PROCEEDINGS OF THE IEEE 355