Contents lists available at ScienceDirect International Journal of Medical Informatics journal homepage: www.elsevier.com/locate/ijmedinf Review article Ambulatory cardiac bio-signals: From mirage to clinical reality through a decade of progress Thamizhisai Periyaswamy a, ⁎ ,1 , Mahendran Balasubramanian b,1 a Department of Human Environmental Studies, 117 Wightman Hall, Central Michigan University, Mount Pleasant, Michigan, 48859, United States b Apparel Merchandising and Product Development, School of Human Environmental Science, 118 Home Economic Building, University of Arkansas, Fayetteville, Arkansas, 72701, United States ARTICLE INFO Keywords: Ambulatory electrocardiography Wearable healthcare Continuous monitoring Clinical viability ABSTRACT Background: Health monitoring is shifting towards continuous, ambulatory and clinically comparable wearable devices. Telemedicine and remote diagnosis could harness the capability of mobile cardiac health information, as the technology on bio-physical signal monitoring has improved significantly. Objectives: The purpose of this review article is (1) to systematically assess the viability of ambulatory elec- trocardiography (ECG), (2) to provide a systems level understanding of a broad spectrum of wearable heart signal monitoring approaches and (3) to identify areas of improvement in the existing technology needed to attain clinical grade diagnosis. Results: Based on the included literature, we have identified (1) that the developments in ECG monitoring through wearable devices are reaching feasibility, and are capable of delivering diagnostic and prognostic in- formation, (2) that reliable sensing is the major bottleneck in the entire process of ambulatory monitoring, (3) that there is a strong need for artificial intelligence and machine learning techniques to parse and infer the biosignals and (4) that aspects of wearer comfort has largely been ignored in the prevailing developments, which can become a key factor for consumer acceptance. Conclusions: Cardiac health information is crucial for diagnosis and prevention of several disease onsets. Mobile and continuous monitoring can aid avoiding risks involved with acute symptoms. The health information ob- tained through continuous monitoring can serve as the BigData of heart signals, and can facilitate new treatment methods and devise effective health policies. 1. Introduction Notable breakthroughs occurred in the technology forefront over the past decade that led to high-density electronics with transistors being sized at ˜10-20 nm [1], with computational efficiency reaching ˜100 GFLOPS per watt of power [2], and inexpensive data storage that costs < $0.01/GB of digital data [3]. Wearable sensing systems and mobile data collection systems took advantage of these growths and have emerged as attractive health monitors. Human health monitoring was previously confined to clinical settings, but with increasing pro- liferation of reliable, miniaturized digital electronics, real-time con- tinuous monitoring has gathered attention. These emergent technolo- gies could offer unprecedented opportunities towards measuring, monitoring and governing our daily-life signals that have remained intractable. But, why long-term monitoring of vital signals becomes necessary? The simplest argument is that having more data can assist evidence-based decision process in medical diagnosis and in prognosis. Furthermore, it could detect and transmit emergency situations to clinics or care-takers for immediate medical attention. Then, how technology and its associated research have shaped up signal mon- itoring, Cardiac signals in particular, during the past 10 years? While the mechanism to monitor the heart has remained the same, i.e., based on an electrode interface that monitors ionic voltage changes, technology has shifted the paradigm from clinical to ambulatory. Even before fo- cusing on to cardiac signals, it is worth mentioning that there are several other disease symptoms that can benefit from continuous monitoring. Notably, diabetes, renal diseases, respiratory diseases, cancer, post-surgery rehabilitation, sports medicine, neurological dis- order, stress disorder, and cardiovascular diseases are potential plat- forms for wearable signal monitoring [4–7]. There are pros and cons to https://doi.org/10.1016/j.ijmedinf.2019.07.007 Received 5 October 2018; Received in revised form 5 June 2019; Accepted 8 July 2019 ⁎ Corresponding author. E-mail addresses: periy1t@cmich.edu (T. Periyaswamy), mb048@uark.edu (M. Balasubramanian). 1 Authors have equal contribution in the submitted work. International Journal of Medical Informatics 130 (2019) 103928 1386-5056/ © 2019 Elsevier B.V. All rights reserved. T