Combination of ECG and PPG Signals for Healthcare Applications: A Survey Mahmoud M. Bassiouni 1* , Islam Hegazy 2 , Nouhad Rizk 3 , Sayed A. El-Dahshan 1,4 , Abdelbadeeh M. Salem 2 1 Department of Computer Science, Faculty of Computer and Information Technology, Egyptian E-Learning University (EELU), 33 El-messah Street, Eldoki 11261, El-Giza, Egypt 2 Faculty of Computer and Information Science, Ain Shams University, Abbassia 11566, Cairo, Egypt 3 Computer Science Department, Director of Undergraduate Studies, Houston University, Texas 77204, USA 4 Department of Physics, Faculty of Science, Ain Shams University, Cairo 11566, Egypt Corresponding Author Email: mbassiouni@eelu.edu.eg https://doi.org/10.18280/ama_b.641-409 ABSTRACT Received: 10 July 2021 Accepted: 5 November 2021 Most health care systems use various physiological signals to provide an accurate diagnosis performance. The main common signals functional in health care applications are the electrocardiogram (ECG) and photoplethysmogram (PPG). ECG signal represents the electrical cardiac activity of the heart, while the PPG signal measures the changes in the blood volume. There are several applications in which the ECG combined with PPG can be used in the field of medical health care. This survey illustrates the various applications that combine features from the ECG and PPG signals. The review manifests the techniques, methodologies used in the data acquisition, pre-processing of the signals. The feature extraction and classification phases for both ECG and PPG are explained. The limitations, challenges, and future directions for the combined application of ECG and PPG are clarified to solve the medical problems that existed, presented, and feasible. This study aims to increase the interest in applying the combination between ECG and PPG signals in more applications and to obtain optimal measurements related to cardiac activity. Keywords: healthcare, biomedical signal processing, electrocardiogram (ECG), heart rate variability (HRV), photoplethysmogram (PPG), health care systems, applications 1. INTRODUCTION Health Care Systems are defined as systems for the enhancement of human health through monitoring various organs in the human body [1]. These systems improve human health through diagnosis, prevention, and cure of specific diseases, recovery, and treatment of physical and mental losses in the people [2]. Two of the main common heart signals used in the health care systems are the electrocardiogram (ECG), photoplethysmogram (PPG). On one hand, ECG is one of the most salient attributes for continuous monitoring of health. One cardiac cycle is composed of five main peaks known as P, Q, R, S, and T [3]. The obstacles of the ECG are related to the heart location, size, fat or thin body because if the body is fat, it will affect the readings, users’ anatomy, and the electrode location concerning the structural location of the heart it can produce inaccurate or abnormal heart rate (HR) readings [4]. Therefore, to overcome these obstacles, ECG technology depends on the electrical impulses of the heart instead of the blood volume. On the other hand, PPG is defined as an optimal methodology for the measurement of the changes in the blood volume at the surface of the skin. It is considered to be the nearest alternative to the ECG signal as it holds essential cardiovascular information [5, 6]. All these merits of the PPG suffer from the inaccurate estimation of the HR and many other limitations such as the skin tone, different skin types, motion noises, and signal crossovers among others [7]. It is necessary to combine the capabilities of the ECG and the PPG signals to get the overall benefits of the features obtained from both signals. Figure 1 illustrates the ECG and PCG signals. Figure 1(a) shows the ECG signals in terms of waveforms, and the black circle represents the main ECG peaks while Figure 1(b) shows the PPG signal and its waveforms, and the black circles in subfigure (b) show the systole and diastole peaks. Figure 1. The ECG and its corresponding PPG signal Various studies have shown that several features obtained from the PPG and ECG signals and mutual information between the PPG and ECG are necessary. In this study, the applications in which the ECG and PPG signals combined will be illustrated. One of the applications in which the PPG and ECG signal can be applied is hypertension [8]. Hypertension is a group of disorders in the blood vessels and the heart. It sometimes has some symptoms such as headache, chest pain, difficulty in the breath, but most of the people that have hypertension do not have any symptoms at all, therefore, it is sometimes called a “silent killer” [9, 10]. Another application in which the ECG and PPG must be combined to have a better Advances in Modelling and Analysis B Vol. 64, No. 1-4, December, 2021, pp. 63-70 Journal homepage: http://iieta.org/journals/ama_b 63