# PPG and Bioimpedance-Based Wearable Applications in Heart Rate Monitoring--A Comprehensive Review
###### Abstract  
The monitoring of hemodynamic parameters, such as heart rate and blood pressure, provides valuable indications of overall cardiovascular health. It is preferable that such monitoring is non-invasive and in real time via an affordable, compact and small-scale device for maximum convenience. Numerous literature sources have exploited derivations of these parameters from photoplethysmogram (PPG) and electrical bioimpedance (EBI) signal measurements through the use of calculation algorithms of varying complexity. Compared to electrocardiogram (ECG), these measurement techniques have a merit of well-established practices of designing a wearable device that could conveniently be put on a wrist. The current paper provides a comprehensive review on the use of PPG and EBI measurement techniques in the context of hemodynamic parameter monitoring using a wearable device. A special emphasis is placed on the most basic hemodynamic parameter--heart rate--describing different algorithms of heart rate detection and monitoring. The last section provides an overview of commercially available and in-home wearable device technologies based on PPG and EBI measurements, their design challenges, and future prospects. bioimpedance; photoplethysmography; heart rate; wearable device; monitoring +
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Multiple long-term studies have shown that a higher resting heart rate (HR) is linked to cardiovascular diseases [5]. Supplementary information on symptoms, such as stress, physical activity, eating, sleeping, etc., traditionally has been verbally obtained from patients. This additional information is crucial in medical decision making. However, the information obtained in such a way may be subjective and misleading especially in case of patients with memory or communication difficulties. On the other hand, data measured with a wearable device can provide objective information continuously in real time [6]. The integration of computing algorithms and communication functionalities within the wearable device allows patients to share important computed biomarkers, such as HR, blood pressure, glucose levels, and others with their doctor in an Internet of Things (IoT) framework [7] to aid the medical diagnosis [6]. Wireless connectivity such as WiFi permits the constant upload of data to cloud storage, enabling continuous and simultaneous monitoring of multiple parameters [6] (see Figure 1). A challenge for implementing wearable technology for applications in intelligent IoT devices for monitoring health conditions is the need for a reliable energy source that can ensure long-term operation [8]. Traditionally, the most popular non-invasive methods used to monitor HR have been electrocardiograms (ECGs) through electric signals and photoplethysmography (PPG) through optical measurements [9]. However, wearable ECG monitoring systems are difficult to design, despite offering means to detect irregularities in the cardiograms and being an overall gold-standard technique for cardiovascular diagnostics [9]. Compared to chest-strap electrodes that have traditionally been used in HR monitoring, PPG-based measurements from wrist-worn devices provide continuous monitoring with greater convenience. Moreover, the latest devices report HR detection error within five percent bounds compared to telemetry [10]. Another technique with a great potential for monitoring of cardiovascular biomarkers is bioimpedance. Even though it is still not completely established in clinical practice, its versatility, simplicity and low cost facilitate its integration into wearable systems for remote monitoring of physiological parameters [11], including HR. This paper aims to bring about a comprehensive review on existing studies of PPG and bioimpedance-based vital bioparameter monitoring using wearable devices, both commercial and designed by different research groups. The following chapter discusses biological signals for a wearable-based HR devices.